MATURATION Oi1 OVUM AND DEVELOPMENT OF ALLOPORA. 579
On the Maturation of the Ovum and the Early
Stages in the Development of Allopora.
By
Sydney J . Hickeon, M.A.Cantab., D.Sc.toml., Sec,
Fellow of Downing College, Cambridge.
With Plate XXXVIII.
MY investigations may conveniently be considered under the
following headings:
1. General summary of events.
2. The formation and fate of the trophodisc.
3. The changes of the germinal vesicle
4. The formation of the embryonic ectoderm.
5. The history of the yolk.
6. General considerations and conclusion.
Contrary to general custom, I propose to commence with a
summary of events. The reason for this is that the history of
three structures growing and changing simultaneously has to
be considered: namely, the trophodisc, the yolk, and the
germinal vesicle; and it will be convenient to consider briefly
the three together before proceeding to describe them separately
in detail.
I. Summary of Events.
1. The ova and young embryos are, so far as my experience
goes, only found in the younger branches of the colony. The
sperm-morulse and spermatozoa are only found in the older,
thicker branches. The youngest ova are first seen either (a)
580
SYDNEY J. HICKSON.
in the endoderm of the canals, or (b) in the degenerated cellmass of the trophodisc of an escaped embryo (Plate XXXVIII,
figs. 2 and 3).
2. As an ovum growing in one of the ordinary canals
enlarges it pushes out the endoderm and ectoderm of the
canal in which it is formed, and thus forms for itself a diverticulum connected with the canal by a narrow aperture. At
the same time the endoderm of the canal wall in the immediate
neighbourhood of the aperture of the diverticulum becomes
thickened and throws out five radial pouches, which embrace
the proximal pole of the diverticulum containing the ovum
(PI. XXXVIII, fig. 5).
3. The five radial pouches thus formed throw out secondary
pouches and become flattened against the proximal pole of the
ovum, and thus form a nourishing lenticular mass of cells—
the trophodisc1 (PI. XXXVIII, fig. 7). The protoplasm of the
ovum is at first clear, but with the formation of the trophodisc
it becomes filled with small yolk-spheres. It possesses no
trace of a vitelline membrane, but is surrounded and protected
by a chorion, the attenuated ectoderm and endoderm of the
primitive diverticulum. The germinal vesicle is large, spherical,
or oval in shape, limited by a well-marked membrane, and
contains a large germinal spot, and a sparse chromatin meshwork with nodal thickenings (PI. XXXVIII, fig. 6).
4. When the trophodisc is fully formed and the lumina of
its constituent pouches obliterated, the germinal vesicle becomes oval or irregular in shape and travels towards the distal
pole of the ovum. At this stage all the yolk-globules are
spherical in shape, the larger ones are found only in the
proximal and peripheral regions of the egg, those found in the
immediate neighbourhood of the germinal vesicle being smaller
and very numerous (PI. XXXVIII, fig. 7).
5. The germinal vesicle travels until it reaches the chorion
covering the distal pole of the ovum, when two polar bodies are
protruded. The germinal vesicle then flattens itself against
the chorion, and apparently remains in that position for some
1
This name was kindly suggested to me by Professor Lankester.
MATtJBATION OF OVUM AND DEVELOPMENT OF ALLOPOKA. 5 8 1
time. Fertilisation was not observed, but it seems probable
that it takes place at this stage (PI. XXXVIII, figs. 10, 11).
6. The germinal vesicle, or perhaps it ought to be called the
oosperm nucleus, next withdraws from the chorion, and is then
seen to be hemispherical ou its proximal aspect but irregular,
and provided with several amoeboid, finger-like processes on its
distal side (PI. XXXVIII, fig. 12).
7. The membrane surrounding the oosperm nucleus gradually
disappears, and it becomes impossible to distinguish its protoplasm from the general protoplasm of the ovum. At the time
of the disappearance of the oosperm nucleus certain changes
may be observed in the character and arrangement of the yolkglobules, the most important of them being the distribution of
a number of small globules in the peripheral region of the
distal hemisphere of the ovum (PI. XXXVIII, fig. 13).
8. In the next stage one or two irregular lumps of nuclear
protoplasm may be found in the distal hemisphere of the
ovum, and in their neighbourhood a few spherical, oval, or
dumb-bell shaped fragments of the same material (PI. XXXVIII,
fig. 14).
9. A little later the irregular lumps of nuclear protoplasm
disappear and the smaller spherical ones increase in number.
All stages in the division of these smaller nuclear fragments may be observed, and it is very probable that their
multiplication in this manner is very rapid. In ova of this
stage vacuolation of the protoplasm usually takes place and
the larger yolk-globules become irregular in shape and show
signs of splitting into smaller ones. The lumina of the trophodisc are at this stage quite obliterated and the organ diminishes in actual size. A little later it degenerates into a mere
syncytium crowded with nuclei (PI. XXXVIII, fig. 15).
10. As the vacuolation of the ovum proceeds, a thin membrane of clear protoplasm containing only a few small yolkspherules may be seen to separate from the distal periphery
of the young embryo. Into this a number of the nuclear
fragments wander and arrange themselves side by side.
(PI. XXXVIII, fig. 18). This is the primitive ectoderm.
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SYDNEY J. HIOKSON.
11. The primitive ectoderm spreads over the periphery until
it entirely encloses the central protoplasm and yolk-mass. The
last point to be enclosed is that immediately in contact with
the trophodisc (PI. XXXVIII, fig. 17). The central mass of
the young embryo at this stage consists of a highly vacuolated
protoplasm containing numerous scattered nuclei and yolk
bodies.
12. The embryonic ectoderm cells are formed by the splitting up of the protoplasm into columnar epithelial cells, each
containing a single nucleus (PI. XXXVIII, fig. 21).
13. When the embryo possesses a complete columnar ectoderm it is ready to escape.
14. The mode of escape has not yet been observed, but
there can be little doubt that it squeezes its way out through
a channel that is prepared for it by the absorption of a part
of the superjacent calcareous skeleton (PI. XXXVIII, fig. 4).
II. The F o r m a t i o n and F a t e of the Trophodisc.
The female gonophores of Allopora have not been previously
described, but cup-shaped structures bearing ova are figured
and described by Moseley (1) in E r r i n a l a b i a t a and Pliob o t h r u s s y m m e t r i c u s . In Errina " the spadix is at first
cup-shaped, the walls of the cup being composed of a very
thick layer of endoderm. The cavity of the cup is directed
towards the surface of the coral and within it rests the single
large ovum with its distinct germinal vesicle and spot." In
Pliobothrus " each ovum is developed within the cup of a cupshaped spadix."
From this it is evident that a structure comparable to the
trophodisc of Allopora also occurs in other Hydrocorallines j
but the term spadix must be abandoned, for there is sufficient
evidence to show that this structure is not homologous with
the spadix of sessile Medusae. The " trophodisc " suggested by
Professor Lankester is a far better word to use as it expresses
thefunction it performs without indicating any homological comparison. Before describing the stages in the development of the
trophodisc of Allopora it is necessary to run through briefly
MATURATION OF OVTJM AND DEVKLOPMENT OF ALLOPOKA. 5 8 3
the main features in the histology of the general canal system.
The general arrangement of the canals is very similar to that
described by Moseley in A. profunda. " T h e ccenosarcal
canals form a fine superficial reticulation at the surface of the
coral beneath the surface layer and spring from a deeper
meshwork of larger canals which, as in the Stylaster already
described, have a mainly longitudinal course within the thickness of the walls of the pore system
"
The canals vary considerably in size, the larger ones being
about *05 mm. in diameter, the smaller ones '02 mm. in
diameter. In young branches the soft or fleshy parts fit very
accurately into the corresponding channels in the calcareous
coenosteum. The soft parts are composed externally of a thin
ectoderm and internally of a thick endoderm. The two layers
are separated from one another by a thin homogeneous
mesoglcea. The ectoderm (PI. XXXVIII, fig. 1) is a thin sheet
of tissue divided into cell areas by fairly well-marked limits,
each area provided with a spherical, oval, or flat nucleus.
Both in the cell substance and nuclei a protoplasmic spongewotk may be clearly seen in well-stained preparations. The
endoderm is practically a solid or tubular rod of cell substance
bearing at intervals large spherical nuclei. I have not yet
been able to distinguish in any of my preparations any traces
of the division of this cell substance into cellular areas. The
endoderm bears a very well-marked protoplasmic spongework,
and the nuclei a chromatin spongework with nodal thickenings.
The mesogloea varies in thickness in different places; it always
stains deeply and never exhibits anything but a clear homogeneous structure. The young ova, whose early history I
shall describe more fully under section 3, cause a swelling or
protrusion of the endoderm. Now, as I have previously pointed
out that the fleshy canals of young branches of AUopora fit
accurately into the canals of the skeleton, the swelling of the
canals must be accompanied by a process of absorption of the
skeleton in the immediate neighbourhood of the ovum. This
process of absorption continues until the large cavity is formed
called the ampulla, in which the embryo lies before it is dis-
584
SYDNEY J. HIOkSON.
charged to the exterior. That absorption of the calcareous
skeleton does actually take place in the formation of the
ampullae seems clear enough. In E r r i n a labiata a very
complicated process of absorption must occur as the ampullaa,
when they contain the ripe embryo, project on the surface
of the branches. Moseley says : " The ampullae are, in this
genus, conspicuous bodies, since they appear as hemispherical projections from the surfaces of the branches of about
the size of a mustard-seed. In vigorous specimens they are
closely crowded together in masses on both sides of the branches
and branchlets in various regions of the flabellum. The
ampullae commence as small cavities in the surface layer of
the coenosteum of the branches, and gradually enlarging in
accordance with the development of the ovum contained in
each, project more and more until those containing mature or
nearly mature planulee appear as conspicuous projections above
described. A hemispherical cavity excavated in the surface of
coenosteum corresponds with each ripe ampulla, but the excavation is usually not deep enough to render the entire ampullar
cavity spherical in form
In accordance with the
gradual expansion of the ampullar cavity, its outer wall, which
is finely reticular in structure, becomes thinner and thinner
until no doubt it at last breaks away entirely, allowing the
escape of the imprisoned planula."
I have no very conclusive evidence to bring forward of the
way in which this absorption is brought about, but I am inclined
to believe that the work is done by certain large ectoderm
cells—calycoclasts—that I have occasionally found in the surface of very young gonophores. It is quite possible that the
calycoclasts are generally carried away during the process of
decalcification. This would account for their paucity in thin
sections of decalcified specimens.
When the diameter of the young ovum is approximately as
large as the canal in which it is formed, a diverticulum of one
wall of the canal is pushed out and the ovum retreats into it.
At first this diverticulum is open to the canal by a wide mouth,
but the lips soon become pleated or pouched and the proximal
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MATURATION OF OVUM AND DEVELOPMENT OF ALLOPOEA. 5 8 5
region of the mouth narrowed. I have found only one specimen with the mouth of the diverticulum widely open. The
pouching of the lips of the diverticulum is accompanied by an
increase of endodermal nuclei, and it appears that there is a
concentration of endodermal tissue in this region. In the three
specimens that I was able to examine satisfactorily there were
five pouches, but I am unable to assert positively that this
number is constant in Allopora.
In this manner, then, the first rudiments of the trophodisc
are formed. It is essentially a structure formed by the lips
of the diverticulum for the support and nourishment of the
ovum,
Soon after the five pbuches of the young trophodisc are
formed small yolk-globules appear in the ovum.
The cavities of the five pouches open into a common vestibule, which communicates, on the one hand, by a very narrow
aperture with the diverticulum containing the ovum, and on
the other with the canal from which these structures were
formed.
In woodcut 1 a diagrammatic transverse section of the
WOODCUT 1.—Schematic transverse section of a young trophodisc.
five primary pouches.
P P. The
young trophodisc is shown. Such an appearance is never seen
in actual sections because the trophodisc is cup-shaped or
medusa-shaped ; but if it were possible to flatten it out artificially, and then cut an accurate transverse section exactly
586
STDNET J. HIOKSON.
through the middle of it, it would present approximately the
outline given in the figure.
As the ovum increases in size the trophodisc becomes more
complicated. The five primary pouches are divided longitudinally, by infoldings of the endoderm and mesogloea, into ten
pouches, and by a similar process secondary endodermal
pouches are formed on the inner and outer faces of some or all
of the primary pouches. The diagrammatic figure given in
woodcut 2 explains the formation of these pouches. It must
not be supposed, however, that anything so clear and simple
as this can be seen in any sections through the trophodisc. In
thin sections through preserved specimens the pouches have
always the appearance of having been considerably squeezed
WOODCUT 2.—Schematic vertical section through the ovum and trophodisc at
a somewhat later stage than that represented in Woodcut 1. Ov. Ovum.
P. Primary pouch, p1. External secondary pouch, p2. Internal secondary
pouch, c. Primary communication between diverticulum and canal.
and pressed together. I am not able to assert that in fresh
specimens this is also the case, as I have not yet had the good
fortune to obtain any specimens alive; but when we consider
that the ovum and trophodisc are, as a rule, bounded by the
calcareous walls of the ampulla, it is quite possible that the
distorted appearance of the trophodisc presented by sections of
preserved specimens is not very materially different from the
natural appearance.
The diagram was composed by drawing the outlines of several
MATURATION OP OVUM AND DEVELOPMENT OF ALLOPORA. 5 8 7
complete series of sections with the camera lucida, and comparing them with one another.
When the ovum is mature the cavities of the pouches of the
trophodisc become obliterated, and soon afterwards their outlines become obscure, and the whole structure degenerates into
a multinucleate mass of endoderm (PI. XXXVIII, fig. 14). In
its earlier stages the ovum is enclosed in a complete chorion
composed of ectoderm, mesoglcea, and endoderm, each of these
tissues being directly continuous with the same tissues of the
canal in which the ovum first began to grow. As the egg increases in size these membranes become much thinner, but they
may always be distinguished on the distal side of the egg, even
when it is fully grown (PI. XXXVIII, fig. 18). On the proximal
side, however, the ectoderm disappears from the area covered
by the trophodisc as soon as the latter comes in contact with
it, and at the same time the ectoderm of the outer walls of the
cup of the trophodisc becomes continuous with the ectoderm
covering the distal side of the ovum. In the later stages then
the ovum and trophodisc are completely enclosed in a continuous ectodermal sheath.
When the embryo escapes to the exterior the ampulla in
which it developed is lined internally by a sheath of ectoderm,
mesogloea, and endoderm, and on the proximal side of it, i. e.
the side nearest to the central axis of the branch, there is an
endodermal syncytium—the degenerated trophodisc—in which
new young ova are frequently to be found.
I I I . The Changes of the G e r m i n a l Vesicle.
For a long time I was unable to discover any of the ova of
Allopora younger than those already sunk in their diverticula.
My attention was at last directed, however, to certain large
nuclei occasionally to be seen in the endoderm of the canals,
more especially in canals in the neighbourhood of already established trophodiscs and ova. These nuclei can be distinguished from ordinary endodermal nuclei, not only by their
large size, but also by their transparency, their large nucleolus,
588
SYDNEY J. HIOKSON.
and well-defined chromatin meshwork. The cell-substance
surrounding each of these nuclei is distinguished from the endodermal cell-substance in being considerably more transparent
and homogeneous, and in taking up the staining reagents less
readily.
I failed entirely to find even a trace of a limiting membrane
between the cell-substance surrounding them and the general
endoderm. When I came to examine these bodies with greater
care, and to compare their structure with that of the youngest
ova I had previously found, no doubt remained that they were
very young ova. I could find no trace of any such structures
in the ectoderm, and no satisfactory evidence that they undergo
any process of migration in the endoderm of the canals. In
some cases, it is true, they exhibit amoeboid processes (PL
XXXVIII, fig. 5); but these cannot be taken as an infallible
sign of movement, as it is quite possible they are merely used
for the purpose of feeding more readily upon the fluids of the
endodermal tissue.
The nucleus or germinal vesicle of the young ovum undergoes no noticeable change except a slight increase in size until
the ovum approaches maturity. The first change to be noted
is that it shifts its position from the centre towards the distal
pole, and at the same time becoming more or less oval in
shape, its longest diameter being approximately at right angles
to a line drawn from the proximal to the distal pole of the
ovum, it loses its regularity of outline.
On the distal side a small papillate process makes its
appearance (PI. XXXVIII, fig. 8) which contains a concentration of the chromatin network. This process represents
the portion of the germinal vesicle that is subsequently discharged with the first polar vesicle. The germinal spot, even
in the younger ova a very prominent structure, attains at this
stage its largest size. It always stains deeply in boraxcarmine. When examined by a high power a regular tesselated appearance can be made out, which seems to be due
to the presence of an intra-nucleolar chromatin meshwork
(fig. 9).
MATURATION OF OVUM AND DEVELOPMENT OF ALLOPOBA. 5 8 9
When the germinal vesicle reaches the distal pole of the
ovum, two polar vesicles are successively thrown out. I am
unable to find any evidence that this is accompanied by
karyokinesis. There seems to be a concentration of the
chromatin meshwork in the part of the germinal vesicle that
is protruded, but that is all. I can find no stars or loops at
any time in the history of the germinal vesicle of Allopora.
After the discharge of the polar bodies the germinal vesicle
becomes flattened to a hemispherical shape on the surface of
the ovum, and from the large number of ova that can be
found of that shape in that position I infer that it remains so
for a considerable time (fig. 11). It is while the germinal vesicle
is in that position that I believe fertilisation takes place. I
have not been fortunate enough to find any steps in the process
of fertilisation, nor to discover the precise way in which the
spermatozoa reach the ovum. It should be noted here, however, that as the ampulla grows in size it breaks into various
canals of the superficial meshwork of the ccenosteum, and consequently the ovum is brought into close contact at various
points with the superficial canal systems. If therefore we
are justified in supposing that a number of spermatozoa are
driven by ciliary currents into the canal systems by way
of the mouths of the gastrozoids, there is little difficulty
in understanding the way in which the spermatozoa reach
the ova.
In the next stages the large nucleus which must now be
supposed to be formed of the conjugated germinal vesicle and
sperm nucleus retreats from the surface of the ovum. At
first it has a rough resemblance to the germinal vesicle in the
stage just before it reaches the periphery, but a careful examination is sufficient to avoid confounding the two stages (fig.
12). When theoosperm nucleus retreats, it is regularly hemispherical on the proximal side but irregular and provided
with four or five delicate amoeboid processes on the distal side.
The membrana limitans of the oosperm nucleus becomes very
indistinct, especially on the proximal side, and in the later
stages entirely disappears. By these characters it is possible,
590
SYDNEY J. HI0ESON.
with a little experience, to distinguish at a glance sections
of the oosperm nucleus and of the germinal vesicle at the
stage represented in PL XXXVIII, fig. 12; but the determination can in all cases be confirmed by the presence or absence
of the remnants of the polar bodies.
When the membrana limitans disappears it is impossible to
distinguish the outline of the oosperm nucleus. The ovum
then bears, in the middle of the distal side near the periphery,
a protoplasmic portion, surrounded by numerous very small
yolk-spheres irregularly distributed, that stains rather more
deeply than the rest of the protoplasm of the egg. This
portion contains, so far as I can observe with high powers, no
other histological elements than a few scattered fragments of
chromatin fibres.
In the next stage observed I found one or two irregular
lumps of nuclear substance close to the distal side of the
young embryo, and seven or eight oval or spherical deeply
stained nuclear bodies ("005 to '01 mm. in diameter) scattered
about in the protoplasm of the distal pole. In the next stage
the irregular lumps have disappeared, and the nuclear bodies
increased in number (PL XXXVIII, figs. 14, 16.
I have paid particular attention to these stages in the
development of Allopora, and 1 can find no trace whatsoever
of any division of the protoplasm in the neighbourhood of the
nucleus, and no evidence that would lead me to suppose that
I have missed any stages of regular segmentation either of the
nucleus or the egg protoplasm.
The evidence before me, so far as it goes, seems to show
that in Allopora the oosperm nucleus, after losing its membrana limitans, simply breaks up into fragments, and that
from the fragments a number of embryonic nuclei are formed
that wander into the region of the embryo that might be
called the blastoderm, where they rapidly multiply by a process
of growth and simple division. The larger irregular lumps of
nuclear substance found only in the earliest stages I take to
be portions of the oosperm nucleus that have not so fragmented,
MATURATION OP OVTJM AND DEVELOPMENT OF ALLOPORA. 5 9 1 •
IV. The Formation of the Embryonic Ectoderm.
At about the time that the first embryonic nuclei make their
appearance the protoplasm becomes considerably vacuolated.
The exact time and the extent of the vacuolation varies in the
two species I have examined, and indeed in different individual
cases of the same species. In very young embryos it is sometimes difficult to distinguish true vacuoles from those artificially
produced by the contraction of the protoplasm under treatment
with reagents. I shall consequently lay but little stress in
this paper on the time and mode of appearance of the vacuoles
in Allopora; but I hope to treat this point more fully when I
have had an opportunity of comparing my results with those
obtained in working out the development of some other
Hydrocorallines.
At the stage represented in PI. XXXVIII, fig. 17, the
embryonic nuclei have increased enormously in numbers, and
may be found scattered throughout the embryonic protoplasm.
The vacuoles are numerous, especially in the distal hemisphere,
and in some cases they are separated from one another by
simple thread-like branching strands of protoplasm. The
larger vacuoles are never quite superficial, but the periphery is
bounded by a thin continuous membrane of protoplasm bearing
a few very small yolk-spherules. Into the substance of this
membrane some of the embryonic nuclei wander and take
up their positions side by side (fig. 18).
This peripheral membrane with the nuclei forms the first
beginning of the embryonic ectoderm. It commences at the
distal pole and gradually spreads all round the young embryo.
When the nuclei have wandered into the peripheral membrane
on the proximal side and formed there the first beginning of
the ectoderm, cellular bodies of protoplasm, each containing a
single nucleus, and together forming a well-defined, thick,
columnar epithelium, have developed on the distal side. At
the sides of the embryo the ectoderm is in a condition intermediate between these two extremes. It must be obvious, then,
that in an embryo at this stage (PI. XXXVIII, fig. 20) it is
592
SYDNEY J. HTOKSON.
possible to study all the phases in the formation of the ectoderm
by the examination of one good section.
The ectoderm is formed in this manner: as soon as the
nuclei have taken up their position in the peripheral protoplasmic membrane, the latter increases considerably in thickness. The protoplasm then breaks up into blocks, each block
containing a single nucleus (fig. 21A). These blocks or cells,
as they may now be called, are separated from one another by
lacunar spaces. I can find no protoplasmic threads connecting
them together nor any residue, so that I think we are justified
in assuming that in life they are filled with a thin watery fluid.
The ectoderm still continues to increase in thickness, and the
cells are in consequence drawn out into spindles, isosceles
triangles, and many other elongated shapes (fig. 21B). At the
external and internal extremities, however, the cells are
continuous with one another.
When the ectoderm has completely developed all round, the
embryo is ready to escape. A passage is prepared for it by
the absorption of the superjacent calcareous walls of the
ampulla, and it escapes.
From the commencement of the formation of the ectoderm
to the time of the escape of the embryo no noteworthy change
occurs in the structure of the central mass. It consists
from first to last of a mass of spongy protoplasm bearing in
its meshes numerous embryonic nuclei and yolk bodies. There
is certainly a considerable increase in the number of the
nuclei, and in the number of the vacuoles; but I cannot
detect even in the oldest embryos any arrangement of these
nuclei and the surrounding protoplasm to form an endoderm,
no invagination of the primitive ectoderm, and nothing
comparable to a process of delamination.
V. The History of the Yolk-Spheres.
It is clear from the description above and the figures given
that the yolk of the ova of Allopora is not formed without,
nor by buds from the germinal vesicle. Neither the cells of
the phoriou nor the endoderm cells of the trophodisc exhibit
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MATURATION O¥ OVfJM AND DEVELOPMENT OF ALLOFOliA. 5 9 3
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at any time highly refracting granules of any kind that could
possibly be mistaken for yolk. The germinal vesicle moreover
shows no sign of budding until the yolk is fully formed.
The yolk-spherules of Allopora are solely produced by the
activity of the egg protoplasm. When they can first be clearly
distinguished as yolk-spherules they are distributed equally
through the protoplasm of the egg, the largest being from g005
to *01 in diameter. At the time that the ovum is approaching
maturity the largest yolk-spheres are on the proximal side of
the egg. In the neighbourhood of the germinal vesicle they
are much more numerous than elsewhere, but smaller. They
are all perfect spheres. The average size of the largest spheres
is '02 mm., but occasionally one may be found with a diameter
of-025 mm.
As soon as the oosperm nucleus retreats from the periphery,
some curious changes occur in the character of the yolk-spheres.
The smallest spherules are not confined to the neighbourhood
of the nucleus, but scattered over the periphery of the distal
hemisphere, and the largest ones are not without exception
perfect spheres as they were before, but some of them are oval
or irregular in shape. Some careful measurements I made
of the irregular yolk bodies gave me the following results :
-OB mm. x -02 mm., '0275 mm. x -02 mm., 025 mm. x -02
mm.; the numbers representing the longest and shortest
diameters respectively (PI. XXXVIII, fig. 19). The largest
perfectly spherical yolk bodies in the same section were "01
to '0125 in diameter. There can be little doubt from the
appearance of the yolk bodies at this stage that a considerable
disturbance is going on in the substance of the ovum; some
of the yolk spheres are breaking up, others are fusing together.
The rapid degeneration that takes place in the structure of the
trophodisc, together with the curious changes that take place
in the character of the yolk-spheres, suggest that at the time
of maturation of the ovum it ceases to be nourished from without, and that the food material required for the further development is entirely procured by the breaking up and distribution
of the stores (yolk bodies) already within the substance of the
594
SYDNEY J. HICKSON.
egg. From the time when the embryonic ectoderm first makes
its appearance until the embryo escapes no marked changes
take place in the character of the yolk bodies. There is
undoubtedly a considerable diminution in the gross amount of
yolk, but in the oldest embryos I have seen there was always a
large number of these bodies in the central endodermic (?)
mass. At no stage in the history of the ovum is there any
structure comparable to the yolk nucleus of other lecithal
VI. General Considerations and Conclusion.
Two points of special interest may be noticed in the development of Allopora, namely, the trophodisc and the fragmentation
of the oosperm nucleus. It is perhaps premature to discuss
fully the suggestions they afford until we have further information about other Hydrocorallines.
I have now at my disposal some excellent specimens of Distichopora from Torres Straits, for which I am indebted to the
generosity of Professor Haddon, and I hope to obtain before
long some specimens of Stylaster. If there are any naturalists
possessing well-preserved specimens of any Hydrocorallines
(including all species of Millepora except M. plicata) who
could spare me a few branches for my investigations I should
be most grateful to receive them.
When I have had the opportunity of working up this
material I shall discuss more fully than I am able to do now
the bearing of these investigations. So far as our knowledge
goes, however, it seems probable that the trophodisc is not a
reduced Medusa. The position of the ovum within the cavity
of the structure that would be called, if it were a reduced
Medusa, the manubrium, the number (5) of the primitive
pouches, the absence of any structure in development comparable to the " Glockenkern," and other considerations, all
tell against the view that the trophodisc was at any time in the
phylogenetic history of the Hydrocorallines a free-swimming
Medusa.
The fact that we find in the group of the Hydrocorallines one
MATURATION OF OVUM AND DEVELOPMENT or ALLOPOEA.
595
1
genus, Millepora, (2) without any trace of yolk in the ovum,
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and others, AUopora (Hickson), Distichopora (Hickson), Errina
(Moseley), Pliobothrus (Moseley), and others with abundance
of yolk in the ovum, is in itself one of considerable interest.
It is another example of the unequal distribution of yolkbearing ova in the various orders of the animal kingdom.
In a recent paper (2) I expressed the opinion that the phenomena observed in the early stages of the development of
M i l l e p o r a plicata do not justify us in assuming that the
ovum of this species at any time in its phylogenetic history was
charged with yolk. The early stages in the development of
AUopora seem to me to support this view. The fact that the
oosperm nucleus of AUopora breaks up into fragments in a
manner comparable with, although not perfectly similar to, that
of Millepora, without any trace of segmentation of the ovum,
shows that the phenomena observed in the latter are not due to
loss of food-yolk. Furthermore, if Millepora at any time in its
philogenetic history produced yolk-bearing ova, we might expect
to find some trace of the trophodisc, a structure that is present
both in the male and in the female sexual organs of all the
the other Hydrocorallines that have at present been observed ;
but no such structure has been discovered.
I obtained the material for these investigations from two
sources. Professor Lankester kindly placed at my disposal
some excellent specimens of AUopora oculina that he
dredged in the Hardanger Fiord, and preserved partly in absolute alcohol alone and partly in absolute alcohol after treatment
with corrosive sublimate; and to Professor Moseley I am
indebted for some specimens of AUopora norvegica given
to him by Mr. Murray from the Triton collection. As it is
impossible to tell the exact age of the ova of AUopora before
they are cut into sections, I found it necessary to make an
immense series of preparations before I could find all the stages
described above. The investigation has consequently taken me
1
This statement is based upon my own researches on M. plicata. It is
quite possible that other species of Millepora, such as M. Murrayi, may
possess a small amount of yolk in their ova.
596
SYDNEY J. HIOKSOtt.
over two years to bring to the state in which I am able to
publish it. The first few preparations were made in the Morphological Laboratory at Cambridge,the research was continued
during 1888 in the anatomical department of the University
Museum at Oxford, and completed in the Laboratory of Comparative Anatomy at University College, London. I cannot close
this paper without expressing my sincere thanks to Professor
Lankester for allowing me the use of a table in his laboratory,
and for the many valuable suggestions he has made to me
during the course of my investigations.
BIBLIOGRAPHY.
1. U. N. MOSELEY.—" Report on Certain Hydroid, Alcyonarian and Madreporarian Corals procured during the Voyage of H.M.S. 'Challenger'
in the years 1873-76," '"Challenger" Reports,' vol. iii.
2. S. J. HIOKSON.—" On the Sexual Cells and Early Stages in the Development of M i l l e p o r a p l i c a t a , " ' Philosophical Transactions,' 1888.
DESCRIPTION OP PLATE XXXVIII,
Illustrating Dr. Hickson's memoir " On the Maturation of
the Ovum and the Early Stages in the Development of
Allopora."
The following Reference Figures are used throughout.
ect. General ectoderm of the parent colony, ect'. Embryonic ectoderm.
end. Endoderm., m. Mesogloea.
ov.t ov1., ov3. Ova.
tr. Trophodisc.
ch. Chorion. g. v. Germinal vesicle, g. s. Germinal spot.
n. Nucleus.
y. Yolk-globules. /. Lumen of canal, p. b. Polar bodies.
FIG. 1.—A small portion of the canal system of Allopora. In the larger
canals, a and 6, a lumen may be seen ; but in the smaller ones, c d, this is not
apparent. The endoderm is traversed by a coarse protoplasmic spongework,
with nodal thickenings. There are no traces of any boundary membranes
between endoderm cells. The ectoderm is like the endoderm in general
structure, but it is distinctly divided into separate cells each containing a
single (frequently flattened) nucleus. Between the endoderm and ectoderm
there is a thin layer of mesoglcea.
MATURATION OF OVUM AND DEVELOPMENT Of ALLOPORA. 597
FIG. 2.—A small portion of the canal system of Allopora, showing a young
ovum lying in the endoderm. The ovum is not surrounded by any trace of a
vitelline membrane, and the structure of its periphery is very similar in
character to that of the endoderm. It is consequently very difficult to
distinguish precisely the exact outline of the ovum at this stage. In the
neighbourhood of the germinal vesicle the protoplasm of the ovum is rather
more cloudy in appearance, and the meshwork it contains is finer and closer in
texture than it is in the endoderm. The germinal vesicle is large, and contains a sparse chromatin meshwork and a large germinal spot. It is surrounded by a fairly well-defined membrane. At N. a large nucleus may be
seen in the endoderm. It is possible that this is the nucleus of a still younger
ovi-cell, the outline of which cannot be distinguished.
FIG. 3.—A young ovum lying in the degenerated syncytium of the trophodisc of an embryo.
PIG. 4.—A small portion of a branch of Allopora, showing a young ovum
and trophodisc growing in the ampulla (sp.) formerly occupied by an embryo.
ap. Aperture by which the embryo escaped. In the walls of the trophodisc,
and in a large canal lying immediately below it, may be seen young ova
(ov>. ov*.).
FIG. 5.—The young trophodisc, the ovum, and a part of the subjacent canals
of the last figure more highly magnified. The young ovum in the trophodisc
may be seen in this figure to be surrounded by a chorion (cA.) of ectoderm
mesoglcea, and eudoderm continuous at x, with the ectoderm mesoglcea and
endoderm of the trophodisc.
J?I(J, 5_—A stage rather more advanced than that shown in Fig, 5. The
trophodisc is more involved, and the substance of the ovum filled with small
spherical yolk-globules.
J I G . 7,—A stage still more advanced than that shown in Fig. 6. The
germinal vesicle is irregularly oval in shape, and is situated not centrally, as
in the earlier stages, but close to the distal pole of the ovum. It is surrounded
by numerous small yolk-spherules. The trophodisc is a flattened disc-shaped
structure, and the lumina of its folds are almost obliterated.
Ji & , 8.—The germinal vesicle of the ovum represented in Fig. 7, more
highly magnified, to show the large germinal spot, the chromatin meshwork
with its nodal thickenings, and the amceboid process on the distal side.
FIG. 9.—Germinal spot of the same, still more highly magnified.
FIG. 10.—The germinal vesicle and a small portion of the substance of the
ovum surrounding it, at the time when the polar bodies are being thrown out.
FIG. 11. Stage in which the germinal vesicle, after the polar bodies have
been thrown out, becomes flattened on the distal pole of the ovum.
FIG. 12.—Later stage of the germinal vesicle (oosperm nucleus) when it
retires from the distal pole of the ovum into the yolk again.
fiu, 13. Stage in the development of Allopora when the germinal vesicle
VOL. XXX, PART 4 .
NEW SER.
QQ
598
SYDNEY J . HIOESON.
(oospertn nucleus) becomes obscure. The smaller yolk-spherules, which in
the earlier stages are principally collected round the germinal vesicle, are in
this stage scattered over the periphery of the distal hemisphere of the ovum.
The larger yolk-spheres lose their regular spherical shape and appear to be
breaking up. The trophodisc is reduced to a mere nucleated syncytium iu
connection with the canal system.
FIG. 14.—Young embryo of Allopora, showing the first appearance of
embryonic nuclei after the disappearance of the germinal vesicle (oosperm
nucleus). These nuclei are separated from one another by considerable
intervals, but are all found within the distal hemisphere. Some of them are
spherical in shape, some dumb-bell shaped, and a few larger ones are irregularly amoeboid in shape.
FIG. 15.—Some examples of the earliest embryonic nuclei yet found.
FIG. 16.—Stage in the development of Allopora, rather later than that
represented in Fig. 14. The embryonic nuclei are more numerous, and there
are no large irregular-shaped one3 characteristic of that stage. The protoplasm of the embryo is here considerably vacuolated.
FIG. 17.—Stage in the development of Allopora, rather later than that
represented in Fig. 16. The embryonic nuclei are still more numerous and
scattered throughout the embryonic protoplasm. On the periphery of the
distal hemisphere there is a thin layer of unvacuolated protoplasm, in which a
considerable number of the nuclei have ranged themselves. This is the first
beginning of the ectoderm of the embryo.
FIG. 18.—A small portion of the periphery of an embryo at the stage
represented in Fig. 17, more highly magnified.
FIG. 19.—Some examples of the larger yolk-globules, found in the embryo
represented in Fig. 17.
FIG. 20.—Last stage observed in the development of the embryo of Allopora. Soon afterwards the embryo escapes. The embryonic ectoderm cells
are now completely formed over the periphery of the distal hemisphere of the
embryo. On the proximal side the protoplasm of the embryonic ectoderm has
not yet broken up into cell-masses round the nuclei.
FIG. 21.—Two stages in the formation of embryonic ectoderm. A. The
younger stage. B. The older stage.
FIG. 22.—A small portion of the protoplasm of the inner portion of the
embryo represented in Fig. 20.