Some Observations on the Behaviour of
Amoeba proteus.
By
C. W. Parsons, B.A.,
Department of Zoology, University of Glasgow.
With 9 Text-figures.
ALTHOUGH the body-form of A m o e b a p r o t e u s shows
so much diversity in any healthy culture, the differences that
may be observed conform to certain types. If these are
assembled, they fall into groups which may be supposed to
indicate physiological variation in the organisms ; for they
are then reacting in different ways to the stimuli provided by
the same external medium, and such behaviour can best be
explained on physiological bases.
The investigation of the groups is approached in the following
pages by two different routes. The first is experimental and
consists in a re-examination of the normal behaviour of adult
Amoebae in the aquarium medium in which they are best
cultured (see Monica Taylor, 11), followed by an analysis of
their reactions to simple changes in their environment. The
second is deductive. It rests upon a hypothesis relating the
degree of activity of these organisms, as measured by the
rapidity and ease with which they extend their pseudopodia,
with their health. It recognizes the presence of healthy forms,
in unfavourable as well as in favourable environments, and puts
forward the view that some Amoebae are more adaptable to
change of medium than are others. A link is here established
with the well-known fact that while the Amoebae in a culture
cease to reproduce by fission after a time, many of them do not
enter on the cycle of encystment. They die out for no obvious
630
C. W. PARSONS
reason. Food remains abundant, overcrowding is avoided, and
the pH throughout the culture may be adjusted to give conditions normally ideal for multiplication. The attempt is here
made to correlate this peculiarity in life history with a conspicuous variation in body-form, and the types exhibiting this
variation are defined.
Material.—Adult Amoebae having chromosome blocks in
the periphery of their nuclei, from a nourishing sub-culture of
Dr. Monica Taylor's culture ' 19 ', type ' B ' (12, pp. 187 and
120).
A p p a r a t u s i n c l u d e d — plunger pipettes, i.e. capillary
pipettes fitted internally with drawn-glass rods and sealed with
rubber tubing. (See Brooker Klugh, 5.)
Solid watch-glasses, 1 | in. square; J in. deep for supporting
cover-slips in high-power work ; ^ in. deep for general purposes.
Sarensen phosphate buffer solutions with indicators, made
up to 10 e.c. and 2 c.c.
A. B e h a v i o u r of A d u l t A m o e b a e in C u l t u r e
Media free from D e b r i s . pH 6-8.
The Amoebae are readily seen as white specks if debris containing them is taken from the parent culture and placed in
a shallow dish on a black background. They may be picked
up singly with the plunger pipette and transferred for observation to a solid watch-glass containing clean culture medium.
If there is rather less than 2 c.c. of fluid they may be focussed
with a Zeiss ' A ' objective (f in.), and their behaviour recorded
by notes and drawings taken at suitable intervals.
High-power observation of single living Amoebae in a drop
of fluid was carried out on a clean glass cover-slip supported in
the well of one of the very shallow watch-glasses (£ in. deep).
If the drop of fluid is small, the Amoebae may be focussed with
a Zeiss ' D ' objective (£ in.), and the drop can be renewed as
required from a fine-drawn pipette. When set aside, rapid
evaporation may be prevented by placing another solid watchglass on top of the one already holding the cover-slip, and the
Amoebae remain alive in the drop.
BEHAVIOUR OF AMOEBA
631
When they are picked up with the plunger pipette the
Amoebae withdraw their pseudopodia, and it thus comes about
that they are delivered to the solid watch-glasses as rounded
masses of protoplasm. If they are kept floating—by gentle
agitation of the fluid—they extend numerous blunt pseudopodia (Text-fig. 1). When allowed to settle, however, they
adhere to the glass surface in an orderly fashion. First, one
of the pseudopodia reaches the substratum and adheres to it by
TEXT-FIGS. 1, 2.
Tig. 1.—Typical body-form of a floating A m o e b a p r o t e u s i n i t s
aquarium culture medium. The figure shows a radial disposition
of pseudopodia of roughly equal size.
Fig. 2.—Contact with the substratum is first established by adhesion
to it of one of the radial pseudopodia, which is distinguishable
now as the major pseudopodium.
the tip. Then it receives the remaining protoplasm (Textfigs. 2, 3, and 4) and, leading the organism down to the substratum, it becomes the directive pseudopodium in the new line
of advance. The Amoebae adhere closely to the glass surface
as a rule, and move rapidly over it in a digitate form. A broader
extremity crowned with active pseudopodia lies in the main
axis of advance, and a slight constriction commonly marks the
' tail' (Text-fig. 4, t).
The ' tail ' is a conspicuous feature of most Amoebae even
in experimental media. It is seldom the seat of pseudopodia
632
C. W. PARSONS
formation, a point which is emphasized by observing theeffect of mechanical stimulation upon one of these adheringforms in culture medium. Schaeffer has shown how they
respond when a glass needle is agitated near to them by initiating
the ' feeding mechanism' (10, p. 229), but if the medium isdisturbed in this way immediately in front of the advancing
pseudopodia, an avoiding reaction is induced. The flow of
TEXT-FIGS. 3, 4.
Fig. 3.—Further stage in the transition from a floating condition
to one of adherence to the substratum.
Fig. 4.—A completely adherent active Amoeba of the type shown
floating in Text-fig. 1, with a body-form frequently observed,
comprising three main pseudopodia, an elongate body, and a
slightly constricted ' tail' (t). Same scale as fig. 1.
granules is at first arrested and then reversed. After receding'
a little, the Amoeba moves away afresh at an angle less than
a right angle from its previous direction of flow. If now left
undisturbed the pseudopodia gradually deflect into the original
alinement and draw the animal into a path parallel to but
slightly removed from its former course. On the other hand,
this movement may be checked soon after its commencement
by disturbing the medium again in front of the pseudopodia
when they begin to diverge. The original course is then
BEHAVIOUR OF AMOEBA
638
reversed : not, however, by the formation of pseudopodia
at the hinder end, but by movement of lateral pseudopodia into
the new position which draw the ' tail' round after them. This
behaviour is typical of the most active adult Amoebae in culture
media.
Many adult Amoebae prove to be relatively inactive when
taken from the parent culture. They are frequently more
consolidated than the more active forms and may be very much
shrunken. There seems, in fact, to be a gradation between
the most active and least active Amoebae, marked by a change
in their appearance under transmitted light from grey to black.
It is notable also that the darker forms tend to lose their
adhesion to the sides of the vessel containing them with greater
ease than do the active grey ones. This suggests that granularity and adhesiveness on the one hand, and readiness of
movement on the other, may be complementary features.
B.
B e h a v i o u r of A m o e b a p r o t e u s in P u r e M e d i a .
The water used in the following observations was distilled
once from Glasgow tap-water in a glass condensing apparatus.
A little potassium permanganate and a trace of mineral acid
was added before distillation to oxidize impurities. The
distillate, called hereunder glass distilled Avater, had a steady
pH of 5-8 before prolonged exposure to the atmosphere.
The Amoebae may reasonably be separated into three types
on the basis of their reactions to this medium.
1. The majority quickly extend numerous pseudopodia to
relatively great distances. (Text-fig. 5.) The reaction is the
same in well-aerated distilled water, and has no relation therefore
to the recognized paucity of oxygen in untreated glass distilled
water. On theoretical grounds Mr. C. F. A. Pantin informs me
that the ratio of surface to volume in Amoeba is so great, that
equilibration with oxygen in the external medium is effected
in an extremely short space of time. The small increase of
surface produced therefore by great extension of pseudopodia,
will not avail to compensate for a law oxygen content in the
medium; although of course the great variation in ' per-
634
C. W. PARSONS
nieability ' of living protoplasm to oxygen is not disputed
(cf. ' Air-bladder of Fishes ').
TEXT-FIG. 5.
The exaggerated floating Jorm ot an active Amoeba suspended in
distilled water. The attenuated pseudopodia suggested the distinction of Amoebae that react in this way from others which are
less active, and it was convenient to describe them as type ' A '
Amoebae.
Amoebae in glass distilled water do not, as a rule, fix themselves to the substratum. They float with extended pseudopodia in a most grotesque manner, and are extremely active
BEHAVIOUR OF AMOEBA
685
at first. Their energy is slowly dissipated, however, and in
8-10 hours they become sluggish. As time elapses they shrink
and grow darker in appearance, but they may remain alive
5-7 days before cytolysis. The death of the organism is frequently preceded by its assumption of a more or less spherical
form in accordance with its subjection to the ordinary forces
of surface tension. Distension of its outer layers—owing to the
decrease in efficiency of the contractile vacuole which normally
Illustrates a body-form not infrequently observed amongst active
Amoebae that are floating in pure media. Movement is steadily
maintained in the outer layers, which may therefore differentiate
sharply as at a, from the endoplasm. Movement in the latter is
also vigorous, but it is checked periodically and results in exceptionally blunt pseudopodia, b. Same scale as fig. 1.
counteracts the diffusion of water into the Amoeba may be
observed, and the appearance then presented is that of a very
large vacuole with a small quantity of granular cytoplasm about
it. After this has been realized, the actual cytolysis may be
delayed for a period varying in individual cases between a clay
and only a few hours. If returned to culture medium within
the first 8 hours, or during their active phase, the Amoebae
recover rapidly and completely : but if this is delayed until
they have become sluggish, recovery is partial and gives rise
to dark, slow-moving forms in the majority of cases.
A noticeable variation in the ease with which the elongated
pseudopodia are extended, is to be seen in these experiments..
686
C. W. PARSONS
Movement in some Amoebae proceeds jerkily. At a, Textfig. 6, a narrow streak of the clear outer layer is put forward,
while the flow of granules goes on in the cytoplasm behind it.
The granules accumulate in great numbers as a result and
swell out the ectoplasm about them into ' beads '. These
disperse suddenly when the pressure releases, to form exceptionally blunt pseudopodia (b, Text-fig. 6). This type of bodyform is described by Gruber (3, p. 256), with the suggestion that
it indicates abnormal consolidation of the outer surface layers.
TEXT-FIG. 7.
The poor response to cuange in environment or one of the darker
inactive Amoebae that occur in culture media, illustrated by the
ragged appearance of 6ne of them in distilled water. It is a typical
type ' R ' Amoeba. Same scale as fig. 1.
2. The darker inactive Amoebae, which have already been
mentioned because they occur in culture media, do not respond
so well to the stimulus of glass distilled water. Only short
pseudopodia are extended when they are transferred to this
fluid. The outer layers are abnormally distinct in them, and
they float with very ragged outlines and limited powers of
movement. (Text-fig. 7.)
3. Exceptional Amoebae remain. These rare forms make no
response at all to the alteration in environment. They are
typically clavate, and adhere lightly to the substratum in
culture medium. In distilled water they retain this capacity
to some extent. (Text-fig. 8.)
BEHAVIOUR OF AMOEBA
687
When these forms are contrasted, the desirability of splitting
the active and inactive groups into types is apparent.
Active attenuating forms .
. type ' A ' (Text-fig. 5).
Inactive ragged forms
.
. type ' E ' (Text-fig. 7).
Inactive clavate forms
.
. type ' C ' (Text-fig. 8).
The types ' E ' and ' 0 ' are more abundant in cultures wherein
multiplication of Amoebae by fission has been in full progress
TEXT-FIG. 8.
A clavate irresponsive form of Amoeba, which is rare in culture
media, figured from distilled water; the type ' C ' Amoeba.
for many months. This, correlated with the fact that type ' A '
Amoebae from these cultures are less responsive to the immediate effects of distilled water, suggests that somewhat unfavourable conditions hasten the change from type ' A ' to type ' E ' ;
which under normal vigorous circumstances will proceed very
slowly. This does not mean that the types ' E ' and ' C ' are
absent from the most flourishing cultures. They may be found
in them, but are rare.
688
C. W. PARSONS
In glass distilled water containing measurable traces of acid
or base, Amoebae react in a manner emphasizing the above
point with regard to unfavourable conditions. Their behaviour
in these solutions is influenced by four factors :
1. The type of Amoeba, ' A ', ' E ', or ' C '.
2. The duration of experiment.
3. The acid or base employed.
4. The change in pH involved in transferring the Amoebae
from the culture medium to the artificial medium.
1, 2, and 3 may be controlled by selection ; 4 requires an
adjustment of the pH of the artificial medium, which may be
carried out by the following process : 10 c.c. of glass distilled
Avater are measured from a burette into each of a dozen clean
test-tubes. Ten drops of the indicator proper to the desired
range of pH follow in the concentration given in Clark and
Lubs's list of indicators (2, p. 80), and drops of a very dilute
solution of the pure acid or base selected for experiment.
Any one colour given in the same amount of phosphate mixture
by the same indicator can thus be accurately matched, and the
pH of the Avater in the test-tubes is standardized. If the number
of drops of the acid or alkaline solution required to obtain a
desired pH exceeds five, a slightly stronger solution should be
employed.
It was at first desirable to use experimental media free from
indicators. The number of drops of acid or base at the giA7en
dilution necessary to bring 10 c.c. of glass distilled water to the
desired pH Avas calculated by this method, therefore, and then
added to 10 c.c. of fresh distilled water. The pH of the resulting
solution was checked by removing 2 c.c. of it, adding 2 drops of
indicator, and comparing the colour obtained with that of 2 c.c.
of phosphate mixture with indicator. It was found, hoAvever,
that the presence of so small a quantity of indicator made
little difference to the behaviour of the Amoebae.
It is not possible to maintain a standardized pH for long
observation in the region of the neutral point. In dealing Avith
alkali, for example, media in the absence of buffering salts
readily absorb CO2 and other acid gases from the atmosphere,
BEHAVIOUR OP AMOEBA
639
and tend to become less alkaline with increasing rapidity as the
pH decreases. This can only be partially remedied by basing
every judgement on a number of observations, by renewing the
fluid as often as possible, by recording the pH before and after
each experiment, and by making up such solutions in glass
distilled water that has been exposed to the atmosphere until
the pH has risen to some steady point. In the Glasgow laboratory it commonly rises from 5-8 to 6-2.
For comparison, the records of the behaviour of the Amoebae
over a definite range of pH are best obtained simultaneously.
A number of solid watch-glasses are placed under microscopes
accommodating the solutions of different pH, and to each are
then added four or five Amoebae. These are selected from
distilled water according to the type required, and transferred
to the solid watch-glass after a preliminary wash in the experimental solution. If the experiments are started at 20 min.
intervals, a general idea of the behaviour of the Amoebae in
each watch-glass is gathered by observing them continuously
for the first 15 min., and afterwards at intervals determined by
their activity. A wide range of pH cannot of course be
attempted. Six values are about the maximum that may be
covered, and even then it is barely possible to give the necessary
attention to the six microscopes when the whole series is under
observation.
The results of such observation, covering a period of one hour
for each watch-glass, are given in Text-fig. 9. The degree of
attenuation of the pseudopodia, which attains a maximum in
this time as a rule, is expressed by the size of the crosses. The
pHs of the experimental solutions are given in the horizontal
row of figures, and in this instance the Amoebae selected were
of the type ' A ' after a short immersion in distilled water—
pH 5-8. It is important to emphasize the fact that they were
derived originally from a culture of pH 6-8 ; for Amoebae
flourish in cultures considerably more acid and more alkaline
than this, and their behaviour in the experimental solutions
is determined by the extent of the contrast between the pH
of the culture and that of the solution to which they are transNO. 280
UU
640
C. W. PARSONS
ferred. If returned to the culture medium at the end of an
hour, Amoebae that have not been brought to the point of
maximum attenuation recover rapidly and completely.
Eecovery is also possible beyond this point over the range of
pH covered by the diminishing crosses in Text-fig. 9 ; but it
then gives rise to forms which approximate to the types
' E ' and ' C', and the changes are not therefore completely
reversible.
If the period of immersion in an experimental solution is prolonged, recovery becomes progressively less possible. The
Amoebae die in the more acid and more alkaline solutions within
the first 5 hours. In the less abnormal solutions they become
sluggish and consolidated in the same way as they do when
floating in distilled water, and will make a partial recovery if
returned to culture medium while movement of granules in
their endoplasm can still be traced. The time when such movement ceases and when the changes become irreversible varies
with the acid or base employed, and, to some extent, with the
individual resistance of each Amoeba. In acid solutions there
is a tendency for this time to be marked by local distensions
of the outer layers, which give to an Amoeba a ' beaded '
appearance. In some alkaline solutions, e. g. sodium hydroxide,
the cessation of all movement is attendant on the formation
of perfectly spherical bodies ; and in both instances the actual
rupture of the cell may be delayed two or three days after this
has happened. Transference to a new medium in this condition,
however, is almost always followed by immediate cytolysis.
Ammoniacal solutions, on the other hand, induce cytolysis at
similar pHs in 4-6 hours, and in them changes involving sudden
cessation of movement may obviate the possibility of adopting
a spherical form.
The reactions of Amoebae belonging to the types ' E ' and
' C ' to these experimental solutions suggest a correlation
between their reduced surface areas and their lesser irritability.
There is some attenuation of type ' E ' Amoebae in the early
stages of immersion, but they round off more readily than do
the type ' A ' Amoebae. Type ' C ' Amoebae, on the other hand,
BEHAVIOUR OF AMOEBA
641
remain unaffected until they, in common with the types ' E '
and ' A ', round off and cytolize at the higher concentrations
of acid or base.
It may be concluded that the effect of these solutions is to
TEXT-FIG. 9.
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A representation, by the size of crosses, of the degree of attenuation
of type ' A ' Amoebae in very dilute pure solutions of various
acids and bases.
accentuate the reactions typical of Amoebae to distilled water.
They are stimulants ; at fairly well-marked critical concentrations (pH 4-6 for acetic acid, 4-8 for hydrochloric acid, &c.) they
become injurious to Amoebae derived from a culture of pH 6-8,
and their effects are cumulative. Some reagents, e. g. ammonia,
bring about early cytolysis, and with others this occurs after a
uu2
642
C. W. PARSONS
variable period of delay. It is also noticeable that the cell
penetrative fatty acids have a slightly less inhibitory effect upon
attenuation in type ' A ' Amoebae; an indication, perhaps, that
they involve in them less radical surface changes.
DISCUSSION.
1. Of A d h e s i o n . — I t is clear that while the types of
Amoebae ' A ', ' E ', and ' C ' differ mainly in respect of
mobility, their behaviour in culture media is further complicated by the power they possess of adhering to the substratum.
When floated in culture media the majority send out spreading
pseudopodia all round the central mass of protoplasm as in
Text-fig. 1. In pure media they react in essentially the same
way (Text-fig. 5), but with excessive attenuation of the pseudopodia owing to their abnormal environment. The loss of
adhesion reveals therefore a characteristic floating form which
is accentuated in the grotesque appearances presented by the
floating type ' A ' Amoebae ; and it follows that adhesive
capacity is an important property of the surface in normal
healthy Amoebae. The nature of this surface is a matter of
controversy. It is generally recognized that it cannot be the
single uniform structure that the term ' ectoplasm ' may be
held to imply, but there is no general agreement as to the
nomenclature or structure of the layers that are supposed to
comprise it. The problem has received the attention of several
workers who in recent years have studied amoeboid movement,
either with the view of supplementing the surface tension
hypothesis, e.g. Schaeffer (9, p. 89), or of replacing it with new
theories, e.g. Jennings (4), Mast (6), and Pantin (7). Schaeffer
recognizes a fluid surface tension layer capable of work (9,
p. 74) over parts of an Amoeba that are not in contact Avith the
substratum. He explains thus the movement of grains of soot,
carmine, &c, over the surface of an Amoeba in the direction
of flow. His hypothesis is supported by experiments with
type ' A ' Amoebae in water containing particles of washed
carmine. The grains of carmine adhere to and move over the
surfaces of Amoebae that are floating in distilled water although
BEHAVIOUR OF AMOEBA
643
there is a verj' definite loss in them of the capacity for adhering
to a substratum. Some justification may therefore be claimed
for the conclusion that the mechanism responsible for fixing
an Amoeba to the substratum is distinct from that by which
small particles are carried over its surface. The thin tenuous
layer called by Chambers a ' pellicle ' (1, p. 279) probably
represents the former, while the latter, a thin fluid film, would
not be demonstrable separately by microdissection. On this
hypothesis, the capacity for adhesion depends upon the condition of a layer beneath the surface tension layer. Circumstances which may render it unusually fluid will impair this
capacity therefore, and it seems reasonable to assign to such a
cause the difficulty of inducing adhesion of type ' A ' Amoebae
in pure media. In active attenuating forms the so-called
' pellicle ', and any gelating layers associated with it, must be
in an abnormally fluid condition ; and once they have lost
from this cause the opportunity of adhesion to the substratum,
further contact with it is prevented by the buoyancy they
acquire from their elongating pseudopodia. Conversely, consolidation of the outer layers of an Amoeba beyond a certain
point will also injure its power of adhesion. The types ' B ' and
' C ' have been described as comparatively immobile forms in
culture media, and processes of consolidation will therefore
influence their surface layers for a greater period of time than
is normally the case. This offers an explanation of a fact of
behaviour that has been noticed, namely, that these types
readily lose their adhesion in culture media, and seldom adhere
to the substratum in pure media although they retain contact
with it.
'2. Of A t t e n u a t e d P s e u d o p o d i a . — T h e remarkable
activity of type ' A ' Amoebae in pure media directs attention
to the elongation of their pseudopodia. Schaeffer calls them
' pseudopodia of position ' to distinguish them from the broad
pseudopodia of movement seen in normal adhering Amoebae.
The tendency to adopt a ' radiose ' form was figured by Verwornfor A m o e b a l i m a x (13, p. 185), and obviously presents
itself here with the difference that in A m o e b a p r o t e u s the
644
C. W. PARSONS
pseudopodia are always blunt. The reason for this attenuation
of pseudopodia lacks explanation however. It is not simply
a question of pH, for Amoebae can appear in flourishing cultures at pHs as low as 4 (Taylor, 12, p. 139) and at least as high
as 7-6. Further, if the pH was ordinarily an important factor
in determining the form of these organisms, the type ' A '
Amoebae would only be obtainable from cultures at the most
suitable pH, and when the types ' E ' and ' C ' occurred at all in
the same cultures, they would be localized in areas which, by
reason of the metabolism of other organisms, were beyond the
range of favourable pHs. There may actually be a small
variation in pH over the bottom of a standing culture, but it
is so small as to make no difference whatever to the variety of
forms that may be yielded from different areas of it.
The colloidal nature of protoplasm is an accepted principle ;
and in active attenuating forms the change of phase from
the fluid internal sol to the gelated condition of the surface
layers must take place very rapidly. Schaeffer points out
(9, p. 90) that this is accompanied by dispersion in a high degree
of the internal phase of the latter, and depends upon the
amount of water in the protoplasm. It seems probable, therefore, that transference of a type ' A ' Amoeba from culture
medium to distilled water is followed by imbibition and
diffusion of water through an ever-increasing surface to increase
the fluidity of the protoplasm. This, combined with the loss
of adhesion and absence of ions essential for normal contractility, e. g. calcium ions (Pantin, 8) expresses itself in great
attenuation.
3. Of t h e E e l a t i o n s h i p b e t w e e n t h e t y p e s ' A ' ,
' E ', a n d ' C '.—Their mobility leaves little doubt that the
type ' A ' Amoebae are the most healthy forms in culture
media. They are more likely to reproduce by fission and to
produce encysted young than either of the types ' E ' or ' C '
therefore, and the comparative unhealthiness of the latter may
be indicated by the approximation of the type ' A ' Amoebae
to their form, after long residence in distilled water. The
inadaptability of the types ' E ' and ' C ' to changes in the
BEHAVIOUR OF AMOEBA
645
environment, their occurrence in small numbers in flourishing
cultures, and their abundance in stale or overcrowded ones,
points to the same conclusion. Cultures that are deliberately
stocked with these forms fail altogether, and it is for these
reasons that it seems probable that the bulk of those Amoebae
which fail to enter on the cycle of encystment and which were
mentioned at the commencement of this paper, belong to the
types ' E ' and ' C '.
SUMMARY.
1. The behaviour of the adult A m o e b a p r o t e u s has
been studied in their aquarium culture media, in distilled
water, and in dilute pure solutions of various acids and bases
of known and graded pHs.
2. Amoebae belonging to three physiological types are
described from the cultures. The majority are large, active,
digitate forms, which adhere closely to the substratum, and are
grey in colour when seen in transmitted light. On transference
to pure media they become attenuated, floating forms, to
which the name type ' A ' Amoebae has here been applied.
The remainder are darker in appearance and less adherent in
the culture media. They belong to two types, which from their
behaviour in pure media have been separated as type ' R.' and
type ' 0 ' Amoebae.
3. The association between the floating form and the capacity
for adhering to the substratum is stressed.
4. The problems of adhesion, of attenuation in pseudopodia,
and of the relationship between the types of Amoebae are
discussed.
I wish to express my gratitude to Dr. Monica Taylor, S.N.D.,
without whose splendid material this investigation could not
have been pursued, and my sincere thanks for their kind
criticism and advice to Professor J. Graham Kerr, Dr. G. S.
Carter, and Mr. C. P. A. Pantin.
March 1926.
646
C. W. PARSONS
BIBLIOGRAPHY.
1. Chambers, Robert.—' General Cytology.' University of Chicago Press,
1924.
2. Clark, W. Mansfield.—' The Determination of Hydrogen Ions.' Williams and Wilkins, Baltimore, 1925.
3. Gruber, Karl.—" Uber eigenartige Korperformen von Amoeba proteus ", 'Arch. Protistenkunde', Bd. 23. Jena, 1911.
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