TNTRACELLTJLAB DIGESTION Ol1 INVERTEBRATES.
89
Researches on the Intracellular Digestion of
Invertebrates.
By
Dr. Ellas MetsehniKoff.1
I HAVE long been of opinion that many questions connected
with the genealogy of the. Metazoa are not to be solved by the
methods of purely morphological embryology. Morphology
certainly considers as of primary importance the structure of
organs which have either lost their function by retrogression,
or which, being not yet fully developed, have not acquired
their full functional activity; but in the determination of the
phylogeuetic importance of such organs, a knowledge of their
physiological history is often indispensable. The embryonic
history of an animal or an organ shows us a series of phenomena, often extremely complicated, among which mere embryology cannot in many cases choose out those which are
primitive from those of secondary value. Every student
of comparative embryology knows how hard it is to determine
in any given case the genealogical meaning of a certain phenomenon, and how often the standard used is purely subjective.
These difficulties are increased by the fact that the primitive
Metazoa have all disappeared, so that the gap between the
Metazoa of to-day and the Protozoa is wide indeed.
From what has been said, it is evident that, in attempting to
discuss the evolution of the alimentary canal, one of the oldest
and most widely distributed of all the organs of the Metazoa,
one must collect not only embryological evidence as to the
mode of formation of the endoderm, but also physiological
evidence as to its function.
1
Translated from the " Arbeiten a. d. Zoolog. Instit. Wien.," 1883.
90
DR. ELIAS METSOHNIKOFF.
When it became known that all the lower Metazoa, such as
Sponges, Coelenterates, and Turbellarians, possessed an intracellular' digestion, the conclusion was obvious that this mode
of nourishment was one of the few characters in the organisation of the Metazoa, which had been directly transmitted to
them from the Protozoa, and so constituted a link, however
small, between the two groups. Now, since the colonial
Monads—organisms which most closely resemble the lowest
known Metazoa, their embryos and larvae—show no kind of
division of labour, no separation into nutritive and locomotive
individuals, the question arises whether the lowest Metazoa
have not retained the power of using any or all the cells of
their body for the purpose of ingesting food.
In order to answer this question I undertook, in the course
of last year, and especially during a six months' residence in
Messina, a series of investigations, the chief results of which
will be described in what follows. That portion of my work
which relates to intracellular digestion in the endoderm itself,
I reserve for a future paper.
I.—INTRACELLULAR INGESTION BY ECTODERM CELLS.
Sponges, in which, seeing that they are the lowest Metazoa
at present known, one would most especially look for some
kind of ectodermal ingestion of food, are not suited for detailed
observation; because the ectoderm in the living creature is
either invisible or else can be seen only imperfectly. "When
Krukenberg speaks of a digestion of albuminoid bodies
by the "ectodermal covering" of many sponges, it is_ not
evident whether he really means the delicate sheet of flat
epithelial cells which constitutes the true epidermis of these
animals. In any case, it is not possible, by such experiments
as his, to determine the part played by wandering mesoderm
cells immediately below the thin ectoderm. Von Lendenfeld,
in a recently published memoir on Australian Aplysinidse,1
1
" Ueber Coelenteraten der Siidsee," ii, ' Zeitschr. f. wiss. Zool.,' Bd.
xxxviii, 2, 1883, p. 253.
INTRAOELLULAR DIGESTION OF INVERTEBRATES.
91
speaks much more precisely. He states that in these sponges
the ectoderm is capable of taking up foreign particles, but not
of digesting them, the ingested matter being simply passed on
to the wandering cells of the mesoderm. However plausible
this view may seem, it appears, from Von Lendenfeld's statements, to need confirmation.
My own researches, conducted on A s c e t t a p r i m o r d i a l i s
and H a l i s a r c a l o b u l a r i s , two sponges with strongly developed ectoderm, gave a negative result. I have lately reexamined the last-named species, but again unsuccessfully:
particles of carmine suspended in water were taken in large
quantity into the entoderm and mesoderm cells ; but the ectoderm remained entirely free from them.
From this I can only infer that, however probable a p r i o r i
the ingestion of food by ectoderm cells in sponges may be, it
is at present by no means proved. More favorable objects
for these researches are the true Coelenterates. A digestive
ectoderm has, as far as I am aware, been described in only a
single one of these. Merejkowsky,1 in describing a Bougainvillea in which the alimentary canal was rudimentary, has put
forward the supposition that in this Medusa the food was taken
in entirely by ectoderm: but he most carefully asserts that he
never found solid particles in the ectoderm of the abnormal
Bougainvillea, which he supposes to be nourished entirely by
organic matter in s o l u t i o n in the sea water.
It has long been known that the ectoderm of hydroid polyps
protrudes pseudopodia, which frequently anastomose to form a
kind of plasmodium; and it occurred to me that these pseudopodia might have the function of picking up food particles. I
have, so far, been able to verify this supposition only in the
single case of the so-called nematocalyces of Plumularia. I have
worked with P. setacea, Ellis, and with a species bearing very
large gonophores, and nearly allied to P. halecioides.
If powdered carmine be suspended in the water surrounding
a Plumularia, it will, after some little time, be evident that a
1
" Sut une anomalie chez les Hydrome'duses et sur leur mode de nutrition
au moyen de l'ectoderme." ' Aroli. Zool. Bxp.,' viii, 1879 et 1880.
92
DR. BLIAS MBTSCHNIKOFP.
considerable quantity has entered the substance of the ectoderm of the iiematocalyces. I have many times repeated
this experiment, and always with the same result. Though
one cannot conclude, from this one observation, that the
nematocalyces fulfil the function of collecting food, yet the
fact was striking enough to invite further investigation. Examination shows that these organs send out from their projecting extremities various kinds of pseudopodia, which attach
themselves either to a calyx, or to its contained polyp, or more
frequently flatten themselves out round the stem, so as almost
to encircle it. The ectoderm cells of these free extremities
are not distinguishable one from another, but fuse into a
common protoplasmic mass, which sends out some few pseudopodia. The slow, creeping movements of the ends of the
nematocalyces probably serve to clean the neighbouring polyps
—a function which accounts for the frequent presence of foreign
particles in their ectoderm. Striking results are obtained by
studying colonies which have been for some little time in. a
watch glass. Plumularia polyps, like so many of their allies,
are very delicate organisms, which only live a short time after
gathering. The whole colony does not, however, die, but only
the polyp-heads; the ccenosarc and nematocalyces survive,
and will, under favorable conditions, produce new polyps. The
last-named organs, in these circumstances, serve to eat up the
dying hydranths—an operation I have witnessed repeatedly in
both the species of Plumularia I have examined. After the polyp
has retracted its tentacles, and become a mere rounded mass, the
free end of a nematocalyx creeps into the theca, and gradually
absorbs, by means of its ectoderm, the whole contents of the cup.
So that on the second day after gathering, when most of the
polyps have vanished, the ectoderm of the nematocalyces is seen to
contain a large number of foreign particles of different kinds.
It is further evident, on examining the loaded nematocalyces,
that the ingested material remains in the ectoderm, and is not,
as seemed to me possible, passed on to the endoderm. For a
few days after this, no marked alteration takes place, so that
I can only put it forward as a hypothesis that the ingested
INTRACELLULAR DIGESTION OF THE INVERTEBRATES.
93
granules are digested in order to provide material for the building up of new polyps. The so-called nematocalyces must
therefore be classed among organs whose chief function is
prophylactic; they eat up necrotie parts of the colony, and
also continually explore the organs in their vicinity, in order to
render harmless by devouring them any injurious bodies which
may be present. Any offensive or defensive function seems to
be purely secondary; at least the larger of my species had no
nematocysts in its " nematocalyces."
It seems probable that the peculiar organs described by
Weissmann1 in B n d e n d r i u m r a c e m o s u m , and the tendrils
observed by Fraipont 2 in C a m p a n u l a r i a a n g u l a t a , serve
the same function as that of the nematocalyces of Plumularia.
The Actinias give us another case of the ingestion of solid
food by the ectoderm. Three years ago, in my work on the
development of A c t i n i a m e s e m b r y a n t h e m u m , I showed
that the tentacles of this creature habitually take up a
large number of carmine granules. During the past year I
have studied the viviparous edible Actinia of Pantano (which is
identical with the new B u n o d e s s a b e l l o i d e s of Dr. Andres),
and I found that the larvae of this species also contained in
their ectoderm a large quantity of foreign matter.
The
younger the larva, the more abundant were the extraneous
granules. One can often find Gastrulae, whose bodies are
asymmetrically swollen, and dirty looking.
Examination
shows that this dirtiness is uot due to particles adhering to the
outside, but that it arises from the presence in the ectoderm
cells of foreign bodies, sometimes black, irregular or angular,
sometimes rounded and fatty or albuminoid in character. The
periphery of the ectoderm (which seems from examination
of the living larva to have all its cells fused into a common
homogeneous ectoplasmic mass) is free from granules, all the
ingested matter being in the deeper parts of the protoplasm,
either just in front of the nucleus, or behind it. The particles
1
' Mittheil. aus d. zool. Stat. zu Neapel/ iii, 1882-, p. 1.
'TLecherches sur l'organ. histol. et le develop, de la Campan angul.' ' Arch.
Zool. Exp.,' viii, 1879, 1880, p. 442.
3
94
mt. ELIAS METSOHNIKOFF.
are usually embedded in the protoplasm itself; but are sometimes surrounded by a vacuole indicating the occurrence of
some digestive process. The ectodermal granules are identical with those existing in the endoderm and in the gastric
cavity of the larva, showing that they are not to be considered
as products of the metabolism of the organism itself. In
fact, we are forced to believe from what has been said that
the larvae of an Actinia, when in the body of the parent, are
commensal parasites, living on the food taken in by the mother.
If the larvae be removed from the mother, and put into water
containing carmine in suspension, the carmine granules are
eaten by the ectoderm cells, being seized by means of short
pseudopodia extended from the free surface.
After the development of the gastric pouches, the number of
foreign particles in the ectoderm is much smaller. Young
larvae, still in the body of the mother, in whose ectoderm no
granules can be seen, retain the power of ingesting carmine
granules, especially in the ectoderm of the tentacles and disk.
It is exceedingly difficult to follow the further history of ingested granules, whether in ectoderm or endoderm ; but it is
hardly conceivable that they should be ingested with any other
object than that of subsequent absorption.
As a further example of ectodermal nourishment, may be
cited the ovarian ova of those animals whose generative cells
are ectodermal—for example, those of Tubularia, and, according to Korotneff, of Hydra. In the first-named animal I have
seen the young amoeboid ovum eat and digest the neighbouring
follicular cells. Korotneff1 asserts, without giving any proof
of his statement, that during the winter the young ectoderm
cells of Hydra devour the older ones.
II. — INTRACELLULAR INGESTION AND DIGESTION
WANDERING MESODERM CELLS.
BY
While the taking up of nutriment by ectoderm cells can only
be observed in rare and exceptional cases, nothing is easier
' ' In bis Russian memoir on Myriothela and H^dra.' Moskow, 1882,
p. 43.
INTRACELLULAR DIGESTION OF INVERTEBRATES.
95
than to find amoeboid cells of the mesoderm, which both ingest
and absorb food particles. It has long been known that
sponges are nourished by means of amoeboid cells; but the
morphological value of such elements was unknown. After I
had suggested1 that they were to be compared to the true
mesoderm of other creatures, F. E. Schultze2 proved, by the
discovery of a true ectoderm, and by a careful histological examination of the cells in question, the correctness of my view,
which has since been generally accepted. It is also generally believed that these mesoderm cells play an important part
in the nourishment of the organism; some observers (Balfour,
von Lendenfeld) attributing the function of ingestion to these
cells alone, while others allow the endoderm some share in the
process. I will not stop to discuss this point, because, in the
first place, it is for the moment of no importance to us whether
the endoderm ingests or not; and, secondly, in a future paper
on the endoderm, I hope to enter fully into the question. I
have only mentioned sponges at all in order to remind the reader
that ingestion by the mesoderm is an established fact among
them—a point which will be of use to us in discussing the
higher forms.
Leaving, then, the Porifera, I must refer to the observations
on the ingestion of granules of colouring matter by blood-corpuscles among the lower Invertebrates. Haeckel 3 was the first
to show that when a Tethys is injected with indigo, the granules are taken up by the blood-corpuscles. Later on he proved
the occurrence of similar phenomena in the blood of various
Invertebrates. Haeckel, however, did not perceive the close
analogy between this process and the mesodermic nutrition of
sponges; indeed, he subsequently* contradicted the statements
of Lieberkiihn as to the functions of the wandering cells of
Spongilla. His observations on the blood-corpuscles became
1
" Zur Entwicklungsgesch. d. Kalkschwamme," 'Zeitschr. f. wiss. Zool.,'
xxiv, p. 10.
3
" U e b . d. Bau u. d. Entwickl. von Sycandra Raphanus," 'Zeitschr. f.
wiss. Zool.,' xxv, suppl., p. 258.
« ' Radiolarien,' 1862, p. 104.* ' Die Kalkschwainme,' 1872, Bd. i, p. 372.
9G
DR. ELTAS METSCHNIKOFtf.
the starting-point of a large series of important researches in
vertebrate histology and pathology ; but by the pure zoologist
they have remained unnoticed.
The ingestive and nutrient functions of wandering mesoderm
cells are very various. 1 Their importance in the resorption of
parts which have become useless or harmful has already been
alluded t o . I have been able to observe this property most
easily in Echinoderm larvae, especially in the Auricularia of
Synapta, and in the so-called Bipinnaria asterigera. In
both these forms large numbers of amoeboid cells appear
between entoderm and ectoderm, giving rise to all the skeletal
structures, and the cutis of the adult, and to the oral musculature of the larva. Their function, however, is not purely
morphogenetic. At the period of metamorphosis, which is, as
is well known, extremely complicated, and associated with the
loss of many larval organs, these mesoderm cells ingest the
Cellular debris of the disappearing organs, and finally absorb
them. In Auricularia numbers of these amoeboid cells collect
beneath the ciliated rings just before metamorphosis, because in
this region the phenomena of resorption are mostpronounced.
The ciliated cells then break down into albuminoid globules,
which are devoured by the amoeboid elements below. Auricularia is so transparent, and so easily obtainable, that it is not
difficult in this form to watch the process of ingestion and
absorption of debris in a single cell. The albuminoid
granules may remain for some time in contact with the amoeboid
cells, lying on their pseudopodia, and then be suddenly swallowed ; or the swallowing process may be so gradual as to allow
of the various stages being seen and drawn. The absorption,
like the ingesticm, of these granules seems to vary greatly
in rapidity; in some cases it commences at once and is soon
completed, while in others one can watch an ingested granule
for hours without noticing the slightest change.
Resorption phenomena, similar to those just described, can
1
I may here remark that by " wandering mesoderm cells " I mean the socalled amoeboid connective-tissue cells, as well as lymph- and blood-corpuscles.
INTRACELLULAR DIGESTION OF INVERTEBRATES.
97
be seen at two stages in the life-history of Auricularia. They
first occur at the assumption of the so-called pupa stage,
when a large part of the longitudinal ring of cilia is lost—that
is, is disintegrated and devoured by the mesoderm. At this
time every amoeboid cell of the middle layer is generally
loaded with enormous numbers of debris granules, which are
slowly absorbed during the pupa stage, so that the cells which
contained them become filled with clear vacuoles. On the
metamorphosis of the pupa into a young Synapta, the cells
begin again their devouring work, collecting as before beneath
the ciliated rings, and eating up the products of disintegration. The appearances seen during the process at this time
are exactly those seen at the time of its first occurrence.
Similar phenomena can be observed in Asterid larvse, where
large tracts of larval tissue atrophy during the metamorphosis.
In this case, also, the disintegrating cells break up into albuminoid granules of various sizes, which are gradually eaten
and absorbed by mesodermal elements.
I have found these appearances so constantly to accompany
metamorphosis, that I cannot but regard them as normal and
necessary events in the life of an Echinoderm larva; and
I am therefore forced to the conclusion that the wandering
mesoderm cells of which I have spoken play the same part
in the resorption of larval organs as that played by osteoclasts
in the resorption of Vertebrate bone. I have never seen,
however, in Echinoderm larvae, any formation of multinuclear
masses similar to those seen during the resorption of bone.
It is hardly possible to believe that this resorbent function
of the mesoderm should be confined to Echinoderms. I rather
incline to the belief that it occurs in all animals whatever which
undergo any great degree of metamorphosis. I have reason to
believe that wandering cells play an essential part in the complicated larval changes of Ascidians. I have not, indeed, been
able to prove this in the case of Ascidia i n t e s t i n a l i s ; but
only, I believe, because of the small size of the cells in
question; but I have frequently seen wandering cells loaded
•with debris. If this should prove to be the case, we should have
VOL. XXIV.
NEW SEK,
G
98
.
DB. EL1AS METSOHNIKOFF.
a simple explanation of such appearances as the transformation
of the degenerating nervous system into a heap of blood-corpuscles, which is at present believed to be due to a direct
morphogenetic change in the ganglion cells.
It may here be pointed out that Ganin, in 1876, attributed
to amoeboid mesoderm cells an important part in the so-called
histolysis of muscle-fibres in flies.1 He says : " During the
early stages of development I have often seen free amoeboid
mesoderm cells in the cavity of the foot attach themselves to
the surface of a muscular mass, into which they often bored
deeply, appearing to nourish themselves from the substance of
the larval muscle." Viallanes, the latest student of histolysis,2
has not sufficiently considered G-anin's observations. Though
the very few drawings which he gives point strongly to the
conclusion that there is a considerable ingestion of food by
mesoderm cells during metamorphosis, he expresses himself in
his text in favour of a totally different view. Thus, he describes
as " degenerated blood-cells, like pus-corpuscles," elements
which have probably only loaded themselves with albumin
granules; while his " cellules musculaires," arising out of
muscular debris, are also, in all probability, nothing but overloaded mesoderm cells.
Though this ingestive power of the mesoderm is specially
pronounced during metamorphosis, it occurs on other occasions. In 1880 Schneider 3 showed that resorption of the
generative products by amoeboid cells resembling blood-corpuscles occurred in Hirudinea; an observation which I have
repeated f o r A u r e l i a a u r i t a . Many of the ovarian ova of this
medusa become surrounded by amoeboid cells, and completely
devoured. These cases can be compared with the following,
which have been observed by me. If a Pilidium be left for
some time in a watch-glass, the rudimentary nemertine
1
' Beitrage zur Kentniss der postembryonalen Entwickelung der Insecten.'
Warschau, 1876 (Russian).
3
" Recherche5 sur l'histologie des Insectes." 'Ann. Sci. Nat.,' v, xiv, 1882,
pp. 135—158.
3
" Ueber die Auflosung der Eier und Sperrnatozoen in den Geschlechtsr
organen." ' Zool. Auz.,' 1880, p. 19.
INTRAOELLULAR DIGESTION OP INVERTEBRATES.
99
atrophies, being devoured by amoeboid mesoderm cells around
it j so that there remains in the watch-glass an apparently
normal Pilidium, only with its rudimentary nemertine replaced by a number of amoeboid cells, full of food-granules.
In all these cases the material eaten by mesoderm cells has
been formed from the body of the animal itself, and has only
become useless at the moment of its being devoured. These
cells can easily be proved capable of ingesting, and usually of
digesting, material altogether foreign to the organism in which
they live. If a large number of transparent creatures (larval
or adult), which possess a mesoderm, be taken fresh from the
sea, it is easy to find, among a number of empty mesoderm
cells, some containing foreign particles which may be of very
different kinds. In the mesoderm cells of Echinoderm larvae
I have often found empty thread-cells.
Similar structures
can often be found in Ctenophora and Pilidium. I believe
that bodies such as these, when found in the mesoderm cells,
have pierced through the body wall, and then been swallowed.
Just beneath the epidermis there is generally present a whole
multitude of amoeboid cells, ready to take up anything which
may pierce the body wall. In my early investigations on the
intra-cellular digestion of Ctenophores 1 I saw that carmine
granules suspended in water passed, not only into the endoderm cells, but also into those of the mesoderm; though I
could not then determine the exact mode of their entry.
These phenomena—on the one hand the process of resorption, on the other the frequent enclosure by mesoderm cells of
foreign bodies—show how extremely well developed is the
power of ingestion and absorption in these cells. I have,
therefore, made several experiments with the object of defining
more clearly the extent of this property. I chose for this
purpose B i p i n n a r i a a s t e r i g e r a and P b y l l i r h o e bucep h a l u m , because these animals are not only transparent, but
large enough to admit of the performance upon them of simple
1
"Ueber die intracellulare Verdauung bei Coelenteraten." ' Zool. Anz..'
1880, p. 262.
100
DT?, EL1AS METSOHNIKOFF.
operations, which they are also hardy enough to survive for
some time.
If water holding carmine or indigo in suspension be injected
beneath the epidermis of the animal under observation, the
particles of colouring matter are after a very short time taken
up by the amoeboid cells. In Phyllirhoe, there are two kinds
of amoeboid cells, of which only the smaller ingest colouring
matters in this way. The larger cells which often assume very
curious forms, are distinguishable by their vacuolated protoplasm, and by containing no foreign bodies; in a Phyllirhoe
which had been injected with powdered carmine, these large
cells contained rosy patches, caused by dissolved carmine. In
spite of repeated trials, I could not ascertain the way in which
the carmine entered them. The smaller granules of solid
carmine were all eaten by the small cells, in a manner precisely similar to that already described; the larger masses were,
on the other hand, surrounded by a sort of plasmodium of
small cells, which came one by one to each lump, and flattened
themselves upon it, fusing with neighbouring cells as these
arrived. In this way arose plasmodia of very different sizes, some
even visible to the naked eye, which may be compared to the
giant cells so often described in Vertebrates. This observation,
which I have often repeated, confirms the opinion of Weiss,1
Koch,2 and others, that giant cells are often found in the
neighbourhood of foreign bodies. In all cases in which I have
found giant cells in Invertebrates, they have arisen round
foreign bodies, and. always by the fusion of separate cells. My
results are therefore opposed to the view, held by some pathologists, that giant cells are formed by absorption of pus cells, or
by a process of incomplete fission, arrested after division of the
nucleus. Other observers, who have studied Invertebrate bloodcorpuscles outside the body, have described their great tendency
to form plasmodia, without the presence of foreign particles.
1
" Ueber die Bildung und die Bedeutung der Riesenzellen, &c.," ' Virchow's
Archiv,' Bd. lviii, p. 13.
1
'Berliner klinische Wochenschrifl,1 1882, No. ]5.
INTRACELLULAB DIGESTION OF INVERTEBRATES.
101
1
Haeckel , for example, has observed this in Echinoderms, and
Geddes3 in the lymph cells of Lumbricus, in Mollusks, and in
Pagurus and other Decapods. These observations are entirely
confirmatory of the views of those observers who regard giant
cells as true plasmodia, due to a fusion of several distinct cells.
Another example of the formation of mesodermal plasmodia
(as I shall henceforward call giant cells) was seen in an
Asterigera, which had had a drop of human blood injected
beneath the skin. Most of the corpuscles were ingested, each
by one or two separate cells; but the larger clots were each
surrounded by several cells, which fused to form plasmodia.
These latter, though loaded with masses of corpuscles, were yet
able to move by means of large pseudopodia.
The nuclei were
so obscured that it was impossible to observe them in the living
state; I therefore treated the plasmodia with alcohol and borax
carmine, finally clearing with oil of bergamot. By this treatment the nuclei came out distinctly ; they were all situated in
the peripheral part of the plasmodium, the centre of which was
filled with great balls of fused blood-corpuscles.
We should conclude, from the above described observations,
that when mesoderm cells are confronted with a large mass of
food material, which they cannot devour singly, they fuse into
a plasmodium, which eats up the whole available food. This is
not, however, invariably the case. When the mucous tissue of
Phyllirhoe was filled with large bodies, such as the boiled eggs of
S p h s e r e c h i n u s g r a n u l a r i s , or boiled cells from the cotyledons
of peas, I could see no formation of plasmodia. Shortly after
the introduction of these bodies into the tissue, small amoeboid
cells collected round them in great numbers, and remained for
several days—till the death of the animal—closely surrounding
them, but without the slightest sign of fusion one with
another. The cells surrounding the tissue of the pea remained inactive during the whole time; for, being unable to
perforate the thick cellulose wall, they could not commence in1
' Radiolarien,' p. 103, Anns.' 2.
" On the Coalescence of Amreboid Cells into Plasmodia," ' Proc. Hoy.
Soc.,' 1880, p. 252, pi. v.
2
102
DE. ELIAS MBTSOHNIKOi'F.
gestion. Those around the echinus egg, on the other hand,
became filled with small particles of yolk, while yet retaining
their complete independence one of another. I have seen the
same thing occur on introducing large bodies, such as glass
spicules, rose thorns, echinus spines, &c, beneath the skin of
Bipinnaria, Tethys, and Terebella, and into the deeper layers
of the mantle of Ascidia i n t e s t i n a l i s . Soon after the
foreign substance appeared within the body, the amoeboid cells
(connective-tissue corpuscles in Bipinnaria and Tethys, lymph
corpuscles in Terebella, test-cells in Ascidia) began to collect
around it, finally surrounding it completely, and forming a
mass so large as to be easily visible to the naked eye, while
remaining perfectly distinct from one another, and so not
forming a true plasmodium. In Tethys I once saw a partial
fusion of cells into a complex, the component elements of
which were still, however, easily recognisable. It was perfectly
easy, by means of sections to make sure of the absence in
Tethys of true plasmodia in these "inflamed" regions (I
made use of the ear-shaped tentacles in my observations).
It follows from this, that while mesoderm plasmodia, when
they arise in the animal body, are formed round foreign substances, yet that this formation is not a necessary consequence
of the presence of such substances, it being perfectly possible
for the reaction of the organism against intruded matter to
take place without any such formation. It has also been shown
that one function of amoeboid mesoderm cells is to eat up those
parts of the organism which have become useless, and also any
foreign bodies which may have pierced through the ectoderm;
or, if it be not possible to eat up such bodies, to surround and
isolate them. It is obvious that the process of removal of
small masses of detritus, or minute grains of carmine, on the
one hand, is fundamentally identical with that of surrounding
larger foreign bodies, on the other. Glass rods, atoms of dust
or carmine, are surrounded or devoured by aggregates of cells
in exactly the same way. It is also undeniable that the
results of introducing a glass spicule, or other irritant, into
the body of an Invertebrate, bear no small resemblance to
INTRAOELLULAR DIGESTION OF INVERTEBRATES.
103
the phenomena of inflammatory exudation in Vertebrates. In
both cases a number of mesoderm cells collect round the irritating body, and act upon it as best they may. The difference
between the two cases is only one of degree. In Bipinnaria,
which has as yet no trace of a vascular system, we see a gradual accumulation of the numerous amoeboid cells, which are
scattered throughout the mesoderm; while in molluscs the
lacunar blood-vessels play a purely passive part in allowing the
corpuscles to flow through them. In Terebella, with its closed
blood system and red plasma, inflammation1 affects only the socalled lymph corpuscles of the body cavity; if the blood-vessels
be not injured by the intrusive body, no transudation occurs,
as is evident from the absence of coloration. Therefore, from
the point of view of comparative pathology, Cohnheim's dictum,
" without vessels no inflammation,"2 which has been accepted
by many pathologists, does not hold. Inflammation is a phenomenon phylogenetically much older than blood-vessels, while
exudation is a comparatively late development. From this
point of view, it is evident that the white corpuscles of Vertebrates must be regarded as of more importance than has been
thought to be the case, thus justifying the views of Thoma.3
Our observations onresorption during metamorphosis among
Echinoderms (which are in complete harmony with the results
of histological and pathological investigation on Vertebrates4)
have taught us that mesoderm cells are able to take up and to
digest albuminoid granules. This conclusion is strengthened
by the following observations.
If we follow the fate of (human) blood-corpuscles, after their
1
I have so far had no opportunity of examining the phenomena of inflammation in those annelids which possess well developed blood-corpuscles; I
hope shortly to fill up this gap in my observations.
8
" Ueber Entziindung und Eiterung," ' Virchow's Archiv,' 1867, Bd. 40 ;
compare also his ' Neue Untersuchungen iiber die Entziindung/ Berlin, 1873,
p. 11, 42, 62, 67, 71.
3
" Uebec Entziindliche Storungen des Capillarkrieslaufs bei Warmblutern,"
• Virchow's Archiv,' Bd. 74, p. 386.
4
Summarised by Ziegler in his ' Lelirbuch der pathologischen Anatomie,'
2 Aufl., Bd.i, 1882, pp. 167—175.
104
DE. ELIAS HKTSCHNIKOIT.
ingestion by the mesoderm cells of Bipinnaria, we see that
they are completely absorbed. Within the cell they swell up
and become clearer; the haemoglobin is then dissolved out,
and finally the whole, corpuscle disappears. (I need hardly
say that corpuscles which have not been eaten do not undergo
this series of changes.) The corpuscles of Discoglossus,
injected into Phyllirhoe, behave somewhat diiferently. The cellbody, and the nucleus, of such a corpuscle, becomes somewhat
irregular in shape after ingestion; a crumpling process goes
on, which results in the breaking up of cell-body and nucleus
into several fragments. At this stage the central part of the
mesoderm cell becomes slightly coloured. The whole process
resembles the resorption within the so-called blood-corpusclecontaining cells of Vertebrates, which must really be regarded
as a process of feeding on the part of a mesoderm cell.1
Milk injected beneath the skin of Bipinnaria and Phillirhoe
has the same fate. The milk spherules are eaten by wandering cells, lose their shining appearance, and break up into
small granules, which are distributed throughout the cell
substance. I have not yet observed any noticeable alteration in
ingested starch grains.
In order to ascertain whether the mesoderm cells exercised
any choice in the particles they absorbed, I injected mixtures
of different kinds. For example, I filled the mucous tissue
of a Phyllirhoe with a mixture of milk and indigo, carmine
and starch grains—nutritious and useless bodies together—
and I found that all these bodies were equally absorbed,
some cells eating all four kinds of food at the same time.
From this it would be supposed that the mesoderm cells ate
everything provided for them, without power of distinction.
The following experiment, however, seems to contradict such a
view. On injecting into a Phyllirhoe living ovarian ova from a
Sphserechinus granularis, it was found that neither
young ovarian cells, nor ripe ova which had extruded polar
bodies, were eaten by the mesoderm cells; on the contrary,
1
G. Langerkaus " On the Resorption of Extravasations and the Formation
of Pigment within them," 'Virchow's Archiv,' 1870, Bd. 49, pp. 81, et seq.
INTBAOELLULAR DIGESTION OF INVERTEBRATES.
105
ova in all stages seemed to live much longer when taken from
the ovary and placed within the tissues of Phyllirhoe, than
when simply placed in sea water. My observations lasted on
one individual six days; that is, until its death. I was also
able to fertilise these eggs within the Phyllirhoe, normal
segmentation and a normal blastosphere being produced. If,
however, boiled eggs of Sphoerechinus were introduced,
amoeboid cells immediately fastened upon them, and began to
devour their yolk. This might be taken as proof that the
amoeboid cells eat only dead matter, were it not that in the
cases already described, of the ingestion of red blood-corpuscles, some, at least, must have been alive when they were
surrounded or devoured. As a further experiment, I introduced a drop of living semen of Sphoerechinus g r a n u l a r i s
beneath the skin of Phyllirhoe. The spermatozoa slackened
their movements, and were soon surrounded and eaten by the
mesoderm cells. A few remained for two days undevoured, and
retained their power of fertilisation.
So that the amoeboid cells do not take up everything that is
offered them, and we probably should not deny their possession
of some means of distinguishing between desirable and undesirable substances. But why they made no attempt to attack the
living ova in the case described, seems at present inexplicable.
It has been sufficiently proved, that the cells of the mesoderm have important prophylactic functions; a result which
invites further investigation. Observations on necrotic organs
of several Invertebrates, especially of B i p i n n a r i a a s t e r i gera, have shown it to be one function of the mesoderm cells
to devour the dying elements of such organs. The long arms
of Bipinnaria end in orange coloured points, covered by an
ectodermal pigmented epithelium, and containing more or
fewer mesoderm cells. These latter generally contain rounded
pigment granules, derived in all probability from the ectoderm. When the animal has been for some time in a watchglass, the ends of the arms become worn out and broken,
so that large pieces frequently fall off. In the ectoderm of
such unhealthy arms are numbers of rounded granules, which
106
DR. BLIAS METSOHNIKOFF.
are eaten by mesoderm cells, just as are the similar granules,
found during metamorphosis; so that the amoeboid cells of
the' arms are often crammed with debris. In addition to
the clear, feebly refracting degradation products, one often
finds in the ectoderm of B i p i n n a r i a masses of small,
round, strongly refracting bodies which are devoured by
the mesoderm cells. I have not been able to determine
exactly the nature of these bodies, but it seems possible that
they may be spores of bacteria, since they closely resemble
undoubted spores, while it can be shown that the mesoderm
cells will readily ingest such bodies. If fluids containing
bacteria be injected beneath the skin of Bipinnaria or Phyllirhoe, or if they develop spontaneously in the wounds of such
animals, they will soon be found within the substance of many
amoeboid mesoderm cells. Both still and motile forms are
thus ingested, and they may be found either embedded
in the protoplasm of the absorbent cell, or surrounded by
a vacuole. Individual bacteria are often seen which retain
their power of movement even after ingestion, while in
other cases the motility is lost at once, and the whole
bacterium becomes so delicate as to be scarcely visible.
We may then consider that bacteria are habitually ingested
by mesoderm cells, when they make their appearance in the
organism;—a fact which obviously increases the prophylactic
importance of these cells. These phenomena can perhaps be most
easily seen in Botrylhis, colonies of which, when freshly gathered,
contain almost invariably large quantities of bacteria within
the test; I found especially a Spirochosta, closely resembling
the S. O b e r m e y e r i o f relapsing fever, and a small Bacillus,
like the L e p r a bacillus, which had a spore at each
end. Both these forms were pursued by the wandering cells
of the Botryllus, and were found ingested and absorbed by
them in various stages of development. The victory was not,
however, all on one side; here and there were found mesoderm
cells to all appearance dead, with long bacterial filaments projecting from them.
The same thing has been observed in Vertebrates, where
INTRAOELLULAK DIGESTION OF INVERTEBRATES.
107
Koch has found both B a c i l l u s a n t h r a c i s and the bacillus of
septicaemia in the mouse enclosed by white blood-corpuscles,1
while tubercle bacilli have been seen by him in the interior
of giant cells.2 So that throughout the whole animal kingdom
the wandering cells of the mesoderm make use of their ingestive power for the destruction of bacteria and similar organisms,
which need for their development a suitable (necrotic) nidus.
In Metazoa with an undeveloped mesoderm, this function is
performed by the ectoderm (Plumularia) or by the endoderm.
Most embryologists agree that the Metazoa are to be derived
from ancestors closely resembling the colonial Monads. Now,
the individuals composing these colonies are all exactly similar
one to another, so that we can find among them no trace of
that division of labour which is the first step in the differentiation of germinal layers. There is even less differentiation
among the colonial Protozoa than among their chlorophyll containing allies, the Volvocinese. The attempt to form some exact
idea of the origin of the lowest Metazoa, as the basis of a comparative biology, is, therefore, extremely difficult. Observers are
agreed in considering the blastula stage to represent an ancestral colony of Monads; but they differ fundamentally in
the meaning attached to the formation of the germinal layers.
Some, as, for example, Balfour, unhappily so early lost to
science, suppose that the blastula cells (or rather, the individuals
composing the colony which this stage represents) were early
differentiated into two kinds; so that the transitional form
between Protozoa and Metazoa consisted of a hemisphere of
nutrient amoeboid cells, joined to a hemisphere of ciliated
locomotive cells.3 I have supposed that some of the blastula
1
' Unters. ueb. d. Aetiologie d. Wundinfectionskrankheiten,' 1878, pp. 44,
72. Koch's opinion that bacteria force themselves into the white corpuscles
in order to multiply there does not seem proved, and does not agree with my
results.
2
' Berliner klinische Wochenschrift,'1882, No. 15. Zopf's interpretation
of Koch's observations does not seem probable (' Die Spaltpilze,' Berlin, 1883,
p. 67).
1
" . . . The larva of Sponges is to be considered as a colony of Protozoa,
108
DE. BLIAS MBTS0HNIKOFP.
cells, which, like all the others of the colony, had the power of
obtaining and ingesting food, travelled from various parts of
the periphery of the blastosphere to the interior, losing their
columnar character, and becoming amoeboid. These amoeboid
cells I regard as the rudiment of a parenchymatous endomesoderm, which afterwards differentiated into true endoderm
and other structures.1
The discovery of ectoderm cells, which retain the power of
ingesting food, in various groups of Coelenterates, seems to
me totally incompatible with an early differentiation of the
blastula cells, such as is assumed by Balfour, and rather goes
to show that at the stage in question the whole ectoderm
habitually performed those functions which are now limited to
the endoderm. It is very difficult to suppose that so fundamental a property as the nutrient power of a Monad cell should
have disappeared in so short a time as that occupied by the
differentiation of the two halves of the ancestral Sycandra.
Further, I am aware of no reason, embryological or otherwise, which prevents our assuming that the germinal layers
were differentiated at a time when all the cells of the organism
retained the power of taking up food. Later on, this power
was restricted to the parenchyma cells, as in Sponges, where
the food is taken in by a parenchymatous mesoderm, which
cannot be clearly distinguished from the endoderm, as even
in the adult cells are continually passing from one layer to
the other. Only later in phylogeny was a sharp distinction
established between the two divisions of the primitive parenchyma, or, as it may be called, phagocytoblast. But in the
one half of the individuals of which have become differentiated into nutritive
forms, the other half into locomotor and respiratory forms. , . . That
• the passage from the Protozoa to the Metazoa may have been effected by such
a differentiation is not improbable on a p r i o r i grounds."
' Comparative
Embriology,1 i, p. 122. [This is quoted from the English edition. In the
German translation, quoted by Metschnikoff, Balfour is made to assume the
passage of all Metazoa through a form like that of the Sycandra larva, an
opinion which he certainly never expressed.—TRANSLATOK..]
1
" Spougeologische Studieu," ' Zeitsch. f. w. Zoologie,' 1879, Bd. xxxii,
p. 375, et s e q .
1NTKA0ELUJLAB DIGESTION OP INVERTEBRATES.
109
lower Coelenterates we can even yet hardly speak of a raesoblast. Though many Acraspeda have amoeboid cells in their
gelatinous tissue, others have no trace of such elements; and
their presence in Craspedota is altogether exceptional. It is
worthy of note that in Medusae these cells, when present,
appear only late in life. If we consider the supporting cells
of the tentacles in Medusae and hydroids as mesoderra, we are
still unable to draw a line between them and the endoderm.
In higher forms, the distinction between the two layers is
much clearer; the endoderm has assumed the function of
utilising the food brought to it from without, and an intracellular absorption is gradually replaced by a process of
enzymogenesis (a change which will be more fully discussed in
the third section of this work). The mesoderm does not,
however, lose its primitive powers, but employs them against
useless and harmful bodies, so that it retains the intracellular digestion, as well as many other of the characters of
the Protozoa—not only the power of throwing out Pseudopodia, but also that of forming Plasmodia. This last property has persisted less in the ectoderm than in the other
layers, being only seen in the epidermis of Sponges, Hydroids,
and perhaps a few other Coelenterates; it is well preserved
in the endoderm of animals with an intracellular digestion,
such as Coelenterates and Turbellarians. Mesodermal Plasmodia are, however, found even in the higher animals, not
excepting Man himself. The cells of the mesoderm have
best preserved their primitive independence one of another,
so that one can truly say that the protopsychic condition
here persists.
Many embryologists, in their endeavours to elucidate the
history of the middle layer, have attempted to determine its
special function. Some (Hatschek) have believed it to be
specially connected with reproduction, while others (Rabl) consider that it arose in connection with a locomotive apparatus.
Considering the facts described in this paper, and, above all, if
we remember that in many animals with a mesoderm both
generative organs and musculature are derived, not from this
110
DE. BLIAS METSCHNIEOFP.
layer, but from the ectoderm or endoderm (generally/perhaps,
from the ectoderm), it is evident that the primitive function of
the mesoderm must be nutritive, and its relation to the various
tissues and organs purely secondary. Apart from any other
argument, I would point to such a creature as Halisarca, where
there is no musculature at all, and where the greater part of
the mesoderm is entirely given up to the performance of nutritive functions.
When I speak of the phagocytoblast, as a whole, I do so because development shows us how intimately the mesoderm, or,
at least, the greater part of it, is connected with the endoderm.
Apart from the facts in the development of Sponges, which I
have elsewhere described, I will mention a few points in the
formation of the amoeboid mesoderm cells in Echmoderms,
Pilidium, &c, which support this view. But I must state, at
the outset, that I do not exclude the ectoderm from all share
in the formation of the middle layer. I would rather suppose
that, in earlier times, when the ectoderm had not so completely
lost its ingestive power, and when the phagocytoblast was still
partly derived from it, amoeboid cells were frequently budded
off from the ectoderm to join the other devouring cells (phagocytes) in the body. In this way may be explained the ectodermal origin of a part of the mesoblast in the larva of Halisarca.
We must also bear in mind that other elements, besides phagocytes, acquired a secondary connection with the mesoderm,
such as reproductive, muscular, and other cells. So that what
we call mesoderm is really a heterogeneous mixture of elements
acquired from various sources at various times; and the origin
of the whole of these, as a single germinal layer, must be regarded as a comparatively late event. In this view of the
mesoderm I agree with Balfour1 and the brothers Hertwig.2
In speaking, however, of the mesoderm as a nutritive cellcomplex, I have done so because I regard this as its primitive
and most important function. A detailed phylogenetic history
of the mesoblast throughout the Metazoa seems at present im1
' Comparative Embryology,' vol. ii, p. 286.
J
" Die Actinien," ' Jenaisohe Zeitschrift,' 1879, vol. xiv.
INTRAOELLULAR DIGESTION OP INVERTEBRATES.
I l l
possible; but whether this be so or not, I am convinced that
such a history can never be obtained by purely morphological
and histological methods, such as those employed by the Hertwigs in their " Ccelomtheorie."
Since we have seen that the power of intracellular ingestion
and absorption is used as a protection against harmful bodies
arising within an organism on reaching it from without,1 it
follows that septic organisms (Bacteria, Chytrideae, Entomophtora, and other parasites) are a very old source of trouble in
the world; and perhaps many organs and events, whose significance has hitherto been overlooked, may find an explanation
in this way. I would especially mention the nematocalyces of
Plumularia, and the peculiar test of Ascidians. It is evident
that instances of prophylactic organs will soon be multiplied,
so as to far exceed in number the few I have given. Here is
yet another case. On the inner surface of the contractile walls
of the excretory organ of Carinaria there are certain "' granular
cells," in which a formation of renal concretions is believed to
occur, as in the Gastropod kidney. I have, however, been
able to prove that these concretions are not formed in the
cells, but are really foreign particles taken up by the cells,
which are amoeboid, as may be proved by suspending carmine
or indigo in the water in which the Carinaria lives. Since
the excretory organ pumps water into the pericardial cavity,
these cells are posted on its walls to prevent the entrance, with
the water, of bodies harmful to the organism.
The great advances made in Pathological Science during the
past few years cannot fail to benefit pure Zoology, which will,
in its turn, help to solve the problems of Medicine by establishing a Comparative Pathology, based on the doctrine of
Evolution.
1
In this way may be explained the recent observations of Buchner on the
action of inflammation on Bacteria (' Die atiologische Therapie und Prophylaxis der Lungentuberculose,' Miisohen, 1283, pp. 11 and 12).