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HARVARD UNIVERSITY.
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CONTENTS.
CONTENTS OF No. 181, N.S., JULY, 1902.
MEMOIRS:
PAGE
On a Free-swimming Hydroid, Pelagohydra mirabilis, n. gen.
et sp. By Arruur Deypy, D.8c., F.L.S., Professor of Biology
in the Canterbury College, University of New Zealand. (With
Plates 1 and 2) . : : ‘ : : ‘ 1
Studies in the Retina. Parts III, IV, and V, with Summary. By
Henry M. Brrnarp, M.A.Cantab. (From the Biological
Laboratories of the Royal College of Science, London.) (With
Plates 3—5) : : : - : : = 25
Notes on the Relations of the Kidneys in Haliotis tuberculata,
ete. By H. J. Funurs, B.Sc., U.C.W., Aberystwyth. (With
Plate 6) . : ‘ : : . ; See
Notes on the Development of Paludina vivipara, with special
reference to the Urino-genital Organs and Theories of Gasteropod
Torsion. By Isapetta M. Drummonp. (With Plates 7—9) . 97
Is Chemotaxis a Factor in the Fertilisation of the Eges of
Animals? By A. H. Reernatp Buter, B.Sc., Ph.D., Demon-
strator in Botany at the University of Birmingham : . 145
CONTENTS OF No. 182, N.S., SEPTEMBER, 1902.
MEMOIRS:
Maturation of the Ovum in Echinus esculentus. By Tuomas
H. Bryce, M.A., M.D. (With Plates 10O—12) . 5 eee
Studies on the Arachnid Entosternite. By R. I. Pocock. (With
Plates 13 and 14) 225
1V CONTENTS.
PAGE
On the Morphology of the Cheilostomata. By Sipnzy F. Harmer,
Se.D., F.R.S. (With Plates 15—18) . . : . 263
On the Development of Sagitta; with Notes on the Anatomy of
the Adult. By L. Doncaster. (With Plates 19—21) - gol
CONTENTS OF No. 183, N.S., DECEMBER, 1902.
MEMOIRS:
On a Cestode from Cestracion. By Wittiam A. Haswett, M.A.,
D.Se., F.R.S. (With Plates 22—24) . , . 399
The Development of Lepidosiren paradoxa.—Part UI. De-
velopment of the Skin and its Derivatives. By J. Granam
Kerr. (With Plates 25—28) . : : 3 . 47
The Metamorphosis of Corystes Cassivelaunus (Pennant).
By Ropert GURNEY, poe Des F.Z.S. (With Plates
29—31) . : ; . ; . 461
Artificial Parthenogenesis and Fertilisation: a Review. By
Tuomas H. Bryce : : ; ; : . 479
CONTENTS OF No. 184, N.S., FEBRUARY, 1903.
MEMOIRS:
The Movements and Reactions of Fresh-water Planarians; a Study
in Animal Behaviour. By Raymonp Peart, Ph.D., Instructor
in Zoology in the Sa of x ee Ann Arbor, Michigan,
US Ae : 509
On the Diplochorda. _ Part IV. On the one Gane of
Cephalodiscus dodecalophus, Mcl. By A.'T. Masrermay,
M.A., D.Se., Lecturer on Zoology, School of Medicine, Edin-
burgh. (With Plates 32 and 38) : : ' « Waa
On Hypurgon Skeati, a new Genus and Species of Compound
Ascidians. By Icerna B. J. Sotzas, B.Se.Lond. (With Plates
34 and 35) : : : : Z : . 729
The Anatomy of Arenicola assimilis, Ehlers, and of a New
Variety of the Species, with some Observations on the Post-
larval Stages. By J. H. Asuworru, D.Sc. (With Plates 36
and 37) . ! : ' ‘ : : : for
TitLE, INDEX, AND CONTENTS.
New Series, No. 181 (Vol. 46, Part 1). Price 10s.
Wey
JULY, 1902.
Ae: |
oy oe THE
QUARTERLY JOURNAL
OF
MICROSCOPICAL SCIENCE.
EDITED BY
E. RAY LANKESTER, M.A., LL.D., F.R.S.,
HONORARY FELLOW OF EXETER COLLEGE, OXFORD}; CORRESPONDENT OF THE INSTITUTE OF FRANCE
AND OF THE IMPERIAL ACADEMY OF SCIENCES OF ST. PETERSBURG, AND OF THE ACADEMY
OF SCIENCES OF PHILADELPHIA; FOREIGN MEMBER OF THE ROYAL BOHEMIAN
SOCIFTY OF SCIENCES, AND OF THE ACADEMY OF THE LINCEI OF ROME;
AND OF THE AMERICAN ACADEMY OF ARTS AND SCIENCES OF BOSTON;
ASSOCIATE OF THE ROYAL ACADEMY OF BELGIUM; HONORARY MEMBER
OF THE NEW YORK ACADEMY OF SCIENCES, AND OF THE
CAMBRIDGE PHILOSOPHICAL SOCIETY, AND OF THE ROYAL
PHYSICAL SOCIETY OF EDINBURGH ; HONORARY
MBMBER OF THE BIOLOGICAL SOCIETY
OF PARIS;
DIRECTOR OF THE NATURAL HISTORY DEPARTMENTS OF THE BRITISH MUSEUM; LATE FULLERIAN
PROFESSOR OF PHYSIOLOGY IN THE ROYAJ, INSTITUTION OF GREAT BRITAIN.
WITH THE CO-OPERATION OF
ADAM SEDGWICK, M.A., F.RS.,
FELLOW AND TUTOR OF TRINITY COLLEGE, CAMBRIDGE 3
W. F. R. WELDON, M.A., F.R.S.,
LINACRE PROFESSOR OF COMPARATIVE ANATOMY AND FELLOW OF MERTON COLLEGE, OXFORD}
LATE FELLOW OF ST. JOHN’S COLLEGE, CAMBRIDGE 35
AND
SYDNEY J. HICKSON, M.A., FE.RS.,
BEYER PROFESSOR OF ZOOLOGY IN fHE OWENS COLLEGE, MANCHESTER. |
f
WITH LITHOGRAPHIC PLATES AND ENGRAVINGS ON WOOD.
“LONDON:
J. & A. CHURCHILL, 7, GREAT MARLBOROUGH STREET.
1902,
Adlard and Son,] [Bartholomew Close.
CONTENTS OF No. 181.—New Series.
MEMOIRS:
PAGE
On a Free-swimming Hydroid, Pelagohydra mirabilis, n. gen. et
sp. By ArtHuR Drypy, D.Sc., F.L.S., Professor of Biology in the
Canterbury College, University of New Zealand. (With Plates
1 and 2) ; : ‘ : : ; 5 ; : : ; 1
Studies in the Retina. Parts III, 1V, and V, with Summary. By
Henry M. Bernarp, M.A.Cantab. (From the Biological Labora-
tories of the Royal College of Science, Iondon). (With Plates
3—5) ae : 25
Notes on the Relations of the Kihiore in Haliotis tahneentatee
etc. By H. J. Frzurs, B.Sc., U.C.W., eee (With
Plate 6) : : : 77
Notes on the Development of Palauan vivipara, with fe
reference to the Urino-genital Organs and Theories of Gasteropod
Torsion. By Isapetta M. Drummonp. (With Plates 7—9) OF
Is Chemotaxis a Factor in the Fertilisation of the Eggs of Animals ?
By A. H. Reeinatp Buiter, B.Sc., Ph.D., Demonstrator in
Botany at the University of Birmingham : : : : . 145
On a Free-swimming Hydroid, Pelagohydra
mirabilis, n. gen. et sp.
By
Arthur Dendy, D.Sc., F.L.S.,
Professor of Biology in the Canterbury College, University of New Zealand.
With Plates 1 and 2.
ConrTENTS.
PAGE
1. Introduction : d ‘ : ; ; 1
2. Notes on the Living ucwal ; ; ; : : 3
3. The Hydroid:
(a) External Characters : : : 2 ; 4
(b) Internal Anatomy. ‘ : : F ; 5
(c) Histology . ; : . : : : 7
4. The Medusoid ;
(a) Structure. s : : . 5 kG
(b) Development A . : t sae ls
5, Discussion of Results, Heletioneling, Giga 5 ‘ eee!)
6. Diagnosis of New Genus and Family : : ; % yell
7. Description of Plates ; 5 ‘ : : » 92
1. INTRODUCTION.
The remarkable organism which forms the subject of the
present memoir was picked up by myself on the sandy beach
at Sumner, a small watering-place near Christchurch, on the
east coast of the South Island of New Zealand. One evening
in October last (1901), while walking on the shore, I saw
lying at my feet a small gelatinous object which had
evidently just been thrown up by the tide. On placing it in
you. 46, PART 1.—NEW SERIES, A
2 ARTHUR DENDY.
a glass of sea water I soon saw that it was still alive, and
that it exhibited very unusual features, differing widely from
any pelagic organism with which I was acquainted. After
studying it for some time with the aid of a pocket lens I
took it up to my laboratory at Christchurch, and continued
my examination of the living animal the same night. Being
unwilling to risk the attempt to keep it alive until the next
morning, I then killed it by the addition of osmic acid to the
sea water, and preserved it in alcohol. It was unfortunate
that the lateness of the hour prevented me from making a
more exhaustive examination of the living organism, as
more light might have been thereby thrown upon its move-
ments and habits; but it seemed best to try and make sure of
having it well preserved for minute investigation subse-
quently, and in this I was fairly successful. The action of
the osmic acid was, as might have been expected in the case
of so large an organism, very unequal, some of the more
superficial parts being much blackened, while the interior
was apparently not affected at all, and consequently turned
out to be not in so good a condition for minute histological
investigation as I could have wished. Had I suspected how
complicated and remarkable the structure of the interior
really was I might have thought it best to cut the organism
in half in order to allow the osmic acid to penetrate, but as
it was it did not seem to me desirable in any way to mutilate
the unique specimen at that early stage of the investigation.
It was very soon obvious that the organism was an
enormous free-swimming hydroid, from the greater part of
the surface of which numerous little medusoids were being
budded off in groups. Being about to pay a visit to
England, however, I postponed the greater part of the
investigation until after my arrival, when I resumed the
work in the zoological laboratory of the Owens College. It
affords me very great pleasure to express my thanks to
Professor Hickson and his staff for the kind hospitality
which I received at their hands, and for the valuable assist-
ance rendered to me during the progress of my research,
ON A FREE-SWIMMING HYDROID. 3
2. Notes on THE Livinc ANIMAL.
The free-swimming hydroid person of Pelagohydra
mirabilis (fig. 1) is apparently a pelagic organism. The
conditions under which it was found, its subsequent be-
haviour when observed in sea water, and its peculiar organi-
sation, all point to this conclusion. When placed ina glass
of sea water in front of a candle (it was too dark to examine
it by daylight) it floated near the surface with the narrow
proboscis-like portion of the body, bearing the mouth at its
extremity, hanging downwards from the much larger balloon-
like structure, which I propose to call the “float.’? The
latter, though near the surface, was totally submerged.
Subsequently, when placed in a tin can for removal to the
laboratory and kept in the dark, the animal sank to the
bottom, though still alive. Probably, therefore, it has the
power of rising and sinking in the water like other pelagic
organisms, and it may be that it always sinks to some depth
beneath the surface when it is dark. The general colour of
the organism was a very pale bluish tint, and it was of
course translucent. The proboscis, however, was pale pink,
intensified round the margin of the mouth. The manubria
of the medusoids were also pink. During life the hydroid
exhibited some slight power of changing its shape, the float
being at one time oval (slightly elongated vertically) and
at another contracted into a sphere, while the proboscis
exhibited considerable power both of contraction, under
which condition it became slightly trumpet-shaped at the
end, and of flexion. When both elongated, as shown in
fig. 1, the float was nearly an inch in greater diameter and
the proboscis rather more than half as long as the float.
The long, slender, tentacular processes of the float occa-
sionally exhibited spasmodic movements of flexion, like
gigantic flagella, many of them simultaneously, or nearly so ;
and from this I am led to conclude that the animal has the
power of rowing itself through the water by means of these
organs.
4, ARTHUR DENDY.
Whether the medusoids naturally separate from the
hydroid I cannot say from direct observation. They ex-
hibited slight twitching movements of contraction, however,
while still attached to the parent, and the structure of the
larger ones leaves no doubt that they ultimately become free-
swimming. Moreover many of them became detached when
the organismn was killed.
3. Tue Hyproirp.
(a2) External Characters.—The body of the hydroid is,
as compared with the ordinary hydroid type—such as we see,
for example, in Tubularia,—greatly modified in form and
structure, and the modification is such as to bring about the
necessary adaptation to the changed conditions of life. The
usual stalk is entirely wanting, nor is there the slightest
indication of its having ever existed. The aboral portion of
the body is enormously swollen out, and quite evenly rounded
off at the upper pole, forming the nearly spherical ‘ float.”
To the lower pole of the float is attached the cylindrical “ pro-
boscis,’ bearing the mouth at its extremity. ‘The line of
junction between the float and the proboscis is well marked
even externally, and corresponds to an even more pronounced
internal demarcation between the two.
The float carries numerous long tentacles, which are
scattered without any definite arrangement at approximately
equal distances from one another all over its surface. ‘These
tentacles are cylindrical and bluntly rounded at the extremity,
never distinctly knobbed. When fully extended they may be
about as long as the float itself. For the most part they are,
as usual amongst the Hydrozoa, unbranched, but two or three
were observed each with a single branch (figs. 3, 5, B.7.),
this condition being probably abnormal.
The proboscis is differentiated transversely into two por-
tions (fig. 2). ‘The upper part bears no tentacles, and
exhibits an appearance of circular and longitudinal striation,
ON A FREE-SWIMMING HYDROID. 5
The lower part, next to the mouth, bears numerous tentacles
of greatly varying size; these are arranged, not quite
regularly, in transverse rows or whorls, and decrease in size
from the uppermost whorl, which contains the largest,
towards the mouth, around the margin of which the tentacles
are very minute. There is altogether a good deal of irregu-
larity about the size and shape of these tentacles, and here
again one of them was found to be branched (fig. 2, B.T.),
but a better idea of their form and arrangement will be
gained from the illustration than from any description which
I can give.
Scattered all over the surface of the float, between the
bases of the tentacles (figs. 4, etc.), are numerous little
branching processes, which we may term “stolons.” They
branch quite irregularly, their branches remaining short and
keeping close to the surface of the hydroid. On these stolons
are borne groups of very small medusoids in various stages
of development, from minute buds to fully formed bells
apparently just ready to separate.
(b) Internal Anatomy.—The most striking feature of
the internal anatomy is the presence of two large cavities,
completely separated from one another by a thin horizontal
septum, as shown in fig. 5. ‘This septum lies at the level of
the junction between the proboscis and the float, and is
slightly arched upwards. A preliminary examination reveals
the fact that the lower and very much smaller chamber is
the main gastral cavity, while the upper one is apparently
excavated in the enormously developed mesogloea between
the ectoderm and endoderm of the roof of the gastral cavity :
this second and much larger chamber I propose to call the
“cavity of the float.” Its real origin will be discussed
presently. It is not a simple cavity, but is subdivided by
what I propose to term the “supporting membranes.”
On the inner surface of the wall of the float there is a net-
work of canals, which give it a honeycombed appearance.
These canals are lined by endoderm, and are in reality con-
tinuations of the gastral cavity, into which they open at their
6 ARTHUR DENDY.
lower extremities. I shall speak of them as the “endo-
dermal canals.”
The Gastral Cavity.—The main gastral cavity, then,
occupies only the interior of the proboscis, but is continued
upwards into the float in the form of endodermal canals.
The lining membrane of the main gastral cavity is thrown into
numerous very prominent longitudinal folds, forming ridges
which project inwards (figs. 5—9, L.G.R.), and whose edges,
in the contracted specimen, are very sinuous (fig. 6). Ata
short distance below the septum the gastral cavity widens
out somewhat, and the ridges almost die away. At the
junction of the septum with the outer wall of the gastral
cavity a prominent annular fold projects into the latter
(figs. 7, 8, A.F.).
The Endodermal Canals.—Above the fold just men-
tioned, around the margin of the septum, which is otherwise
imperforate, lie the openings of the endodermal canals (figs.
7, 8, Op. End.). From the network which these canals form
on the inner surface of the wall of the float (figs. 5, 6,
End. C.) short branches are given off outwards, which run
into the stolons; but the canals themselves have apparently
no communication with the tentacles (fig. 8).
The Septum.—The septum which separates the main
gastral cavity from the cavity of the float is a thin but firm
membrane. As already stated, it is somewhat arched
upwards. Its two surfaces are both smooth, but to the
upper one are attached some of the supporting membranes
in the chamber of the float (figs. 5—8). It is, as already
stated, imperforate, except for the openings of the endo-
dermal canals, and in this respect differs from either of the
> in the gigantic Branchiocerianthus
imperator, which in some respects certainly resembles our
hydroid.’
The Cavity of the Float.—The cavity of the float is
very spacious, but it is subdivided by numerous very thin,
r ce = ™ 2
two ‘diaphragms
1 Vide Miyajima, ‘Journ. Coll. Sci. Imp. University of Tokio,’ vol. xiii,
p. 258, ete.
ON A FREE-SWIMMING HYDROID. ‘
transparent, membranous sheets, which radiate outwards
from a more solid mass of tissue formed by their union nearly
in the middle of the chamber, and which have their edges
attached to the inner surface of the wall of the float and to
the upper surface of the septum. ‘These remarkable struc-
tures I have called the ‘supporting membranes.” The
inner surface of the wall of the chamber exhibits a honey-
combed appearance, being marked out into roundly poly-
gonal areas by the projecting endodermal canals. In the
centre of each depressed area between the endodermal
canals a knob-like projection may frequently be seen; this
is caused by the tissue which fills the cavity of the tentacle
projecting inwards into the chamber of the float like a plug
(figs. 6, 8, Ten. Pl.). These structures we may call the
“tentacle plugs.”
The Tentacles.—All the tentacles are filled with a highly
vacuolated tissue, composed of sheets or strands of delicate
membrane. In the case of the tentacles of the float this
tissue may, as Just stated, project as a plug into the float
cavity. In the proboscis the mesoglcea in the wall of the
gastral cavity is, in the neighbourhood of the tentacle bases,
much thickened and highly vacuolated, giving rise to cavities
of considerable size, and this vacuolated tissue is continued
into the tentacles (figs. 8,9). The exact nature and origin
of the tissue which thus fills the interior of all the tentacles
are, however, by no means easy to determine, and the
question will be best dealt with under the next heading.
(c) Histology.—Pelagohydra exhibits, for a hydroid,
a remarkable amount of histological differentiation. For
purposes of description it will be most convenient to sub-
divide this part of our subject according to the different
regions of the body, rather than to attempt to follow out each
layer completely before passing on to the next. Indeed, as
we shall see later, in some parts of the body the delimitation
of the layers is by no means always obvious—at any rate, in
the case of the endoderm.
As already indicated, the histological preservation of the
8 ARTHUR DENDY.
internal tissues is not all that could be desired, and it is
greatly to be hoped that an opportunity may arise of working
out this subject more in detail with the aid of material
specially treated for the purpose. It is also highly desirable
that a detailed comparison should be made of the histological
structure of Corymorpha, Monocaulus, and Branchio-
cerianthus, which are evidently related to Pelagohydra,
and, like it, of exceptional size.
Wall of the Proboscis.—The ectoderm is a thick layer
densely charged with small, darkly staining nuclei and
thread-cells irregularly scattered throughout its substance
(figs. 9, 10, Het.).
In section it exhibits numerous fine radial lines running
in at right angles from its outer surface, and perhaps
indicating the boundaries of a single layer of large prismatic
cells. On its inner aspect, immediately contiguous to the
mesogloea, is a well-developed layer of longitudinal muscle-
fibres. In transverse sections (fig. 10) we see that this layer,
consisting of an approximately single row of fibres, is thrown
into longitudinal folds, the mesogloea being produced out-
wards in plate-like ridges between the folds. This arrange-
ment, so well known in the mesenteries of the Actinians, no
doubt serves to increase the extent of the muscular tissue.
The ectoderm decreases in thickness from below up-
wards, and the folding of the muscular layer is especially
conspicuous just above the region of the tentacles, and dies
away as it approaches the upper limit of the proboscis.
Between the bases of the proboscis tentacles the ectoderm is
extremely thick, but thins out greatly over the tentacles
themselves.
The endodermal lining of the proboscis wall is enormously
thick, and throughout the greater part of its extent is thrown
into prominent longitudinal folds or ridges in the manner
already described (figs 5—9). The structure of these ridges
(figs. 9—11) is very peculiar. The mesogloeal supporting
lamella which divides the endoderm from the ectoderm is
uot continued into them, and is indeed sharply marked off by
ON A FREE-SWIMMING HYDROID. 9
another layer of muscle-fibres, which we may consider to be
endodermal in origin. These fibres are arranged in a
circular manner at right angles to those of the ectoderm (fig.
10), and the extent of the muscular layer is increased by
horizontal folds, similar to the vertical folds of the ecto-
dermal layer. These horizontal folds are, of course, recog-
nisable only in vertical sections, while the vertical folds of
the ectodermal musculature are conspicuous in transverse
section (fig. 10).
The free surfaces of the gastral ridges bounding the
gastral cavity are covered with an epithelium of a very
peculiar type (figs. 9—11). It consists of long, slender,
columnar cells arranged at right angles to the surface.
They have a finely granular cytoplasm and distinct nuclei,
and appear in the sections to be collected into small groups,
like bundles of cigars, from the inner ends of which delicate
wavy fibres run obliquely towards the central plane of the
ridge, and thence inwards side by side till they meet the
mesogloeal supporting lamella, where they probably give rise
to the circular musculature. The grouping of the epithelial
cells into bundles is, I think, probably a post-mortem con-
dition due to contraction in alcohol. I imagine that the cells
are normally arranged so that each is continued inwards
into a separate fibre. We may probably regard the endo-
derm of the gastral ridges as glandular-muscular in function,
for no doubt it secretes the digestive fluid. There are no
thread-cells in the gastral ridges, nor, indeed, have I seen
them in any part of the endoderm. On approaching the
annular endodermal fold which marks the upper limit of the
proboscis the gastral ridges gradually die away, and their
epithelium gives place to that which lines the gastral face of
the septum on the one hand, and the endodermal canals on
the other (fig. 8).
The mesogloeal supporting lamella of the proboscis wall
may be regarded as being bounded on the outside by the
1 Compare the structure of the endodermal villi with their muscle-fibres in
Myriothela (Hardy, ‘ Quart. Journ. Mier. Sci.,’ vol. xxxil, p. 505).
10 ARTHUR DENDY.
ectodermal, and on the inside by the endodermal layer of
muscle-fibres respectively. It is continued into the folds of
the muscular layers, and also into the annular fold of endo-
derm. It has the usual clear gelatinous appearance,! and
though everywhere more or less distinct, attains its maximum
development in the neighbourhood of the tentacle bases,
where it appears to become immensely thickened, and at the
same time broken up by large vacuoles into a network of
irregular sheets (figs. 8,9). It may possibly be invaded in
this region by cells migrating from the endoderm, as will be
described later in the case of the supporting membranes of
the float; but this point I have not been able to determine.
Tentacles of the Proboscis.—The larger tentacles of
the proboscis are identical in structure with those of the
float, shown in transverse section in fig. 15. The outer wall
of the tentacle is formed by a single layer of short columnar
cells; it is highly vacuolated, and abundantly charged with
thread-cells in all stages of development ; on its inner face is
a well-developed single layer of longitudinal muscle-fibres.
A more or less distinct layer of mesogloea comes next, crossed
in places by slender strands (of protoplasm ?) extending
inwards from the ectoderm, while the axis of the tentacle is
occupied by an irregular network of sheets continuous with
the vacuolated mesogloea of the proboscis wall. Here and
there over the surfaces of these thin and apparently structure-
less sheets are scattered very well-defined bodies, which may
be either small isolated cells with small nuclei, or, as 1 am
inclined to think, themselves large nuclei with conspicuous
nucleoli, These bodies are flattened against, or perhaps in
the thickness of, the septa which separate the enormous
vacuoles from one another. When seen en face they are
nearly round, and about 0°0125 mm. in diameter. Their
protoplasm stains fairly deeply, especially that of the small
enclosed body, and is scarcely at all granular. It is note-
1 Tt seems probable that the fibrillated character of the mesogleea described
by Allman and Miyajima (loe. cit.) in Branchiocerianthus may be due to
the ectodermal and endodermal muscle-fibres attached to it.
ON A FREE-SWIMMING HYDROID. 1]
worthy that two of these large nuclei may be found lying
close together, side by side, on the same side of one septum,
which seems to indicate that each cavity in the axial tissue is
not simply the enlarged vacuole of a single cell. Though
abundant in the tentacles themselves, the large nuclei are, so
far as my experience goes, not to be found in the vacuolated
mesoglcea with which the axial tissue of the tentacle becomes
continuous in the proboscis wall.
Owing partly to the specimen being somewhat injured in
the neighbourhood of the mouth (possibly by being washed
about by the tide on the sand, with mouth downwards), I
have been unable to make a satisfactory investigation of the
minute structure of the smallest tentacles. It is evident,
however, that these conform much more closely to the ordi-
nary Tubularian type than do the large ones. This may be
chiefly owing to their smaller diameter, which enables the
membranous septa to stretch right across transversely and
more or less parallel with one another, so as to divide the
interior into approximately a single row of chambers, sur-
rounded by a very thick layer of mesogloea inside the
ectoderm. ‘Thus it would seem that the axis of the smallest
tentacles is occupied by a single row of large vacuolated
endoderm cells as usual. Whether even in the smallest
tentacles these axial cells retain their connection with the
endodermal lining of the gastral cavity is extremely doubtful.
In the case of the large tentacles there is no trace of any
connection remaining between the axial tissue and the endo-
derm of the gastral cavity,’ and the origin of this tissue
must remain doubtful. It has probably been originally
derived from the endoderm, but it has become so modified in
structure and so completely disconnected that perhaps only
embryological research can decide the question.
Wall of the Float.—The wall of the float forms but a
comparatively thin shell, enclosing the central cavity with its
remarkable system of supporting membranes. ‘The histo-
logical characters of the ectoderm (fig. 12, Hct.) are very
‘ Compare Miyajima’s remarks on Branchiocerianthus, loe. cit.
12 ARTHUR DENDY.
similar to those of the corresponding layer in the wall of the
proboscis. It is, however, less distinctly muscular. In the
immediate neighbourhood of the tentacles it retains the
characters which it exhibits in the tentacles themselves, being
comparatively thin, and having the muscle-fibres arranged
radially in continuation with the longitudinal muscular layer
of the tentacle. Elsewhere the ectoderm is thick and very
densely crowded with thread-cells.
The Endodermal Canals.—The lining epithelium of
the endodermal canals, directly continuous with that of the
gastral cavity proper, is differentiated into two very distinct
portions, differing greatly in histological character. The
canals are somewhat flattened against the wall of the float ;
their own outer walls form part of the thickness of the latter
(fig. 12), and are lined by a layer of large epithelial cells with
rounded club-shaped ends projecting into the lumen. These
cells have very large vacuoles and small round nuclei, and
their very darkly staining granular contents are collected
together in or near their swollen club-shaped ends (fig. 12,
End O.). They also contain darkly staining spherical globules
of various sizes. ‘The epithelium forming the inner walls of
the endodermal canals, on the other hand, consists of a single
layer of smaller cells, approximately cubical in shape, with
small nuclei and only a small quantity of faintly staining,
finely granular cytoplasm (fig. 12, End. I.).
The Supporting Membranes of the Float.—The thin
transparent sheets of membrane which subdivide the cavity of
the float (figs. 5—8, 12, Sup. Mem.) appear to have a
very remarkable structure and origin. Hach sheet consists
of a thin structureless layer of mesogloea (fig. 13, Mes.),
thickening at the angles where the sheets meet one another.
Spread out on each surface of this mesoglceal sheet is a still
thinner layer of finely granulated, frothy-looking protoplasm,
containing rounded nuclei irregularly scattered through it
(fig. 14). No cell boundaries can be distinguished in my
preparations, but the protoplasm appears to form a vacuolated
syncytium. It may occasionally be collected or drawn
ON A FREE-SWIMMING HYDROID. 13
together into a thick rounded blob or drop, containing many
nuclei (fig. 13), but this condition appears to be of rare
occurrence. Probably the nuclei multiply by division, as
indicated in fig. 14, at a This peculiar tissue appears to
originate, in part at any rate, from the inner walls of the
endodermal canals.! The mesoglceal portion of these walls
may be very thick, and occasionally little groups of cells (fig.
12, End. Bud) may be seen growing into it from the endo-
dermal lining of the canal. These cells have very finely
granular contents and small nuclei. Irregular cavities (fig.
12, D. F. C.) are apparently developed between them,
and gradually enlarge until the nuclei become widely
separated, while the mesoglea is reduced to thin sheets
separating adjacent cavities from one another, and the proto-
plasm of the endoderm cells becomes spread out over these
sheets in the form of a granular syncytium.
Sometimes, where a comparatively thin layer of mesogloea
lies behind the endoderm of the inner wall of an endodermal
canal, threads of finely granular protoplasm may be seen
stretching at right angles through the mesogloea from the
one surface (covered by the finely granular syncytium) to the
other (covered by the endodermal cells of the canal wall).
Thus it appears that the supporting membranes of the float
originate in a peculiar manner from the endoderm. It is not
certain, however, that they do not receive cells from the
external ectoderm also, for thread-cells in various stages of
development may sometimes be observed in places where the
mesoglcea is thick, beneath the external ectoderm and doubt-
less derived from the latter. This inward migration of the
cnidoblasts can hardly be looked upon as normal, but if they
are able to migrate inwards it seems equally possibly that
other ectoderm cells may do the same, and possible eventually
take part in the formation of the supporting membranes.
1 Professor Ray Lankester has pointed out to me that a somewhat similar
method of tissue formation has been observed in the ‘‘ laminar tissue’ of
Amphioxus (vide Pouchet, “On the Laminar Tissue of Amphioxus,”
‘Quart. Journ, Mier. Sci.,’ vol. xx, n. s., p. 421, pl. xxix),
14. ARTHUR DENDY.
The Septum.—The histological structure of the septum
which divides the main gastral cavity from the cavity of
the float is practically identical with that of the inner walls
of the endodermal canals, with which it is directly continuous.
On its lower face it is covered by a layer of lightly staining
cells with small nuclei and finely granular contents, and this
is separated by a moderately thick layer of mesogloea from
the finely granular syncytium which covers its upper surface.
Some of the supporting membranes of the float are attached
to its upper surface, and probably originate from the septum
in the same way as those already described originate from
the inner walls of the endodermal canals.
Tentacles of the Float.—The tentacles of the float are
histologically identical with the large tentacles of the pro-
boscis, as will be seen by comparison of fig. 15 with the
description already given. ‘The peculiar manner in which the
axial tissue seems to project into the cavity of the float in the
form of a cushion or plug has already been referred to. In
the projecting plug, however, when best developed, the
network of tissue is made up chiefly of a finely granular
frothy syncytium, with very little mesogloea and small nuclei.
In the tentacle itself the granular material is hardly recog-
nisable, the septa (fig. 15, S.W.7.) are very thin, and the
nuclei (fig. 15, Nw.) much larger and of a different character,
like those in the proboscis tentacles. Thus the “plug”
seems to be to some extent transitional in character between
the true axial tissue of the tentacle and the very much coarser
reticulation formed by the supporting membranes in the
interior of the float. It is not always recognisable as a
distinct structure, however, and even where best developed
it passes gradually into the axial tentacular tissue beyond,
while its apparent histological differences may be in part due
to the want of penetration of the osmic acid with which the
specimen was hardened.
The endodermal canals come very close to the bases of the
tentacles, and we may be pretty certain that the axes of the
latter are endodermal in origin, though, as in the case of the
ON A FREE-SWIMMING HYDROID. 15
proboscis tentacles, embryological research may be required
before we can say exactly how they arise.
The Stolons.—The stolons are simply branching hollow
outgrowths of the wall of the float in the neighbourhood of the
endodermal canals, which are prolonged into them to their
extremities (figs. 8, 12, St.). The ectoderm (fig. 12, Het.) is
composed of the usual large clear cells, rectangular in
longitudinal section, with small nuclei pressed against their
dividing walls. At its base lies a feebly developed layer of
longitudinal muscle-fibres. Thread-cells are almost entirely
wanting. The mesoglcea is thick, and traversed by slender
threads crossing from ectoderm to endoderm. ‘The endo-
derm (fig. 12, Hnd.) is simply a continuation of the endoderm
which lines the outer walls of the endodermal canals, and,
like the latter, is composed of large cells, often with rounded
extremities projecting into the central lumen, with enormous
vacuoles and darkly staining contents massed together either
in the rounded end or elsewhere. ‘They have small nuclei,
and in addition contain darkly staining spherical globules of
various sizes.
The Thread-cells.—The thread-cells (figs. 16, 17) are of
large size. The actual nematocysts or capsules are approxi-
mately ovoid in shape, but truncated at the somewhat
narrower outer ends, and measure, when fully developed,
about 0°0128 mm. in longer diameter. Each one is more or less
enclosed in a delicate cnidoblast (fig. 17, enb.). When fully
developed the thread-cells lie in the outer parts of the large
ectoderm cells just beneath the surface, and the cnidoblast is
prolonged inwards to the base of the cell in the form of a long
thread—the cnidopod! (figs. 16, 17, Cup.). The enidopod is
remarkably distinct and tough, so much so that when the
ectoderm of a tentacle has been abraded, so that the large
ectoderm cells have disappeared, the cnidopods may still
remain projecting from the surface like hairs, with or with-
out the thread-cells still attached to their extremities.
1 Compare Allman, ‘Challenger Reports,’ “ Hydroida,” Part 2, p. xv, for
the use of this term,
16 ARTHUR DENDY.
I have seen no thread-cells with the threads everted, and
have not been able to make out any details with regard to
the thread itself. No barbs were visible in my preparations.
Smaller thread-cells, in various stages of development, lie in
the deeper parts of the ectoderm.
4, THe MEDUSOID.
(a) Structure.—Although no free-swimming meduse
have as yet been observed, there can be little doubt that
they normally separate from the parent hydroid. As already
pointed out, they exhibit movements of contraction while
still attached, and separate very readily in the process of
killing and preserving. Moreover none of the meduse,
which were found attached to the hydroid in large numbers,
were sexually mature, and the largest were only about 1 mm.
in longer diameter of the bell.
In the largest examples the bell is considerably deeper
than wide, and nearly square, though with rounded angles,
in cross-section (figs. 22—24). The mouth of the bell is
still very narrow (fig. 23), probably expanding considerably
later on. It is surrounded by the velum, around which the
margin of the bell has grown out into four arms or lobes,
arranged in the form of a cross, per-radially, corresponding
to the angles of the bell. Each of these arms bears five
tentacles arranged in a very peculiar manner—a pair of
larger ones, a pair of smaller ones, and a very small odd one ;
the largest being furthest from the mouth, the odd nearest to
the mouth, and the remaining pair intermediate in position,
as shown in fig. 23. All the tentacles are short, even in
the living animal, and they are only very slightly if at all
swollen at their extremities. It is possible that the number
of tentacles increases as the medusa grows older, but their
peculiar and definite arrangement seems to indicate that the
full complement is already present. The tentacles are filled
with solid endoderm formed in the usual manner, while the
arms or lobes upon which they are borne are characterised
ON A FREE-SWIMMING HYDROID. 17
by an enormous thickening of the ectoderm, containing
numerous thread-cells.
At the aboral apex of the bell is a depression, where the
exumbrellar ectoderm dips in to meet an outward extension
of the endodermal lining of the gastral cavity. This marks
the spot where the young medusa is attached to the stolon
(fig. 22, Z.).
The manubrium (figs. 22, 25, Man.) is large, but does not
project beyond the mouth of the bell. Its surface is smooth,
and there are no outgrowths at its extremity.
The subumbrellar cavity is, in the middle, somewhat
octagonal in transverse section (fig. 25), being produced into
four shallow per-radial angles where the ectoderm is attached
to the radial canals, and four deeper interradial angles where
it is attached to the endodermal lamella. Immediately
beneath the subumbrellar ectoderm cells is a layer of trans-
verse (“circular”) muscular fibres, and the entire epithelium
with its musculature is thrown into transverse folds, as shown
in figs. 22—24. Towards the mouth of the bell the cross-
section of the subumbrellar cavity becomes square, the inter-
radial angles alone remaining.
The gastral cavity immediately above the manubrium is
cruciate in transverse section, the four arms of the cross
being produced outwards into the radial canals, and the
endoderm being greatly thickened between them to form four
ridges. Inthe manubrium itself the gastral cavity is squarish
or irregular in section, with a variable number of longitudinal
endodermal ridges.
The four radial canals present no features of special interest,
nor does the thin endodermal lamella by which they remain
connected. Near the margin of the bell they open into the
circular canal (fig. 22, c. can.), enlarged per-radially in the
tentacle-bearing arms and then produced to form the solid
axes of the tentacles.
No gonads are yet recognisable, but the ectoderm of the
manubrium exhibits a thickening all round about the middle
voL. 46, PART 1.—NEW SERIES. B
18 ARTHUR DENDY.
of its length, which probably indicates the position in which
they will subsequently appear.
There appear to be no sense-organs, and I have not satisfied
myself as to the existence of a nerve-ring. In life there isa
pink spot on the outside of the base of each tentacle group,
and the manubrium also is more or less pink in colour.
(6) Development.—The medusz are developed as hollow
outgrowths or “buds” from the branching stolons already
described, and each stolon may bear as many as half a dozen
at the same time in various stages of development. As soon
as one medusoid approaches maturity another bud (fig. 20 a)
appears on the stolon close to its point of attachment, ready
to replace the first when it falls off.
The youngest buds observed are represented in figs. 20 4
and 188; each is a single hollow outgrowth of the stolon,
composed of ectoderm and endoderm, but the thick mesoglea
of the stolon disappears almost if not quite completely in the
bud (fig. 20). The ectoderm and endoderm also change
their character, becoming much more compact and solid-
looking, and staining much more darkly.
In the next stage (fig. 18 c) the endocodon is formed from
the ectoderm at the apex of the bud. There is, in the
section represented in the figure, some appearance of invagi-
nation, but if not at first solid the endocodon speedily
becomes so.
The endocodon grows inwards, and at the same time the
endoderm invaginates as if pushed before it (figs. 18, 19),
forming a deep cup. The bottom of this cup is then pushed
outwards again through the endocodon to form the hollow,
finger-like manubrium, which makes its appearance very
early (fig. 20).
Meanwhile the cells of the endocodon arrange themselves
in a single layer over the outer surface of the manubrium,
the inner surface of the future subumbrella, and the inner
surface of the future velum (fig. 20). These layers are at
first in close contact, but ultimately the subumbrellar cavity
inakes its appearance between them.
ON A FREE-SWIMMING HYDROID. 19
While these changes have been going on the original
gastral cavity of the bud becomes further subdivided by
the union of its inner and outer walls interradially (fig. 21)
to form the solid endodermal lamella, thus defining the four
radial canals and the circular canal. The ectoderm becomes
greatly thickened outside the circular canal, and the tentacles
begin to grow out.
Hitherto ectoderm and endoderm have everywhere re-
mained in close contact (figs. 20, 21), but the transparent
gelatous mesoglcea now appears and forces the layers apart
(fig. 25). About the same time the subumbrellar cavity is
developed and the velum is ruptured in the middle (fig.
20, w.), giving rise to the mouth of the bell (fig. 23).
5. Discussion or Resuits, RELATIONSHIPS, ETC.
Pelagohydra mirabilis is a remarkably interesting
organism from several points of view. In the first place it
forms an excellent example of adaptation to changed con-
ditions of life, showing us how a representative of a group
whose members are normally attached, in the hydroid phase,
to the ends of fixed stalks may become adapted to a free-
swimming pelagic existence. In the second place it exhibits
remarkable structural features, especially in the compli-
cation of the gastral cavity with its endodermal canals,
and the development of the float with its extraordinary
supporting membranes. It also has very striking histo-
logical peculiarities, showing in this respect a degree of
differentiation perhaps unequalled in any other hydroid.!
As a pelagic member of a typically non-pelagic group of
animals we may compare it with Pelagonemertes amongst
the Nemertines, 'Tomopteris amongst the Annelids, and
Pelagothuria amongst the Holothurians, and it may
1 The gigantic Branchiocerianthus imperator probably resembles
Pelagohydra closely in histological features, but requires further investiga-
tion (vide Miyajima, ‘ Journ. Coll. Sci. Imp. University of Tokio,’ vol. xiii,
p. 235, ete.).
20 ARTHUR DENDY.
possibly throw some light upon the origin of that remarkable
pelagic group of Hydrozoa the Siphonophora, although it
will perhaps hardly bear close comparison with any known
member of that order.
That it is an aberrant Tubularian hydroid there can, I think,
be no doubt, and its nearest relations appear to be the
enigmatical Corymorpha and its allies! In the genus
Corymorpha we also find that there is no true stalk, and
the curious prolongation of the body by which the animal
fixes itself in the sand or mud is, I believe, homologous with
what I have termed the float in Pelagohydra. In Cory-
morpha also we have a system of endodermal canals forming
a network around a spongy central mass, and communicating
at one end with the main gastral cavity. Then, again, in Cory-
morpha curious processes are given off from the surface of
the body in the neighbourhood of the endodermal canals,
which may be homologous with the stolons of Pelago-
hydra, or possibly with the tentacles of the float. Little is
known, however, of the minute anatomy and histology of
Corymorpha, and a careful investigation in comparison
with Pelagohydra is greatly to be desired. There are,
of course, sufficiently striking differences between the two
forms, but these are of a more superficial character, and
mainly to be accounted for by the difference in mode of life.
Instead of a float we find in Corymorpha a kind of rooting
process, and the tentacles are confined to one end of the
elongated body, where they are arranged in a proximal and a
distal set, the latter obviously representing the tentacles of
the proboscis in Pelagohydra. The position of the stolons,
between the two sets of tentacles, is totally different; and the
medusz also are quite distinct, for in Steenstrupia, the
medusa of Corymorpha, we find a single odd tentacle,
representing one only of the four tentacle groups of the
Corymorpha medusa. In both cases, however, the meduse
are markedly quadriradiate, and essentially similar in in-
ternal organisation; while in Amalthza, which appears to
1 Allman, ‘ Tubularian Hydroids,’ p. 386, ete.
ON A FREE-SWIMMING HYDROID. 21
be closely related to Corymorpha, all four tentacles are
developed.
It is a very curious fact that two distinct genera of
Tubularian hydroids agreeing in such striking anatomical
peculiarities should have become adapted to two such differ-
ent modes of life, the one swimming freely in the open ocean,
and the other rooting itself in the sand at the bottom. It
would indeed be difficult to find a better example of the
powers of adaptation to divers conditions of life. So far as
I am aware there is no other hydroid yet known which has
become specially adapted to a pelagic mode of life. It is
true that floating hydranths—Acaulis and Nemopsis—
are known, but these have probably become detached from
stalks, and are not structurally adapted to a free-swimming
existence.
6. Diagnosis or New Genus AND FAMILY.
Genus Pelagohydra, n. gen.—Hydroid solitary, free-
swimming; the proximal portion of the body modified to
form a float, supported internally by a system of radiating
membranes of endodermal origin ; the distal portion forming a
flexible proboscis, with the mouth at its extremity. Gastral
cavity continued from the proboscis into the float in the form
of endodermal canals, from which arise branching stolons.
Tentacles filiform, scattered over the surface of the float and
in whorls around the mouth. Meduse developed on stolons
between the tentacles of the float; quadriradiate, symmetri-
eal, probably with gonads in the wall of the simple manu-
brium ; tentacles in four per-radial groups of five (possibly
more in the adult).
The genus may be regarded as belonging to a distinct
family, for which I propose the name Pelagohydridea, and
for which the generic diagnosis may at present suffice. This
family is, however, closely related to the “Corymorphinz”
of Delage and Herouard;’ indeed, some zoologists might
1 «Traité de Zoologie concréte :’ “ Les Ceelenterés,” p. 88,
22 ARTHUR DENDY.
prefer to modify and extend their conception of the Cory-
morphine so as to include Pelagohydra (as the authors
referred to include the Hybocodonide and Monocaulidez
of Allman) in preference to making a new family for its
reception.
7. DESCRIPTION OF PLATES 1 & 2.
Illustrating Professor Dendy’s memoir on “ Pelagohydra
mirabilis.”
EXPLANATION OF LETTERING.
A. F. Annular fold of endoderm around the margin of the septum. B. 7.
Branched tentacles. C.Can. Circular canal. Cnb. Cnidoblast. Cnp.
Cnidopod. D. F.C. Developing float cavities. Hen. Endocodon of medusa
bud. Hct. Ectoderm. End. Endoderm. End. Bud. Buds of endoderm
growing into the mesoglcea from the inner walls of the endodermal canals.
End. C. Endodermal canal. End. I. Endoderm of inner wall of endodermal
canal. Hnd.Z. Endodermal lamella of medusa. Hnd.O. Kndoderm of
outer wall of endodermal canal. EH. U. EH. Exumbrellar epithelium of medusa,
Fl. Float. G.C. Man. Gastral cavity in manubrium. JZ. G. BR. Longi-
tudinal gastral ridges of endoderm. Man. Manubrium. Med. Meduse in
various stages of development. Mes. Mesoglea. M. F. Ect. Ectodermal
muscle-fibres. MM. F. End. Endodermal muscle-fibres. Mo. Mouth. Nu.
Nucleus. Op. Hnd. Openings of endodermal canals into gastral cavity.
Pr. Proboscis. R.Can. Radial canals. Sep. Septum between the main
gastral cavity and the cavity of the float. S.M. 7. Internal supporting
membranes of the tentacles. Sé. Stolons. §.U.C. Subumbrellar cavity.
S.U. EH. Subumbrellar epithelium of the medusa. S.U.M. Subumbrellar
muscular layer of the medusa. Swp. Mem. Supporting membranes of the
float. Syn. Vacuolated syncytium covering the supporting membranes of
the float. Z.C. Thread-cells. Ten. Fl. Tentacles of float. Ten. Pr.
Tentacles of proboscis. Th.A. Thin area of wall of float around tentacle
base. w. The point where the ectoderm of the young medusa ruptures to
form the opening in the velum. @. Nucleus in syncytium apparently
dividing. y. Point of attachment of subumbrellar epithelio-muscular layer to
endodermal lamella. z. The place where the medusa was attached to the
stolon,
ON A FREE-SWIMMING HYDROID. 23
Figs. 1—17 inclusive refer to the hydroid stage of Pelagohydra
mirabilis; Figs. 18—25 inclusive refer to the medusoid stage of the same.
Fic. 1.—The free-swimming hydroid, from a sketch of the living animal.
x 2.
Fic. 2,—External view of a piece cut out of the preserved specimen,
showing the arrangement of the proboscis tentacles, etc. x 7.
Fic. 3.—Three adjacent tentacles of the float, showing variation in shape,
from the preserved specimen.
Fic. 4.—Portion of the surface of the float, much enlarged, showing the
stolons with the developing meduse, lying between the bases of the tentacles.
Fic. 5.—The preserved specimen after removal of a portion of the wall,
showing the gastral cavity, septum, float cavity, supporting membranes of
float, endodermal canals, ete. x 4.
Fre. 6.—Internal view of the piece represented in Fig. 2, showing septum,
longitudinal gastral ridges, endodermal canals, etc. x 7.
‘te. 7.—Portion of the same turned so as to show the under surface of
the septum, with the annular fold of endoderm and the openings of the
endodermal canals into the main gastral cavity. x 7.
Fie. 8.—Diagrammatic longitudinal section through a portion of the wall,
showing the relations of the internal cavities, septum, endodermal canals,
supporting membranes, tentacles, stolon, medusa buds, ete.
Fic. 9.—Part of a transverse section of the wall of the proboscis, through
the bases of the larger tentacles and the longitudinal gastral ridges of the
endoderm. Drawn under Zeiss objective A, oc. 2, camera outlines.
Fic. 10.—Portion of a transverse section similar to and near the last, to
show especially the arrangement of the muscle-fibres. Drawn under Zeiss
objective D, oc. 2, camera outlines.
Fic. 11.—Portion of a transverse section of one of the longitudinal gastral
ridges, showing the endodermal epithelial cells continued into muscle-fibres.
Drawn under Zeiss objective F, oc. 2.
Fic. 12.—Part of a transverse section through the wall of the float,
showing an endodermal canal continued outwards into a stolon, and giving
rise to supporting membranes of the float by means of groups of cells budded
off from its lining epithelium. Drawn under Zeiss objective C, oc. 2, camera
outlines (slightly diagrammatic).
Fic. 13.—Part of a transverse section of a supporting membrane from the
interior of the float, showing the mesoglceal layer covered on each side by a
syncytium, here collected on one side into a rounded multinucleate mass of
protoplasm. Drawn under Zeiss objective F, oc. 2, camera outlines.
Fie. 14.—Surface view of one of the supporting membranes of the float,
24. ARTHUR DENDY.
showing syncytium and nuclei. Drawn under Zeiss objective F, oc. 2, camera
outlines.
Fic. 15.—Part of a transverse section of a tentacle from the float. Drawn
under Zeiss objective D, oc. 3, camera outlines.
Fic. 16.—Part of the ectoderm layer from a section similar to the last.
Drawn under Zeiss objective F, oc. 2, camera outlines.
Fic. 17.—Two thread-cells with their cnidoblasts and cnidopods, from one
of the tentacles of the float. Drawn under Zeiss objective F, oc. 2.
(In Figs. 18—21 inclusive, showing stages in the development of the
meduse, the histology is, for the sake of clearness, rendered diagrammatic-
ally ; the endoderm is shaded; the external ectoderm is unshaded, and the
ectoderm of the endocodon and its derivatives is unshaded but has the nuclei
represented by dots. All are drawn, with the aid of the camera lucida, under
Zeiss objective D, oc. 2.)
Fic. 18.—Two young medusa buds seen in longitudinal section,—B before
the formation of the endocodon; C with the endocodon and manubrium
developing. (Owing to slight obliquity of the sections, the cavity of the
stolon is not shown.)
Fic. 19.—Slightly older medusa bud in longitudinal section.
Fic. 20.—Still older medusa bud in longitudinal section, with a very young
bud also springing from the same stolon at A.
Fie. 21.—Transverse section of a medusa bud a little older than the last,
showing the radial canals, ete.
Fig. 22.—Side view of one of the oldest meduse found. Drawn from
spirit specimen under Zeiss objective A, oc. 1, as a transparent object.
Fie. 23.—Oral view of similar specimen under similar conditions. The
mouth of the bell is now visible in the middle of the velum, between the four
tentacle-bearing arms.
Fic. 24.—Aboral view of similar specimen under similar conditions,
showing the four radial canals, subumbrellar musculature, etc.
Fre. 25.—Transverse section of a medusa of about the same age. Drawn
under Zeiss objective A, oc. 3, camera outlines.
Norz.—The microscopical sections were all stained with borax carmine.
STUDIES IN THE RETINA. 25
Studies in the Retina.
Parts III, IV, and V, with Summary.!
By
fienry M. Bernard, M.A.Cantab.
(Irom the Biological Laboratories of the Royal College of Science, London.)
With Plates 3—5.
Parr Tt.
The Migration of the Retinal Nuclei.
In this third part I had hoped to have dealt further with
the material absorbed by the rods from the pigmented
epithelium ; two important points, however, demand im-
mediate attention. In Part I, in referring to the migration
of the nuclei, I slightly misquoted Borysiekiewitz’s observa-
tions, and in Part II I left a serious gap in the description of
the outer ends of the developing rods. They were shown in
the figures (e. g. Pl. 31, fig. 29) as if truncated, just, indeed,
as they appeared in the sections. This gap I am now in a
position to fill (see Part IV), while Part V will describe the
fate of the absorbed pigment.
Referring to the migration of the nuclei from the middle
nuclear to the outer nuclear layer in Part I (p. 44), astonish-
ment was expressed that it had not been noticed before.
The only observer who, so far as I am aware, had called
1 For Parts I and IL see this Journat, vol. xliii, 1900, p. 23, and vol. xliv,
1901, p. 443.
VoL. 46, part 1.—New SERIES. Bie
26 H. M. BERNARD.
attention to the phenomenon is Borysiekiewitz.| This writer
recorded two evidences of migration (‘“ Ortswechsel”) of
nuclei in the human retina. Nuclei wandered outwards—(1)
from the outer nuclear layer into the basal limbs of the cones,
an observation which was not new; and (2) from the middle
nuclear layer through the outer reticular layer. I then added
that in this latter case “it was the characters of the migrated
nuclei, exactly like those of the layer they had left, and not
at all like those of the layer into which they had moved,
which convinced him that migration must have taken place.”
The similarity of these nuclei had been so often noticed by
myself as a convincing proof that the muclei embedded in the
outer reticular layer were passing through it, to become
transformed into rod nuclei, that after reading Borysiekiewitz’s
two treatises, and finding that he had alse noticed the
migration, I inadvertently attributed to him an observation
which, however, he does not make. He only indirectly
indicates it in his quotation from Dogiel, who recognised a
layer of “ subepithelial nerve-cells”’? in the outer nuclear
layer, i. e. a layer of “cells” exactly similar to those on the
other side of the outer reticular layer, called by Dogiel’
“the bipolar cells of the ganglion retine.”” Borysiekiewitz
remarks that such cells are probably merely his migrated
nuclei,® but rightly adds that they do not form a “ layer.”
Borysiekiewitz was himself convinced of the migration of
the nuclei by finding a tract (I. c., p. 37) in one of his pre-
parations in which the middle nuclear layer changed from two
rows into one row and then back again; but where it was in
a single row, the missing nuclei were visible either in, or on
the outer side of, the reticular layer (for a parallel case see
fig. 19, with description). This valuable observation shows
1 «Weitere Untersuchungen tiber den feineren Bau der Netzhaut,’ Wien,
1894.
2 * Archiv f. mikro. Anat.,’ 38, p. 317.
8 Borysiekiewitz uses the word “ Korn,” which does not exactly mean
nucleus, but in this connection it is practically the nuclei alone about which
anything ean be definitely stated. The point will be dealt with in my next
paper.
STUDIES IN THE RETINA. 27
that the migration of the nuclei from the middle nuclear
layer into the outer, which I described for the Amphibia, occurs
also in the human retina ; and indeed, I may add, it occurs in
all the vertebrate eyes I have yet examined.
Borysiekiewitz’s own theory of the essential structure of
the retina, in the light of which this migration finds no ex-
planation, is very different from mine. According to him,
these nuclei are inside the “ Miiller’s fibres,” in the more fluid
axial portions of which they can move. The outermost ends
of these radial fibres are, according to his view, the rods and
cones. So that the migration of the nuclei beyond the outer
reticular layer is a kindred phenomenon with their movement
beyond the membrana limitans externa into the basal limbs
of the “ cones,” both being mere shiftings outward along the
axes of the “ Miiller’s fibres.” The comparative study of the
“ Miiller’s fibres,” which will be found in Part V, makes the
acceptance of this description impossible. I may add that
the only difference which Borysiekiewitz can see between the
rods and cones of the human retina, is that the latter are
those tips of the Miiller’s fibres into which nuclei have
migrated beyond the membrana limitans externa. There is
no observable difference in the lengths of their outer limbs.
Confining ourselves to the migration of the nuclei, we may
review the position of the argument as far as it was advanced
in PartsTand II. Inthe Amphibia migration from the middle
to the outer nuclear layer can not only be seen—(1) in the
actual passage of nuclei through the outer reticular layer
(Part I, Pl. 3, fig. 5, e, f, and Part IT, Pl. 31, figs. 23, 24, 25),
and (2) in the exact similarity of those in the outer nuclear
layer which are not yet rod nuclei, but still close up against
the outer reticular layer with certain nuclei in the outermost
edge of the middle nuclear layer (see Part IJ, Pl. 30,
fig. 16, b), but is a necessary assumption in order to account
for the number of new rods required by the growing retina.
A short migration within the outer nuclear layer can be
seen in the fact that the “cone” nuclei, as the cones assume
the definitive rod-form, move outwards from near the outer
28 H. M. BERNARD.
reticular layer towards the membrana limitans externa. The
only possible escapes from the assumption that when the
original supply in the outer nuclear layer has been exhausted,
fresh supplies migrate outwards from the middle nuclear
layer are two: (1) if it could be shown that the layer of rods
and cones with their nuclei grows only at the edges ; and (2)
if it could be shown that the nuclei for the new rods are pro-
duced by the division of those already composing. the outer
nuclear layer.
With regard to the former of these alternatives, it is
certainly true that the retina as a whole does grow mainly at
the edges. I am not, indeed, now inclined to lay very much
stress upon the argument used in Part I, that if growth
took place only at the sides, the eye would not keep its
shape, for growth at the sides alone would, I thought,
merely carry up those sides, and the eyes would be funnel-
shaped rather than round cup-shaped. This argument would
perhaps hold if no other facters were present which could
help to keep the retina hemispherical. There is, however,
another traceable factor, the full force of which I did not
then see. I refer to the vitreous humour which, as a collec-
tion of semi-fluid matter in the hollow of the eye, would, if
the supply is kept up at any pressure, compel its flexible
walls to adopt the normal shape. But though this is a
possible factor in keeping the growing retina round, the
argument which refers the persistence of its shape to its own
orowth-processes can hardly be put altogether on one side,
for it is a fact that cones, 1. e. new rods, can be seen forming
over the whole Amphibian retina at all stages of its
crowth, and even in the eyes of adults.
Secondly, the suggestion that the nuclei for new rods
might be supplied by the division of those already present
can be met by a decided negative. In embryonic eyes
(Mammalia), or in amphibian eyes before they are functional
(see fig. 5), 1. e. before any rods are formed, and only the
merest traces of vesicular protrusions are to be seen,
divisions of nuclei occur over the whole retina in the outer-
STUDIES IN THE RETINA. 29
most layers. But as soon as and wherever vesicles are pro-
duced and rods begin to be developed out of them, a
process which always takes place first in the centre of the
retina and spreads outward from the centre, no divisions
normally take place. In order to ascertain this point, I
have examined the retinas of tadpoles (toads and frogs)
killed at almost all hours of day and night.! Nuclear
divisions were very numerous in the tadpoles killed in the
night, and sometimes in those killed in the daytime. A
study of them makes it quite safe to affirm that nuclear
division is normally confined to the edges of the retina,
that is, to those parts where there are no traces, or
only the faintest traces, of vesicular protrusions, although
one may just occasionally be seen dividing a short way within
the zone where the vesicular protrusions are beginning.” This
result is obtained from so many amphibian retinas that I
have no hesitation in affirming that after the rods have begun
to develop, nuclear divisions are never found normally in the
layer of rod nuclei. This is, of course, what we should have
theoretically expected, that cells specialised for some active
function are incapable of mitotic division.
We are, then, debarred from finding the source of the nuclei
for new rods in the nuclear layer itself. Hence the new nuclei
required must come into the outer nuclear layer from without,
i.e. from the middle nuclear layer, by migration through
the outer reticular layer ; and these migrating nuclei, whether
the retina grows mainly at the edges or not, must be many
thousands, considering the great numbers of “‘ cones” found
in the central regions in all stages of its growth.
This, then, brings us face to face with the question, Whence
does the middle nuclear layer obtain the large supply
necessary to furnish the outer nuclear layer with so many?
No one will suggest that the supply could be kept up from
the “ ganglionic cell” layer, which in the central regions is
1 Viz. at almost every hour of the night, from 4 p.m. to 6 a.m.
2 They are occasionally found in young fish retinas, even within the already
functioning central region !
30 Hi. M. BERNARD.
seldom more than one deep. It is true that nuclei from this
layer do pass through the thick inner reticular layer to the
middle nuclear layer. Most sections will show, as has been
often noted before and variously interpreted, one or two
actually within the inner reticular layer. Further, all sections
show a number of nuclei in the innermost rows of the middle
nuclear layer so like the “ ganglionic cells” that they have
been recently freely claimed as being ‘ ganglionic,” i. e.
as of much the same functional activity as the nuclei of the
innermost layer, which has always been the “ ganglionic
cell” layer of authors. Although this resemblance need not
necessarily have anything to do with the question of migra-
tion, there cannot, to my mind, be any doubt but that the
“ganglionic cell” layer is drawn upon by the middle nuclear
layer, and may, indeed, for considerable tracts, be quite ex-
hausted (compare fig. 22, gl. ina, 6, andc). But such
a supply, at its best, would be insufficient to counterbalance
the drain on the middle nuclear layer. Further, as in the
case of the layer of rod nuclei, no mitotic divisions are found
in the “ ganglionic cell” layer after the eye has once become
functional.
We have therefore to seek elsewhere for the supply of
nuclei required by the middle layer to enable it to send so
many outwards through the outer reticular layer to become
the nuclei of the new rods. One would think the most
probable source for these nuclei would be the division of
those already composing the layer, but here again we are
baffled, for divisions do not occur, at least near or in the
places where they are wanted. Indeed, the primary object I
had in view in examining retinas killed at all hours of the night
was to ascertain whether it was not possible that, as no
divisions were ever seen in this layer in retinas killed during
1 [have seen a few traces of fragmentation which deserve attention, but
hardly wide-spread enough to meet the present difficulty. See also Borysie-
kiewitz’s figures (‘Untersuchungen tiber den feineren Bau der Netzhaut,’
p. 19, 1887), which represent “twin ganglion cells.” They certainly suggest
divisions of these cells, but are capable of a different interpretation.
STUDIES IN THE RETINA. 31
the day, they might take place during the night, when
the eye is at rest. This, however, as above stated, proves
not to be the case ; no divisions occur in the middle nuclear
layer, except near the edges of the retina, where it is not
possible to speak of the middle layer because the two reticular
layers which separate the retinal nuclei into zones only begin
where the rods and cones are themselves commencing to
form.
We have, then, no other source, except this undifferentiated
rim, for the enormous number of nuclei required by the middle
nuclear layer in order to keep up the supply of rod nuclei
required by the growing retina. A few, one here and there,
as we have seen, may be obtained from the innermost,
or the “ganglionic cell” layer, but none from divisions of
those already present. The real supply must come, as stated,
from the rim of the retina; and however startling the idea may
at first appear, we have to assume a stream of nuclei from
the undifferentiated edges of the retina towards
the base of the cup. Further, as long as growth lasts,
this streaming must be considerable, for in addition to the
supply of nuclei for the formation of new rods, the thickness
of the middle layer is kept up,' even though the layer itself
has to expamd greatly as the eye grows larger. Indeed, it
has not only to extend as the eye grows, but, as compared
with the bulk of the layer in very young eyes, it may also
greatly thicken. Sections of small retinas (of tadpoles) about
0°5 mm. in diameter may show the middle layer in the central
region only three nuclei deep, while eyes over | mm. in diameter
may show it six nuclei deep; in the adult frog a layer four
deep is very common. But I do not think that much import-
ance can be laid upon these variations in thickness, as they
are probably accidents of nourishment and growth. It is
quite possible that at times the supply of fresh nuclei may be
greater than the immediate demand, in which case the layer
would temporarily thicken; or in times of bad nourishment
1 Apparently in all eyes, except in the “‘ fovea centralis” of human and
ape retinas.
32 H. M. BERNARD.
the supply might be less than the demand, and the layer in
consequence thin away. The really important fact is that we
find ourselves compelled to assume a migration of nuclei
ona very large scale. Not only can it be shown that nuclei
travel outwards through the outer reticular layer in great
numbers to become rod nuclei, but that all the nuclei destined
for this function have, at least after the original supply has
been used up, to travel down from the edge of the retina
along the middle nuclear layer to their ultimate destinations.
In addition to these movements it can be shown that
nuclei of the so-called “ ganglionic cell” layer occasionaily
travel outwards through the thick inner reticular layer until
in old eyes (g.l., fig. 21) they may be almost entirely used up.
Our investigations into the growth-processes of the retinas
of some score of frog- and toad-tadpoles having thus
eliminated all other possible sources for the nuclei of new
rods required by the central regions of the retina except this
immigration from the rim, it remains for us to see what direct
or indirect evidence there is for such an unexpected
phenomenon, not as an occasional, but as a normal growth-
process. It is hardly likely that such a movement could take
place without showing visible traces,—without leaving its
mark on the tectonics of the retina itself. We shall now see
that this surmise is fully justified.
The divisions take place in early growth-stages along the
whole of the rim into the iris, but are most numerous in the
angle between the iris and the cup of the retina. ‘To this
angle, as the iris becomes differentiated, they are usually con-
fined. They also take place chiefly, though not exclusively, in
the outermost row of nuclei, in what I have elsewhere called
the palisade layer. Inthe part where the divisions are active
it is common to find the large, radially arranged, more or
less spindle-shaped nuclei attached either to the internal or
to the external limiting membrane by a frequently thick stain-
ing cytoplasmic strand. The nuclei are usually so numerous as
to obscure the sections, so that one cannot state that these
strands, each with its suspended nucleus, run distinct and
STUDIES IN THE RETINA. 33
isolated from membrane to membrane. All that is really
important at the present moment is to note that the nuclei in
the region of active division are attached to one or other of
the “ limiting membranes ”’ by definite strands which are only
found in this undifferentiated rim.
Now these strands can often be seen showing the following
interesting arrangement :—On the axial side of this area of
nuclear division, and just where the differentiation of the
retina into zones is commencing, the nuclei, still for the most
part having retained their spindle shapes, are seen to be
arranged in slight curves (figs. 1 and 2) ; the two ends of the
curves are attached by these strands to the limiting mem-
branes, and their middle parts bulge outwards towards the axis
of the eye. This curving might easily be passed over, and
when seen it might be considered as a purely accidental phe-
nomenon. It is far more probable, however, that it is normal,
and due to the process we are discussing, viz. the tendency of
the nuclei to travel from the rim towards the functional
axial region of the retina. It is clear that the curving could
be so explained.
Again, comparisons of the different thicknesses of the middle
nuclear layer at different parts of the retina and at different
stages in its growth tell the same story of movement. We
always find that the layer is thickest near the rim where the
nuclei produced by division are crowding into it, and thinnest
in and near the centre where the nuclei are presumably in
most demand. Further, the variations in thickness of this
layer in the central region at different stages of growth
clearly show fluctuations in the numbers and changes in the
positions of its component nuclei.
If the nuclei from the undifferentiated rim have, then, this
tendency to stream inwards towards the axis, it is clearly
those occupying the middle ranks in the retina which wonld
be the freest to move, and which therefore would travel
fastest. Those of the innermost ranks will be more firmly
attached to the internal limiting membrane, and may,
perhaps, be further entangled by the developing nerve-layer,
voL. 46, part 1.—NEW SERIES. C
34 H. M. BERNARD.
while externally the nuclei are for the most part functioning
as rod nuclei. It would thus be only a band of nuclei dowr
the middle which would be freest to travel towards the axis.
This fact gives us a clue to the origin of the zonal arrange-
ment of the retina into alternating nuclear and reticular layers.
It is fairly clear that if a band of nuclei travelled along
between two stationary layers such as the innermost and
outermost layers in an amphibian retina, and if, when they
started, they had cytoplasmic attachments to the limiting
membranes, they would almost certainly leave traces of those
attachments trailed along on each side of the stream, and the
accumulations of the trailings would separate them from
the stationary fringing layers. We get, indeed, in this
somewhat startling and unexpected manner a perfectly in-
telligible reason for the existence of the two reticular layers.
It is unexpected because, considering that all these cyto-
plasmic strands are living protoplasm, it would appear more
natural if they had readjusted themselves in the retina, letting
the nuclei pass on. ‘The evidence, however, shows clearly
that this is not the case, and that they are to a large extent
trailed along and assist in the formation, at least, of the
inner reticular layer. I say “assist” because they appa-
rently only form its cytoplasmic basis; other elements, as we
shall see later on, contribute to the final result.
In studying thin sections of retinas of tadpoles I had
often been struck by the fact that from the extreme end of
the inner reticular layer irregular threads went off and
curved inwards towards the membrana limitans interna. This
is more striking in some cases than in others. Figs. 1 and 2
are sufficient to show what is meant, but in some cases I have
seen it so marked that it looked as if the inner reticular
layer took its origin, at each end of the section, from
the internal limiting membrane, sometimes almost shutting
off the layer of “ganglionic cells” from those of the un-
differentiated rim. ‘This appearance greatly puzzled me
until the discovery of the migration of the nuclei made
it clear that these threads which joined the inner reticular
STUDIES IN THE RETINA. 35
layer to the internal membrane were the remains of the
attachments of the originally spindle-shaped nuclei which
had moved away down the middle nuclear layer.
More conclusive still is the fact that every now and then
a section is found in which the nuclei of the middle layer,
especially near the rim, are actually caught trailing irregular
tangles of cytoplasmic threads in the manner shown i figs. 1
and 2. This can be seen with some frequency, though by
no means always, because it is probably a matter of accident
whether the particular retina happened, at the moment
it was fixed, to be in the exact phase of its life activities
which required such movements. For it is hardly likely that
the inward streamings of nuclei are continuous; periods of
rest would probably intervene. However seldom they occur
there is no mistaking their significance.
Still keeping the movement of the nuclei in view, it is
worth while paying further attention to the inner reticular
layer. We find that the early stages in its appearance
show differences which, though at first disconcerting, are yet
on the whole entirely confirmatory. The earliest stages which
I have so far seen are shown in figs. 5, 6, and 7, which Tinterpret
as follows:—The nuclei, which had been fairly evenly dis-
tributed through the retina, and not tightly squeezed together
(see fig. 7), gradually separate along the line which will be
later occupied by the inner reticular layer, the larger half
migrating outwards. A row, two or three deep, remains
against the internal limiting membrane, although one or two
even of these, in the axis of the eye, may escape outwards,
leaving a gap in the innermost layer (figs. 5 and 6). The great
mass of the nuclei gradually move, as stated, outwards, but the
very outermost can at the most move but a few micromilli-
metres, being arrested at once by the pigment epithelium.
The rest, therefore, leaving a few stragglers, crowd up close
behind, with the result that the irregular but conspicuous rent,
just described, occurs in the previously uniform nuclear ranks.
This rent in its early stages seems to be mainly occupied by
rounded vesicles, at least in the retina from which fig. 7 was
36 H. M. BERNARD.
drawn, but later becomes filled with a rather loose tangle of
staining matter, composed mainly of the cytoplasmic frame-
work in which the nuclei were suspended. ‘These phases not
only reveal the outward movement of the nuclei, but also show
that it is not due to pressure such as might be exerted from
a region of active division. ‘I'he nuclei in these sections
can only have moved outwards under the action of some
attraction. This fact, that the force bringing about these
migrations is attractive, 1s important, though we cannot stop
at the present moment to develop it further.
A slightly later phase in the formation of the inner
reticular layer can be seen in figs. 3 and 4. The irregulari-
ties seen in the layer in its first appearance, as a reticulum
filling up a split among the ranks of the nuclei, asjust described,
have become more definable as tongues running out among
the outwardly pressing nuclei. In one case (fig. 4) several
tongues appear of nearly equal size, although the one which
appeared to be nearly in the axis of the eye was the largest.
In another case (fig. 3) this axial tongue was very much larger
than any of the others. As will be seen from the direction
of the arrows in this latter figure, I explain these phenomena
as due to the migration of nuclei from the sides. ‘lhe
attraction which first drew the nuclei from their original
positions in the embryonic retina to press outwards has
extended and drawn nuclei from the peripheral portions of
the retina which have not yet begun to function. This com-
bined centripetal and outward movement of the nuclei would
naturally give the rudiments of the inner reticular layer the
shapes which they assume in these sections. ‘hat this
movement is taking place in the direction of the arrows
may be gathered from the closeness with which the nuclei
in these sections are packed in the central and more actively
functioning region as compared with their straggling and
loose arrangement elsewhere.
We have to add to this evidence, each item of which seems
fairly conclusive, the fact that the inner reticular layer grows
thicker as it slowly reaches the adult condition, and not
STUDIES IN THE RETINA. 37
only thicker but very much more extensive without the
appearance of any special formative cells which would
account for it.t If, however, the nuclei of the middle layer,
each with more or less cytoplasm trailing behind it, do
actually move along from the rim of the retina towards the
axis, we can account not only for the gradual thickening of
the inner reticular layer as the eye grows, but also for its
curious stratification, which is sometimes very striking.
The layer reaches its definitive thickness when the eye has
ceased to grow and no more nuclei are produced at the rim.
So far, however, we have only considered the inner
reticular layer, but there are two such layers, as there should
be if the mechanics of their formation here sketched be
correct. If correct, it supplies us also with an explanation
of the fact that the two reticular layers are always co-
extensive with the region of rod-formation, only appearing
where the vesicles are being protruded. A slight difficulty,
however, now arises. If these layers are produced by the
nuclei travelling down the middle layer from the rim towards
the centre, why is not the reticular layer on the outer side of
the stream as thick as that on the inner? An answer may
be suggested which is probably correct, although it would
be difficult to bring any evidence for or against it. Great
numbers of the nuclei travelling on the outer side are
arrested as they go and pass into the layer of rod nuclei.
These might be expected to take all the cytoplasm they
could with them as the formative substance of the vesicular
protrusions which they are destined to send out from the
retina for the formation of their rods. Hence it is probable
that the greater part of the cytoplasmic reticulum which
would otherwise be accumulated here as a counterpart of
the inner reticular layer is carried outwards and used up
1 [ have never seen any indication of the rows of small, faintly outlined,
formative cells such as Borysiekiewitz (|. ¢.) describes for the inner reticular
layer in human retinas ; whenever nuclei do occur in the layer, in all the eyes
I have examined, they are always quite distinct, and to be regarded as migrat-
ing outwards from the “ ganglionic cell” layer.
38 H. M. BERNARD.
in the production of rods. This seems to be a_ possible
explanation of the difficulty. Some traces of an outer
reticular layer, however, there always are, and doubtless
here again the retinal cytoplasm forms its basis, and only its
basis. In a future paper I shall show that neither of the
reticular layers is a homogeneous structure; the outer layer,
indeed, presents several difficult problems. In fig. 3, 0.7., we
already see signs of accumulations of deeply staining matter
along the line of the future outer reticular layer. These
accumulations, which we shall meet with again in Part V,
are apparently in some way due to the functioning of the
nuclei, for it is obvious they cannot, from their position, be
due to any merely mechanical streaming movements.
In the very existence of these two reticular layers, as well
as in their stratified texture, in their attachments round the rim
by threads to the membrana limitans interna, and in the shapes
they assume during early growth, we find strong evidence of
the migration of the nuclei, which is the subject we have
specially in hand.!. We may sum up the arguments briefly :
(1) the nuclei of the adult rods protrude a little beyond the
membraua limitans externa ; (2) the nuclei of the cones, which
(in Amphibia) are early stages in the formation of new rods,
move gradually outwards from near the outer reticular layer
towards the membrana limitans externa as their rods de-
velop; (3) no nuclear division takes place in this layer where
rods and cones are developing ; the nuclei for the further pro-
duction of rods come through the outer reticular layer from
the middle nuclear layer; (4) no nuclear division takes place
in this middle layer anywhere near the axial portion of the
retina, and the supply must be kept up by migration from
the sides. A very few may come through the inner reticular
layer from the layer of “ ganglionic cells,” but the bulk of
1 We shall refer to some of the very discordant views which have been put
out as to the origin and constitution of these layers when we come to deal
with them in detail. In the meantime a useful summary may be found in the
Literatur-Verzeichniss to Borysiekiewitz’s first paper, ‘ Untersuchungen
iiber den feineren Bau der Netzhaut,’ 1887, notes 19—27.
STUDIES IN THE RETINA. 39
those required travel along the middle nuclear layer from the
undifferentiated rim of the retina where nuclear division is
active during growth. Thus a stream of nuclei travels
inwards from this undifferentiated rim towards the axis along
the middle nuclear layer; (5) this stream of nuclei lays
the foundation for the two reticular layers of the retina; the
cytoplasmic trailings of the nuclei being, as it were, swept to
the sides of the stream, accumulate, but while the inner
accumulation persists the outer is mostly used up, probably
in the formation of the rod-vesicles.
Before leaving the subject for the present, I should like to
call attention to the conviction which I expressed in Part II,
p. 452, that the retina is a syncytium, in the reticulum of
which nuclei are suspended, and that it is almost impossible
to speak of “cells” in connection with its component
elements. ‘he streaming of the nuclei and the trailing
behind them of cytoplasmic tangles, which trailings
accumulate as the eye grows, may, I think, be regarded as
complete justification for this conviction. I had not for-
gotten and do not forget the large “ ganglionic cells,”
which appear to supply an easy refutation. On the contrary,
it was a prolonged study of these same “ cells” which first
led me to this conclusion, as I shall relate in detail in a
future paper.
Lastly, I should like to venture the suggestion that the
principle here established for the retina may be of wide
application, although I cannot hear of any other exemplifica-
tion of it as yet known. The principle is this: an organ
has to continue to grow after it has begun to func-
tion. Assuming that nuclei or cells are incapable of mitotic
division when once specialised for some highly complex
function, we should be compelled to postulate an undifferen-
tiated region which would persist as long as growth lasts.
From this region, which would be the centre of active nuclear
or cell division, the new elements required by the functioning
and growing area would have to migrate, through longer
or shorter distances according to the exigencies of the
4A H. M. BERNARD.
particular case. Further, as in the case of the retina, these
migrations may have considerable, influence on the tectonics
of the organs in which they can be established.
Part IV.
On the Vesicular Swellings at the Tips of the
“Cones” and some Earlier Form-phases in Rod-
production in the Amphibia.
As was shown in Part I,! the tips of the young cones swelled
into vesicles on reaching the pigment layer. Vesicles or parts
of vesicles were figured (P]. 3, figs. 2,3, and 10), and these justi-
fied the construction of the series of form-changes shown in
the diagram (fig. 4), but they were only certainly seen in eyes
fixed with boiling corrosive sublimate. Other figures on the
same plate (e.g. fig. 12, and on Pl. 31, Part II,? fig. 29)
showed no traces of any such vesicular tips, and in some
cases it was difficult to understand why, if they had existed,
they should vanish so completely from the sections. This
point has now been settled, not, I regret to say, by the
discovery of a new and more perfect fixative, but by a kind
of good fortune. I brought down a few tadpoles from
Table Mountain, Cape ‘own, killed and fixed them in
Perenyi’s fluid at midnight, i.e. when the pigment would
be retracted. The object was to see whether, owing to the
brilliant sunlight of South Africa and the intense heat, the
pigmentation in the retina showed any modifications on that
seen in our indigenous tadpoles, and if so whether any
correlated changes in the retina could be discovered. For
the same reason I made special efforts to obtain baboon’s
eyes (see Part V).
One interesting difference was at once apparent. ‘I'he
pigment in the South African tadpoles is far greater in
1 This Journal, vol, xtiii, 1900, p. 28.
? Ibid., vol. xliv, 1901, p. 443.
STUDIES IN THE RETINA. 41
quantity and of a very much darker brown. The colour is in
striking contrast with the reddish brown which is most
common over here. But this was not all; probably in corre-
lation with this increase in mass and quality of the pigment
the rods were also different (see figs. 8, 9, and 10), in that
the longitudinal striation is so marked that it can be seen at
once with a low power. Cross-sections of rods which seem
to be somewhat thin, 4 u across, often tapering to 3 pa, show
a thick, straggling, branching, and knotted strand running
down the axis of each rod as the representative of the axial
reticulum, and connected irregularly with the dark strix
running down the wall. Here and there the greater part of
the axis of the rod is taken up by a mass of dull grey homo-
geneous matter, in which case the axial reticulum is appa-
rently represented by clump§ at the sides, but it usually
comes into view again on focussing up or down. ‘These grey
masses in the rods are the remains of material absorbed from
the pigment granules (see fig. 8, with description).
Apparently correlated with these strongly developed
staining strie is the fact that the rods, though very thin, are
comparatively speaking tough; for, quite unlike those in our
own species of Amphibia, which break up so easily and
usually part at the junction of the inner and outer limbs, in
these eyes, where the retina and choroid have parted, they
are drawn intact out of the dense pigment.
Turning to the cones, we fortunately find that their vesi-
cular tips share in this greater toughness. Very many of
these latter, it is true, have broken down and have been
reduced to a granular mash which is very conspicuous, but
places such as those figured (figs. 9 and 10) might be
multiplied to any extent. ‘The vesicles are shrunken, and it
is largely owing to the folds in their walls that they are
visible. Some seem to have clear traces of rows of dots
running down them which remind one of the rows of dots on
the longitudinal strize of the rods. In optical section the
wall of the shrunken vesicle could often be traced quite
plainly into that of the cone (fig. 10).
42 H. M. BERNARD.
We conclude, then, that in all young amphibian eyes, in
which the rod layer seems to consist mainly of cones ending
at some distance from the pigment, the apparently vacant
space between the truncated cone-tips and the pigment is, in
life, filled up by a compact mass of swollen vesicles. ‘These
vesicles are, however, so exquisitely delicate that the process
of fixation and hardening destroys them almost completely.
But I should add that now that I have seen the vesicles in
the Table Mountain specimens, I have been able to discover,
in sections of our native forms, several cone-tips running out
into faint diverging threads.
Another peculiarity in the retinas of these Table Mountain
tadpoles deserves mention. In Part I, p. 34, [remarked in a
note that the only long-necked elements which I could find in
frogs’ retinas at all resembling the long-necked “cones” figured
by van Genderen Stort (‘Quain’s Anatomy,’ 10th ed., vol. 11,
part 8, p. 48) were those which appeared in each case as one of
the so-called twin or double cones (see PI. 3, fig. 5). Besides
these, the only elements with long inner limbs were Schwalbe’s
rods,in which the refractive globule had, as a rule, already
disappeared and the outer limbs had already become
cylindrical (Part I, Pl. 3, fig. 4, 7). But in the retinas of
these T'able Mountain tadpoles, cones with striking refrac-
tive globules like those figured by van Genderen Stort are
very plentiful, close down against the pigment layer. The
greater toughness of the walls may account for the persis-
tence of the shape in a phase where it is quite lost in our
native forms (see the phases Part I, fig. 4, c, andr). The
fact that the refractive globule is not so quickly absorbed
may be referred to the great quantities of pigment to be
dealt with (for the origin of this globule see Part II, p. 463).
The transformation of these long-necked cones into rods is,
in some cases, very easy to follow. The conical portion
thickens and shortens, while the swollen vesicle at the tip
becomes cylindrical and the refractive globule disappears.
In fig. 9 elements like those on the right and left hand are
very common; that on the right shows a division in the
STUDIES IN THE RETINA. 43
material filling the rod; the short innermost darker portion
clearly corresponds with the old staining tip of the cone
(cf. the sections indicated by asterisks). The small numerals
1—7 show a continuous developmental series illustrating the
transformation of long-necked cones into Schwalbe’s rods.
It was stated in Part I that the growth-forms (c, cg, cs) of
the new rods shown in PI. 3, fig. 4, of that paper were due
to the fact that each new element on being protruded
had to force its way between tightly packed cylindrical
rods; obviously the new vesicle could only swell into a
sac after getting through, that is, against the pigment.
If this explanation be correct we should expect to find
other form-phases where the rods were not so long. A
much more direct transformation of cones into rods was,
indeed, figured in the series of elements supplied by the
axolotl, Part 1, Pl. 3, fig. 8.! In this animal the rods are
very thick, and, compared with their thickness, very short.
Now it is interesting to note that we have at the sides of
tadpole retinas, where the rods get progressively shorter, a
very similar process of direct transformation of cones into
rods to that which we found in the axolotl. The distal
portion of the protruded cone seems to be neatly rounded off
(fig. 11), as if there had never been any swollen vesicle at
its tip. It is further quite distinctly striated longitudinally.’
Here, then, we have the cones chauging directly into rods by
the absorption of the refractive globule and the lengthening
of the outer limb at the expense of the inner. If we compare
this process closely with that occurring among the long rods
in the central regions of the retina (figs. 9 and 10), we find
that it differs in two points: (1) there is no long thin neck,
and consequently (2) there would appear to be no con-
spicuously swollen vesicle at the tip which would have ulti-
1 To make that figure true to life the tips of the cones in @ and d should
have been drawn with delicate vesicles, but all traces of such vesicles had
been destroyed in the actual sections.
2 Compare the dots seen on the distal vesicles shown in figs. 9 and 10, also
the remarks on the striation of the cones in Part II, p. 455.
44. H. M. BERNARD.
mately to be brought into the typical cylindrical form. It is
easy to see that both these differences are due solely to the
fact that where the rods are long cylinders the protrusion
has to force its way between them, and only swells out into a
conspicuous vesicle after getting through.
Tt will be seen from the study of these details how important
it is to keep the compactness of the layer of rods very clearly
before the mind. The rod layer, in fact, arises as the result
of the thrusting out of great numbers of vesicles from the
retina, the vesicles only gradually assuming the long, cylin-
drical rod shape. The varying forms which the early stages
of new rods assume when first protruded, and until they are
finally developed, depend not only upon the forms, but
also upon the lengths of those among which they
have to force their way. We have now seen two of
these different series of form-changes, and it will be best in
this connection to record the observations made on still
earlier stages of growth, when the new vesicles are protruded,
not among rods, but among other vesicles which have not had
time to become rods. We shall see that whereas, when the
rods are formed, and their shapes fixed, new vesicles have
to adapt themselves entirely to them; while the rods are
still unformed and vesicular the protrusion of new vesicles
is able to modify their shapes. In the changes described in
Part I we saw that the protrusion of fresh cones altered the
shapes only of other cones, helpmg to change long cones
into Schwalbe’s rods, but that they had no apparent effect
upon finished rods,
The first appearance of rod-vesicles begins very early, as
soon as ever the eye begins to function. They can be seen in
various sizes in figs. 3 to 7, as round clear spaces against the
pigment. At first they are scattered and confused, because
all the nuclei do not secrete vesicles simultaneously (see
figs. 16, 17,and 18). A little later a stage is reached when
they are arranged side by side as large sacs mutually com-
pressing one another (figs. 12 and 15). It is at this
stage that our sections usually failus. So long as the vesicles
STUDIES IN THE RETINA. 45
are small and round their outlines are clear, being preserved,
no doubt, by the fixing of the pigment cells on the one hand,
and of the deep staining matter, which usually forms their
proximal walls, on the other (see figs. 17 and 18, where the
shaded vesicles represent deeply stained walls). As soon, how-
ever, as they lengthen out, the walls become so delicate that
they collapse under the violent processes of fixing, hardening,
and preparing the sections. It is common to find in young
eyes great empty spaces where the rod layer should be
between the retina and the pigment, the spaces occasion-
ally interrupted by single, short, thick, deeply staining
rod-like structures, one here and there having survived.
That elements of some form or other filled these gaps is
absolutely certain ; indeed, the ragged remains of membranes
can often be seen fringing the distal ends of the nuclei, and
protruding a little from the membrana limitans externa. A
creat many sections show nothing but this, and one is apt to
become hopeless of ever seeing the vesicles which, in life, had
been crowded together in those gaps. On one occasion I
found one of these spaces occupied by a single large
vesicle with a complete pigment cel!, which had left the pig-
ment epithelium, inside it. In time traces of long vesicles
become more frequent because they are supported and
preserved by being in contact with other more formed and
stronger elements (Part I, Pl. 3, fig. 16). It is when a
number of very fragile vesicles are mutually supporting and
squeezing one another that they disappear from our sections
leaving hardly a trace behind.
In sections of retinas killed at night I have succeeded at
last in finding vesicles intact. ‘They are slightly mottled
and dotted over with stain, and I conclude that they owe
their preservation largely to this fact, viz. that their walls
were strengthened by this staining matter, as appears to have
been the case with the rods and the cone tips in the retinas
above referred to from Table Mountain. Fig. 12 shows a
eroup which have fortunately been preserved intact, and
fio. 13, a—e, are elements from the same retina.
46 H. M. BERNARD.
On looking at these our attention is at once arrested by
figs. a—c. We have apparently typical cones with their
points thrust into terminal vesicles. A little reflection, how-
ever, shows how such appearances could be easily produced
as transitory phases. ‘To make it clear I give a diagrammatic
series, fig, 14, a—d. a represents an unmodified vesicle
protruded as a long oval. ‘The pressure caused by the
gradual protrusion of new vesicles will be exerted upon a in
the direction of the arrows shown in b, with the result
that @ will take the form shown by 6 (ef. the middle vesicle,
fig. 12). In the narrow neck of b staining matter accumu-
lates. Continuation of the pressure further lengthens the
neck, and at the same time the adding of new vesicles forces
back the pigment cells.!
In the stage c I have introduced a refractive globule,
which we may assume to have come out of the distal
vesicle as matter absorbed by it from the pigment, as ex-
plained at length in Part II. At this stage it is again the
turn of the element whose form-changes we are following to
receive another discharge from the retina or, as argued in
Part II, from its nucleus. This discharge drives out the
staining matter which occupied the neck, so that it protrudes
into” the distal vesicle. The three figures 13, a—c, show
three distinct degrees of thrust, quite accidentally selected,
the figures having been drawn in the order shown in the
plate before I was at all clear as to their meaning. In a,
only the narrow tip of the matter from the neck has been
pushed into the sac; in ¢, the tip and a portion of the
refractive globule, in this case the matter composing the tip
itself has been disarranged against the distal end of the
vesicle; in b, a larger portion of the staining matter still has
been thrust outwards into the sac. ‘These curious “ cones,”
1 This lengthening of the vesicles widens the distance between the pigment
layer and the body of the retina. The width is greatest in the centre of the
retina, and in very young eyes diminishes rapidly on either side. This is
certainly due to the greater activity of vesicle formation, i.e. of rod-produc-
tion in the area of most active functioning.
STUDIES IN THE RETINA. 47
then, are due to the driving out of the staining matter which
had accumulated in the neck of the squeezed-up vesicle.
There is no telescoping of the membrane into itself. It is
simply another form of the phenomenon shown by asterisks
in fig. 9, where the original contents of what has hitherto been
thought to be the tip of the cone become the proximal portion
of the contents of the Schwalbe’s rod, which arises as soon
as the vesicle has assumed its cylindrical shape. ‘lhe vesicle
assumes this latter shape apparently in both cases as it
becomes more and more turgid with matter received on the
one hand from the retina, and on the other from absorption
of pigment. The stages cand d in fig. 13 require no con-
necting links, d being the next stage produced by the filling
up of the distal vesicle. Still younger and simpler stages of
transformation of vesicles into rods are shown in fig. 15.
They need no comment.
Many interesting details of observations in relation to this
part of the subject might be added, but the task of dealing
with the retina of the Amphibia alone threatens to lengthen
out so greatly that only points necessary to a clear under-
standing of the essential morphology of the retinas dealt
with can be mentioned.
How necessary it is to understand the minute details of
rod-formation I need hardly insist, that is if we are to make
any progress with our researches into the mechanics of vision,
for the rods are the specific structures which constitute
the retina the specific organ of this sense. Believing, as I
do, that all structures are produced both phylogenetically
and ontogenetically only in response to physiological needs,
I feel confident that in a case like this where the rods are
produced in situ, and only when required, their processes
of formation must throw light upon the mechanics of their
functional activities. Some further details relating to these
activities will be found in the next part.
48 H. M. BERNARD.
_ Parr V.
On the Removal of the Absorbed Pigmentary
Matter from the Rods: an Explanation of the
“™Miller’s Fibres.”
In Part IT I described in detail a set of phenomena which
found their simplest interpretation in the assumption that
the protoplasmic vesicles, known as the rods, protruded by
the retina against the pigmented epithelium, absorbed the
pigmented granules, and at times also the cytoplasm of the
pigment cells. I propose in this paper to describe another
set of phenomena which indicate the way in which the rods
are freed from the excess of matter thus absorbed.
My results differ somewhat widely from any hitherto pub-
lished, and especially from those obtained by the now popular
impregnation methods, and I ought, perhaps, to make some
excuse for not testing those other methods myself. My
answer, I fear, can only be an apology. I selected the
purely comparative method deliberately as the only absolutely
certain way of obtaining light on intricate morphological
problems, but the method is slow and laborious, and I grudge
the time necessary to become an adept in the use of others,
the results of which have still to be interpreted.
The pigmented matter was, as we saw, absorbed through
the walls of the outer limbs, and some of it found its way
through the transverse membranes into the inner limbs,
where it helped to form the bodies known as the ellipsoids.
Part of the absorbed matter, then, finds its way through
the transverse membrane into the inner limb. Here, unless
it can find some further method of escape, it must accumulate
and cause the inner limb to swell. No such swelling of
the inner limb takes place in the Amphibia, but it is a
striking phenomenon in many fish. This is the explanation
giant cones”? which are so startling when seen for
the first time (see figs. 20, b, and 21).
ce
of their
STUDIES IN THE RETINA. 49
Although the rods and cones in the fish are not our special
subjects in this paper, it will be necessary to enter into a few
details with regard to them. In the very young the elements
are seen to be nearly all of uniform size, with apparently the
same form-phases in their production as we described for the
Amphibia, viz. (a) small cones, (b) gradually lengthening
cones, (c) Schwalbe’s rods, and (d) fully developed rods
(fig. 20, a). These are the natural stages in the formation
of new rods in the amphibian retina. But in the fish,
after the earlier stages of growth have passed, we find a very
striking change, which seems to begin somewhere near the
central region! and spread gradually over a large part of
the retina. The change is as follows :—The inner limbs of
the earliest formed rods gradually swell, until, in large and —
presumably old fish, they may be of monstrous proportions.
So that though, while growth is still going on, there may be
room for a few more young cones to protrude or for a few
more of the inner limbs of the Schwalbe’s rods to shorten
while the outer limbs lengthen, that time comes to an end,
and the retinal elements, at least over the modified area,
consist entirely of (a) rods with monstrous inner limbs, and
(b) bunches of Schwalbe’s rods. The thin thread-like inner
limbs of the latter find their ways between the swollen inner
limbs of the “ giant cones,” while their numerous cylindrical
outer limbs fill up the spaces between the comparatively
speaking small outer limbs of the ‘‘ giant cones.” Fig. 20,
a, b, shows comparisons between the conditions of the elements
in young and old eyes in the viviparous blenny. Fig. 22,
a, b, c, shows different parts of the same retina of a young
plaice, a being near the centre, where it has functioned most
actively. Similar results might have been shown from my
sections of trout and stickleback. I had no sections of young
cod for comparison with fig. 21, from an old fish, but we may
judge of the original thickness of the elements by those
which persist as Schwalbe’s rods, a few of which are shown.
1 Without having exactly located the region, I believe it to be the postero-
ventral half of the central region.
vou. 46, part 1,—NEW 8ERIKS. D
50 H. M. BERNARD.
Here, then, we have the very phenomenon we anticipated in
the event of the refractive matter which passed from the
outer into the inner limbs not being able to escape from the
latter, at least as fast as it accumulates. These inner limbs
become swollen with refractive matter. That this is the true
explanation of the “ giant cones” is rendered clear by a study
of large fish like the cod. While in some, especially smaller
fish, the matter filling the inner limbs is often difficult to
define, in the sections I possess of the retina of an old speci-
men of this fish ! the refractive inatter is quite recognisable.
It is often seen in round homogeneous pellets just inside the
transverse membrane, and usually continued some way up the
axis of the inner limb. Round the periphery the contents
are more granular. Here and there, however, the whole
inner limb is one smooth, bright, homogeneous mass. These
smooth, round, refractive pellets, which seem to accumulate
above the transverse membrane, may be compared with the
refractive globules in the cones of the frog. The refractive
matter here, as elsewhere, is deeply coloured by plasma
stains, such as eosin, but easily gives up nuclear stains.
If further evidence were wanted that the material which
swells the inner limb is the refractive matter absorbed from
the pigment granules by the outer limbs, it is supplied by
those cases in which the colour of the absorbed matter is the
same as that of the pigment. Such cases may be purely
individual differences, and depend upon chemical variations in
the pigments, or, perhaps, may be due simply to a too rapid
absorption. Certain it is that though the shape of the pig-
ment granules is lost, the colour of the absorbed matter may
now and then be hardly altered. Among my sections of the
plaice,? for instance, there is one in which the strong reddish
colour of the pigment only slowly vanishes. It pervades all
the outer half of the swollen inner limbs, sometimes extending
some way up the “rod fibres.” Other more striking instances
1 Fixed with corrosive sublimate.
2 Specially fixed and preserved for me by my lamented friend Mr. Martin
Woodward at the Plymouth Marine Laboratory.
STUDIES IN THE RETINA. 51
of the persistence of the colour right into the retina will be
given below.
Before continuing to consider whether and how the matter
can escape from these inner limbs, one or two points may be
noted in passing. (1) The “ giant cones” of the eyes of fish
are not the morphological equivalents of the cones in the eyes
of the frog. The latter are almost the earliest stages of rod
formation; the former are not only fully developed rods, but, at
least so far as growth proportions go, the most highly deve-
loped elements known in vertebrate eyes; their great size
will have some bearing upon the question as to the length of
the life of individual retinal elements when that question
comes to be put.
(2) A very large proportion of these “ giant cones” are
double. Although the dividing line in the swollen inner
limb may become very faint, and sometimes only traceable
in tangential sections, the presence of two nuclei and of
two outer limbs will always show whether any large “ cone”
is double. I have already shown that the peculiarities of the
double cones in the frog are due to the fact that two
protrusions of the retina start side by side almost simul-
taneously, and their forms are due to mutual pressure, both
being subject at the same time to the general pressure
which we have to assume to account for the ordinary cone
phases of rod formation. In these great double “ giant
cones” we merely have two rods very close together, and about
the same age. The fusion of their inner limbs will take place
sooner or later, as these inner limbs swell with matter. The
only part of the phenomenon which requires investigating is
why these rods of similar age should be so frequently in
pairs, or, tracing it a stage further back, why it is that as
new nuclei arrive to send out their protrusions to form new
elements for the growing retina, they leave so many pairs of
nuclei between which they do not or cannot force their
way.
(3) The remarkable change which takes place in the
forms of the elements of the growing’ fish retina, from an
Sp H. M. BERNARD.
early stage with (a) small cones, (b) “Schwalbe’s rods,”
and (c) fully formed rods, into a stage with only two kinds
of elements, viz. (a) rods with enormously swollen inner
limbs, and (b) “Schwalbe’s rods” with long thread-like
inner limbs, fully justifies the appeal which throughout all
these papers we have made to pressure in order to account
for the form-phases of the elements of the bacillary layer.
(4) The fact that in the eyes of all Vertebrates higher than
fish refractive matter no longer accumulates in the inner
limbs, at least so as to swell them to such disproportionate
sizes, apparently justifies the conclusion that these accumula-
tions are not helpful to the specific function of the retina.
Returning to the subject in hand, we must now show how
the refractive matter ultimately escapes from these swollen
inner limbs of fish retinas.
Reference to the sections of the cod leaves no doubt on
this point; the very size of the ‘giant cones,’ and the
coarseness of their connections, reveal what the smaller
elements of other eyes could not so plainly show, at least
until the facts have already been made clear. What I take
to be a thick stream ascends from each of these “ giant
cones,” and ends in a refractive clump against the outer
reticular layer, the ‘ cone ” nuclei being sometimes elongated
in the line of the stream. The terminal clumps form, as it
were, conical expansions where the streams meet the tan-
gentially arranged tissue of the outer reticular layer. Here,
again, microscopic examination of the “stream,” and espe-
cially of its large conical expansion, as seen in the cod, show
at once the presence of the same matter as that in the inner
limbs. It is not meant, of course, that this matter is alone
present, for in what follows it will be seen that this refractive
matter follows the threads and fibres of the cytoplasmic
network of the retina. In this case the stream and its
conical expansion doubtless have a cytoplasmic framework.
These streams with their expansions occur in one form or
another in most if not all eyes, at least as physiological
stages, and are usually described as the “cone fibres” with
STUDIES IN THE RETINA. 53
their intra-retinal terminal swellings.) Fig. 21 shows them
in the cod, fig. 20, a, b, in the blenny, fig. 22, a, b, c, in the
plaice, fig. 24, a, in the trout. These swellings, which are
also specially conspicuous in Ramon y Cajal’s figures from
metal impregnation preparations, have hitherto found no
explanation. They can now be accounted for as the points
where the refractive matter escaping from the rods is
temporarily arrested as it reaches the outer reticular layer.
Confirmatory evidence can be seen in the fact that their size
depends upon the functional activity of the part. Fig. 22,
a, b, and c, shows three parts of the same retina. They show,
as do all the fish eyes I have examined, that the retina is
very unequally used up. The part a from the central region
shows the largest swelling of the inner limbs of the knobs of
the rod fibres, and the most marked using up of the inner
and middle nuclear layers.
Having brought the refractive matter thus far in the eyes
of the fish, we may go back and consider some other eyes in
which it escapes from the outer limbs without swelling the
inner limbs to such monstrous proportions. We can only
refer to two cases, for a full treatment of the subject would
require a close comparative study of the retinas of all the
animal groups other than fish. The two cases chosen are
especially interesting because they present such striking
contrasts: (1) the Amphibia, with the inner limbs of their
adult rods quite small and insignificant as compared with the
outer limbs; (2) the Primates, with their long, rather thick
inner limbs and thin outer limbs not much, if at all, longer
than the inner limbs (see fig. 31, a).
(1) That the refractive matter escapes into the inner limbs
in the Amphibia we know from the invariable presence of
the ellipsoid. But the ellipsoid does not, as a rule, seem to
grow, so that if refractive matter is always exuding through
the transverse membrane, it must as rapidly be transformed
and conveyed away through the retina. Certain it is that
1 I need only refer to the familiar text-book diagrams, such as fig. 52,
p. 46, of ‘ Quain’s Anatomy,’ 10th edition, vol. iii, part 3.
5A H. M. BERNARD.
the inner limb is never swollen up with refractive matter.
This sparing of the inner limbs in the Amphibia may
perhaps be correlated with the enormous size of the outer
limbs, for, so far as I know, no other group of animals has
them so large in proportion. In the frog they are immense
cylindrical vesicles, sometimes as much as 60, long and
9 to 10m in diameter. These, then, form very capacious
reservoirs for the absorbed refractive matter, and, perhaps,
seldom require, during any single period of activity, to
overflow into the inner limb. In this way the matter may
be dealt with by the outer limb itself, and, apart from the
ellipsoid, escape directly into the retina along the wall
of the inner limb without entering it. That the refractive
matter escapes directly from the outer limbs into the retina
along the walls of the inner limbs can sometimes be actually
seen (fig. 25, a, b, c). These cases are all from the South
African tadpoles referred to in Part III, which, owing to the
immense quantity and dark colour of the pigment, are
very instructive in this connection.
(2) Equally decisive for our contention are my sections of a
human retina (the healthy normal eye having been excised
for a morbid growth on the eyelid). This eye had clearly
not been much exposed to light before excision. We conse-
quently find the outer limbs of the rods free from all
refractive matter, and, like the inner limbs, almost clear
vesicles but for the longitudinal fibrils and the granules
taking nuclear stain. ‘I'he fibrils on the inner limbs are dotted
like those of the outer limbs in the Amphibia (see Part II and
figures). Naturally no thick refractive streams can be seen
running up into the retina from these rods. In very strong
contrast with this are fig. 31, a, b, from retinas of the South
African chama baboon,’ which live in the full glare of the
' Kindly fixed in Perenyi’s fluid and preserved for me by the well-known
ophthalmic surgeon, Mr. EK. Treacher Collins.
2 They were generously obtained specially for the purposes of these re-
searches by Mr. J. C. Kous, Tafelberg Station, Cape Colony, through the kind
intervention of our mutual friends, Mr. and Mrs. Mallinson, of the Hex
River Valley.
STUDIES IN THE RETINA. 55
South African sun. Here the rods, and especially the large
inner limbs, are mostly full of pigment, which can be seen
streaming inwards into the retina, no longer forming single
fibrils with terminal knobs, but great tangles of refractive
matter, which, in the eye (31, a) with dark blackish pigment
are dull and blackish, but in the eye (31,6) with bright
yellowish-brown pigment are bright yellow-brown. I may
say that after seeing how the pigmented matter streamed
through the retina in the tadpoles brought from the slopes of
Table Mountain, I was quite prepared to find something of
the kind in the retina of the baboons, but was myself
surprised to see how very obvious the escape of the absorbed
pigmented matter into the retina is in these cases. The pig-
ment is so dense that the colouring matter is not bleached in
the rods, nor, indeed, does it undergo much loss of colour
throughout its passage through the retina, as it usually does,
say, In our indigenous Amphibia.
We have so far, then, traced the matter absorbed by the
rods into the retina as far as the region known as the outer
reticular layer. This is in many respects one of the most
difficult parts of the retina to understand. The matted and
deeply pigmented strands just below this layer in the
baboon’s eye, as well as the conical expansions of the
ordinary ‘‘ rod fibres,” indicate that the absorbed pigmented
matter is temporarily stopped by it. But the exact cause of
the stoppage at this point I have not succeeded in unravelling.
Krause thought that there was a tangential membrane at
this place, his ‘‘ membrana fenestrata,’ ' and certainly the
first time one sees the outermost layer of nuclei of the middle
layer arranged tangentially in a compact row, as shown in
figs. 20, a, and 22, c, it is difficult not to think that Krause was
right; but a study of older eyes (figs. 20, b, 21, and 24, a), or
even of the more used-up parts of younger eyes (fig. 22, a),
will show that these nuclei do not belong to any fixed mor-
phological structure in the retina such as a membrane, but
1] have unfortunately never seen a copy of the book written by Krause
under this title.
56 H. M. BERNARD.
that they are merely nuclei of the middle layer passing out-
wards to become rod nuclei, and apparently flattened against
the same tangentially arranged cytoplasmic tissue as that
which detains the refractive matter in the manner described
above. But the difficulty is not quite so simple as this, viz.
that the stoppage of the nuclei going outwards is due to the
presence of tangentially arranged tissue, or even to a mutual
blocking of the way on the part of the nuclei moving outwards
and of pigmented matter moving inwards. ‘That this latter
is not the cause is clear, because we find the same stoppage
of the refractive matter even when, as in old eyes, nearly all
the middle nuclei have passed outwards (see fig. 21). That
other subtler complications are present can be gathered from
the fact that the “rod fibres” often expand so as to form
chambers in the outer reticular layer, and clumps of matter,
often taking nuclear stains, may be seen in various conditions
within these chambers. The relations of these clumps of
staining matter to the terminal expansions of the “‘ rod fibres”
is not easy to ascertain ; it is clearly necessary to keep them
distinct in our minds. Borysiekiewitz, who, I believe, is
the first to figure these chambers,! took them for a new and
hitherto undiscovered layer of cells, the “nuclei of which may
sometimes be seen dividing.” ‘This description, however, does
not apply to any eye I have yet examined, for I have found
them in all stages of formation, sometimes in patches, some-
times all along the retina (cf. figs. 20, 22, and 24). A com-
parative study has convinced me that they are, as stated,
merely expansions of the inner ends of the “rod fibres ”
round some peculiar mass of staining matter. Similar masses
occur in the cytoplasmic chambers between the rod nuclei
and the outer reticular layer in the frog, as shown in fig. 25,
bandd. I can regard them, therefore, only as form-phases
expressive of some physiological activity, the significance of
which, so far as I have been able to unravel it, will be
explained in a later paper.
But whatever is the real structure of the outer reticular
' Figured in 1887, but only claimed as a new “ cell” layer in 1894.
STUDIES IN THE RETINA. 57
layer, we shall see from what follows that the refractive
matter sooner or later finds it way through it. We shall,
indeed, now proceed to show what very startling effects its
passage may have on the remaining layers.
Every sagittal section of a functional retina will show us
the matter streaming through the middle nuclear layer and
through the inner reticular layer, in which latter, however, the
streams frequently lose themselves. Indeed, as must be
apparent by this time to every student of the retina, | am
putting an entirely new interpretation upon a very familiar
phenomenon, viz. the ‘“ Miller’s fibres.” These, as is well
known, have hitherto always been regarded as sustentacular,
and are said to be formed out of distinct cells with recognis-
able nuclei. Buta survey of many eyes and of eyes of the
same kind at different ages, and of the same eye at different
parts and in different physiological conditions, shows beyondall
mistake that they are only streams of absorbed pigmentary
matter finding its way through the retina. The current
doctrine that they are sustentacular has been based solely
upon their appearances when most developed. Well-de-
veloped streams may be found at almost any age, inasmuch as
their development depends upon the degree of functional
activity of the retina; but according to my experience they
are found in this condition most frequently in very old eyes,
as we shall see in detail below.
This, then, is the next point we have to demonstrate; the
chief difficulty in the way of doing so is how to select from
the abundance of the evidence only that which is the most
conclusive.
First of all, it is best at the outset to record the obser-
vation that the refractive matter seems to be temporarily
arrested by all cytoplasmic strands and membranes which are
arranged tangentially, and only to form definite streams along
strands arranged radially. Hence the rapidity with which
the refractive matter passes through the retina depends upon
the number of suitably disposed radial strands. From the
rods to the outer reticular layer most of the strands are
58 H. M. BERNARD.
radial, e. g. “the rod and cone fibres,” hence accumulations
of amorphous matter seldom take place in this layer. On
reaching the outer reticular layer there occurs, as described,
some temporary obstruction, the exact nature of which we
have not attempted here to unravel. Through this reticular
layer, however, the matter escapes. In young eyes with a
plentiful cytoplasmic reticulum supporting the rows of nuclei,
radial strands can be found in abundance to carry the matter
through the middle layer to the inner reticular layer; but in
older eyes, when the nuclei of the middle layer have been
largely used up and the cytoplasmic reticulum is so reduced
that but few radially disposed strands can be found, the re-
fractive matter tends to accumulate often in large quantities
(m.n. of the figures). In fig. 20, b (blenny), it is seen in small
irregular patches ; in fig. 24, a (trout), in thick tangential
strands just above the outer reticular layer; in others,
again, in immense tangentially arranged sheets. In fig. 23,
a and b (plaice), the accumulations are near the outer
reticular layer; in fig. 21 (cod) near the inner reticular
layer. Many more figures might have been given, but
these must suffice. Fig. 23, a, which was from a very
large old plaice,! should be compared with fig. 22, a,
b, c, which are from a young plaice, six inches long. In
the least used-up part of the retina (c) no traces of these
accumulations can be seen; in b they are beginning ; in a they
are already of considerable size, but in the very old fish they
are enormous, and occur over most of the retine. Of the few
traces which I have so far seen of accumulations of matter
in the retinze of mammals one is shown in fig. 30, where a thick
strand runs along on the inner side of the outer reticular
layer of a mouse which had been exposed to the light of an
arc lamp. As it tapered away it gave off typical ‘“ Miiller’s
fibres”? in the way figured (see also p. 37 and fig. 26).
Before going on to the inner reticular layer, one word as to
the supposed nuclei of the “ Miiller’s fibres.” These are nothing
‘ Specially selected for these researches by my friend the late Mr. Martin
Woodward, while temporarily associated with the Irish Fisheries.
STUDIES IN THE RETINA. 59
but the ordinary nuclei of the middle layer, and are used up
like the rest. The appearances which have led to the sup-
position that they are nuclei of fixed morphological strands
are due to the fact that single nuclei are not infrequently
involved in these streams of matter, and, indeed, may at
times apparently enter into some intimate physiological
association with them. ‘'hey may often be seen drawn out,
and even at times robbed of their chromatic substance (see
fie. 25, c). That they are not the nuclei of preformed sus-
tentacular fibres follows from the fact that a comparative
study shows that no such preformed structures exist, and that
the so-called “Miiller’s fibres” are mere expressions of
functional activity, and great numbers, even when best
developed, have no such involved nuclei (fig. 32, a).
Coming to the inner reticular layer, this also, like the
middle nuclear layer, undergoes changes with age (cf. 7.7,, figs.
20, a and b, and 24, wand b). In very young eyes the reti-
culum is close, and forms what is called the “ Punktsubstanz.”
As soon as the eye begins to function, before which time
there are no “Miiller’s fibres,” streams of refractive
matter begin to pass through it as very thin radial threads.
Under a high power these are seen to be a fine zigzag; they
are clearly not independent strands, but some staining matter
running along the threads of the inner reticular layer. Fur-
ther, they may branch or end suddenly in thin, tangentially
arranged layers, from which new radial strands arise to run
further in. Again, itis evident that these thin radial strands,
which every one would at once call the ‘‘ Miiller’s fibres,” are
not fixed structures, from the fact that in the retina of an
older animal of the same kind (cf. figs. 20, a and b, 24,@ and
b)! they may have disappeared altogether, and instead there
occur thicker streams finding their way in much coarser zig-
zags(fig.21) along the strandsand between the much more open
1 The specimens of the viviparous blenny were fixed in Blés’ fluid
in the St. Andrews Marine Laboratory, and kindly given me by Mr. Wallace,
who had prepared them for his own work, The trout were specially fixed for
these researches by Dr. Kyle, also of St. Andrews.
60 H. M. BERNARD.
meshes of the now altered reticulum. This fact is an absolute
demonstration that these“ Miiller’s fibres” are not independent
preformed structures, but merely cytoplasmic threads of the
retinal reticulum thickened with matter. When they run
quite straight without any zigzag we must regard it as due
to a gradually acquired radial rearrangement of the threads
of the reticulum (cf. fig. 32, a and b).
Then, again, apparently at any point in the inner reticular
layer, these strands may end suddenly, and the staining
matter which was travelling along them may disperse
to right and left (fig. 28,c). Many of the different aspects
of the inner reticular layer are due to the presence of
this refractive matter accumulated in different ways along
its strands. One phenomenon is particularly suggestive; I
refer to the darker zones which are frequently seen in it
running for longer or shorter tracts round the retina. ~ I
have seen them frequently (see figs. 20, a, 22, b, c, 24, a).
Borysiekiewitz has also called attention to them. These
dark zones are, as it were, waves of absorbed matter, records
of former periods of functional activity, passing through the
retina. This is not evident microscopically when the reti-
culum is a close “ Punktsubstanz,” and the matter finely and
evenly dispersed, but becomes quite obvious when the reti-
culum is coarse and open, for then the individual strands of
the affected part can be seen specially thickened (see fig. 24, a).
All these facts become so obvious to any one who will take
the trouble to study the retina comparatively that I feel it
almost unnecessary to discuss the details any further. One
or two points, however, remain to be noted. Just as the
streams end almost anywhere in the inner reticular layer, the
matter dispersing along the tangential strands, so fresh ones
may begin anywhere within the same layer. And this brings
us to the next layer, the nerve-fibre layer, or, as it is more com-
monly but less accurately called, the “ ganglionic cell layer.”
The appearance of the strands which run radially from the
inner reticular layer to the membrana limitans interna is well
known ; they are the typical inner ends of the“ Miiller’s fibres.”
STUDIES IN THE RETINA. 61
Usually comparatively thin as they leave the inner reticular
layer, they expand into a conical arrangement of strands or
membranes until they look in some sections like an arcade of
expanding columns supporting the internal limiting mem-
brane with its subjacent reticulum. Under the arches of
this arcade are found the strands of the optic nerve, and the
“ oanglionic cells.’ My faith in the sustentacular
character of the “ Miiller’s fibres ” was first shaken by finding
that in many of my preparations the majority of these
columns arise from the edge of, or from various depths
within the inner reticular layer itself, and that those which
did so did not apparently differ from those which came
through the inner reticular layer from the outer layers. It is
quite apparent that when they arise from the edge of the
inner reticular layer, they are in a position to collect and
carry away matter from that layer (figs. 20, 6, 21, 24, a,
26, b, 27). It is common also to find them arising in one of
so-callec
the darker zones above referred to, and when once the
suggestion is made that they are, as it were, draining the
inner reticular layer, a flood of light is thrown upon all
their various shapes, for the typical arcade form I have
described, though frequently found, is not invariable.
In my preparations of the retina of a large cod, for instance,
the typical expanding columns are somewhat rare, so
that the matter, not carried away fast enough, clogs any
strands or membranes running tangentially ; see fig. 21, in
which it coats the strands (n.s.) supporting the nerves.
Solid accumulations of this matter are, however, not often
found in the nerve-fibre layer, although the clotting of the
nerve and other strands which partly occupy the layer may
be very dense (see fig. 28, b, from an old rat). Something
more like solid accumulations are found in certain old eyes;
e.g. figs. 26, a, b, and 28, a, show the absorbed matter
accumulating within the conical expansions of the “ Miiller’s
fibres,’ sometimes causing them to change their forms and
become nearly bell-shaped—the trumpet shapes shown in
fig. 26, b, are apparently due to distortion of the sections.
62 H. M. BERNARD.
But these accumulations and the clottings of strands and
membranes are not sufficient to account for the lifelong
streamings of refractive matter into this layer, and we should
have to assume that it escaped finally through the internal
limiting membrane to join the vitreous humour, even if the
microscope did not clearly show us that this is what actually
takes place.
In very few sections will the internal membrane be seen
quite thin and clear; it is usually found thick and apparently
laminated, and layers are frequently found flaking off into
the hollow of the eye. hat these flakes are, at any rate in
part, due to the matter which comes along the “ Miiller’s
fibres” can be seen in the fact that in osmic acid preparations,
in which these streams are usually blackened, the portions
of the internal membrane which cover their conical expan-
sions not infrequently show different degrees of blackening
(see fig. 29, a). This shows that the refractive matter is
certainly deposited on the internal limiting membrane. The
question is, Does it pass through? It certainly passed into
the retina through the external protoplasmic membrane,
pushed out in the form of rods it traverses the whole thick-
ness of the retina, and if it does not pass through the exactly
similar protoplasmic membrane on the inside of the retina it
ought to accumulate in large quantities. The only accumu-
lations which we actually find in connection with this mem-
brane are the above-mentioned lamin, which, as is well
known, belong to the vitreous humour. Some sections,
indeed, show the stained ‘ Miiller’s fibres,” looking like so
many processes rooting the similarly stained remains of the
vitreous humour into the retina. And here let me say
that absolute microscopic demonstration of subtle physio-
logical processes may not be possible as so many separate
details, but when all the facts are taken together the evidence
may become as convincing as if we could prove each detail
separately. This particular detail, however, namely, that the
refractive matter absorbed by the rods passes ultimately into
the vitreous humour, admits of demonstration.
STUDIES IN THE RETINA. 63
This demonstration is afforded us by the fact that in the
baboon’s eyes the pigmented matter retains its colour right
through the retina, being only slightly less bright and
refractive near the internal limiting membrane, where it is
present in enormous quantities. In the youngest baboon’s
retina the congealed vitreous humour was left in situ in the
base of the retinal cup, and appears in the sections. Its
layers nearest the retina are coloured like the
pigmented matter on the retinal side of the internal
limiting membrane.!
Returning to our review of the passage of the matter
absorbed by the rods through the retina, we have seen that
if, instead of the matter having to travel along zigzag paths
on the strands of the cytoplasmic reticulum, it found a sufficient
number of radial strands running in continuous courses right
through, the passage would be much simplified. All the accu-
mulations of matter which we have described in the eyes of fish
might be avoided. ‘he most perfect radial strands which I
have ever seen running through the inner reticular layer occur
in sections of a human retina” which, from the scarcity of the
nuclei in both the nerve-fibre layer and the middle nuclear
layer, and from the condition of the inner reticular layer, I
take to be that of an old individual (see fig. 32, a). Itis hard
to believe that such “ Miiller’s fibres” as these were not per-
manent structural elements ; if they were they had become so
only during life, and to meet special functional requirements,
for in the normal healthy retina of a man of forty-eight,
referred to above, hardly a single straight radial strand can be
found through the whole inner reticular layer. Faint zigzag
streams alone occur here and there (fig. 32, 6), but are not
numerous. ‘hat there should be no pronounced “ Miiller’s
' As this absorbed matter streams through all parts of the retina (except
the blind spot), and during a lifetime of functioning, it is clearly a factor
which no student of the vitreous humour can afford to ignore. It suggests,
for instance, a new and very simple explanation of Stilling’s canal.
2 Purchased many years ago from Messrs. Watson, of Holborn, in a series
of slides to illustrate the structure of the eye.
64 H. M. BERNARD.
fibres,” i.e. streams of matter passing through the inner
reticular layer, in this eye, is just what we should expect, in
view of the fact that for some days prior to excision it had
not been exposed to light; but it is surprising that there
should be no traces of any permanent rearrangement of the
cytoplasmic reticulum so as to form continuous radial lines.
It is possible that this only takes place in very old eyes, when
both nuclei and cytoplasmic framework, all but its radial
strands, seem to be disappearing (cf. the general condition
of the inner reticular layer in fig. 32, a, with that in 32, b).
In the baboon’s retinas, through which an enormous
quantity of matter can be seen to have been passing, and in
which the large inner limbs are filled with the same matter,
all of it the same colour as the pigment, the conditions are as
follows :—In the youngest retina (three months) thick
yellowish-brown streams in immense numbers pass radially
through the compact middle nuclear layer, but when they
reach the inner reticular layer by far the greater number
disappear; the few which seem to run straight through
that layer, on examination with a high power, are seen to have
avery zigzag and interrupted course. On the inner side of
this layer dense streams again form and run towards the inner
limiting membrane, expanding and losing their intensity
before reaching it.
In this young baboon’s eye, then, there are no clear radial
arrangements of the fibres of the inner reticular layer which
could, even under the most strained interpretation, be re-
garded as sustentacular.
In an adult male baboon the same is true, only the pigment
is blackish. We again see what was described above for
other retinas, that the reticulum of the inner reticular layer
has become much coarser than in the younger eye, and
consequently the zigzag of those streams which run con-
tinuously through is much more pronounced.
In an “old, very large male” the streams are still fewer
in the inner reticular layer, apparently because every strand
is clotted with pigmented matter, as is also every strand and
STUDIES IN THE RETINA. 65
membrane between this layer and the internal limiting mem-
brane where the dark brown of the pigmented matter is very
dense. Individual streams can hardly be followed.
In these eyes, then, again, and in spite of the quantity of
the pigment absorbed, we find the same difficulty as we found
in the cod (ef. fig. 21, 7.7.) in establishing direct radial paths
for the escape of the absorbed matter through the cytoplasmic
reticulum ; that such paths do occur and may be very highly
specialised we know from the (presumably old) human retina
shown in fig. 32,a. These, seen alone, certainly appear as
if they were sustentacular.
Other perfect radial tracks seem to occur normally in the
Amphibia, for as soon as the eye begins to function, that
is in quite young tadpoles, there arise distinct, smooth,
nearly straight radial fibres through the inner reticular
layer, and these become so tough in preservation that
they can be isolated intact if a section 1s teased up or crushed
on aslide.! Further, in tangential sections they often appear
running through the inner reticular layer within a clear
passage. It is possible that this clear passage may be delu-
sive, and due to the fact that the adjacent parts of the
reticulum are drained by them of any matter which would
render their delicate cytoplasmic membranes or threads
visible. Compare with these apparently clear courses of the
“ Miiller’s fibres ” through the inner reticular layer, fig. 29, b,
where “ Miiller’s fibres” of a rabbit are shown cut trans-
versely, and the tangential threads or membranes of the
inner reticular layer thickened with matter are seen to be
running into them.
But the important contrast comes later. The establish-
ment of direct radial streams through the inner reticular
layers in the young tadpole is quite natural, for we remember
that in other eyes the nearest approach we found to a straight
course was in young retinas (see fig. 24,b) when the inner
reticular layer is a close “ Punktsubstanz.” But whereas the
' Many of them with nuclei of the middle layer attached to them (see
fir. 25, c).
voL. 46, pART 1.—NEW SERIES. E
66 H. M. BERNARD.
streams in these fish retinas become more and more zigzag as
the meshes of the inner reticular layer get larger and coarser,in
the frog and toad, for some reason or other, the early straight
paths appear to become fixed. Whether this can in any way
be correlated with the other peculiarity pointed out in these
amphibian retinas, viz. that the absorbed matter passes by,
apparently without entering, the inner limbs of the rods, which
consequently remain very small, we are not yet in a position
to decide. It is, of course, quite possible that the physical
condition of the absorbed matter not coming in contact with
the staining matter in the inner limbs might be different, and
consequently its action on the cytoplasmic framework of a
retina might also be different.
It need hardly be pointed out that if a group of such
streams as those shown in fig. 31, a,b, flowing through the layer
of rod nuclei were to combine in or just after leaving the outer
reticular layer (0.7.), and then flow on as one thick stream
through the middle nuclear and the inner reticular layers, we
should have the most developed type of “ Miiller’s fibre,’ such
as that shown in fig. 32, a. Itis these most developed streams,
looking as ifthey were permanent structural elements inthe eye,
which have alone been regarded as typical ‘‘ Miller’s fibres.”
Had all the minor forms of the same streams received equal
notice, the error could never have been made of ascribing to
them any fixed morphological significance. Such a wider
survey would also have saved Borysiekiewitz, to whose works
on the retina I should like here to express my indebtedness,
from his conclusion that the “ Miiller’s fibres” are tubes
conveying the nerve-fibrils to the rod layer.! Only these most
developed strands which seem to rise directly from the rods
could possibly supply the necessary conditions, and, if this
conclusion were correct, we ought to find such developed
strands in all and throughout all the retinas of the whole of
the Vertebrata. This, as we have seen, is very far from
being the case. Equally mistaken, too, are the conclusions
based upon the impregnation method. In Ramon y Cajal’s
1 “Weitere Untersuchungen,’ Leipzig und Wien, 1894.
STUDIES IN THE RETINA. 67
well-known figures of “ fixed morphological elements” re-
vealed by the method, we find not only the “dendrites,” but
also the ‘‘ Miiller’s fibres ” in their most developed form, and
the “rod fibres” with their terminal swellings all equally
clearly shown. The interpretation which we have put upon
the latter two makes it more than probable that a proportion
at least of the “‘ dendrites”’ are also nothing but the parts of
streams already so frequently alluded to in the foregoing
pages. I say a proportion of the “dendrites” for reasons
which will be made clear in another paper, in which I shall
also show that the nerve-paths through the retina can be
demonstrated by ordinary methods of staining, and that they
have no connection whatever with the “ dendrites.”
SuMMARY.
As the results so far attained in the preceding five parts of
these studies are largely hidden under a mass of minute histo-
logical detail, it is better, at this stage, that a summary be
given of the more important. In the next paper we shall deal
with the question of the nerves, which naturally has a much
wider bearing than any detail of retinal structure merely as
such.
The conclusion which of all others now arrived at is of
widest significance from a general point of view, is that the
retina can no longer be regarded as built up of so many
separate “cells,” each with some definite and permanent
morphological value. his view, which has always been taught
hitherto, has recently to all appearance been strongly confirmed
by means of the metal impregnation method. This appears
to reveal several distinct types of cells mainly distinguishable
by their positions and by the different forms assumed by the
ramifications of their respective cytoplasms. It is now main-
tained, indeed, that these cells, to which special names have
been given, have distinct and definite functions, so that if one
68 H. M. BERNARD.
single one were absent, a blind spot would ensue as a neces-
sary consequence.
The results here published, obtained solely by comparisons
not only of different eyes but of the same eye at different
ages, involve a direct contradiction to this interpretation of
the phenomena. If there ever were distinct cells com-
posing the retina, their walls were early lost.1 The func-
tional retina is a continuous cytoplasmic reticulum
in which nuclei are suspended, and the nuclei are
not stationary. (1) A large proportion of those which are
present in the young retina move outwards when it begins
to function to become the nuclei of the new rods required by
growth. (2) Their places are supplied by others migrating
inwards from the rim. (3) The outward movement continues
as long as life lasts, for in old eyes the nuclei of both the
innermost and the middle nuclear layers are found to have
largely disappeared. Whether 3 is for the supply of
new rods or for some regenerative process we have no
means yet of deciding. ‘These migrations, and especially
this using up of the nuclei, in a retina which is all the while
functioning normally, shows clearly that some other value
must be assigned to its structural elements than that which
is needed by the neuron theory as applied to this organ. It
is clear that these nuclei are not the nuclei of cells taking
part in fixed morphological chains, every link of which
is essential. ‘he nearest approach we obtain to anything
like a permanent cell in the retina is the rod with its nucleus ;
that it would be inaccurate to persist in using the term
“visual cell” in this connection will be conclusively shown
in my next paper.
With reference to the retina itself as the specific organ of
vision, by far the most important result obtained is the
discovery of some new details relating to the origin and
structure of the rods, that is of those structures which are
peculiar to the retina as the visual organ. According to the
1 What appears to be the gradual dissolution of cell walls may often be
seen where the young retina is passing into the cells of the iris.
STUDIES IN THE RETINA. 69
usual description they are of the nature of cuticular forma-
tions. ‘This is a very natural summing up of the facts—
(1) that they are almost certainly the end organs of the
nerves, and (2) that their tips are filled with refractive
matter of the nature of keratin. But the parallel with
cuticular cells, although justifiable, is not very close. As
protoplasmic vesicles thrust out against the pigment cells
they absorb the pigment granules and (unless the quantity
absorbed be too great, and its colour too intense) clarify them
somewhat as the stratum lucidum of the epidermis receives
and clarifies the pigment brought to it through the skin.
Here, however, the parallel ceases, for while the cells of the
cuticle perish with the waste matter they receive, and ulti-
mately fall away as horn-cells, the rods get rid of their refrac-
tive contents, which stream away through the retina.
The working out of the finer structural details of the rods,
taken up where the subject was left by Max Schultze thirty
years ago, need not be repeated here, but one or two of the
more important corrections of the current doctrine may be
mentioned.
What are called the ‘‘cones” of the vertebrate eye, to
which special functions distinct from those of the rods have
been assigned, are not always analogous structures.
In the Amphibia they are the early stages in the forma-
tion of new rods, and their form-phases are due to the
squeezing of new vesicles between the already existing rods.
In the fish analogous stages appear in very young eyes,
but in older eyes the inner limbs of the earlier formed rods
swell to such monstrous sizes that the conditions of the rod
layer are altered, and the protrusion of new vesicles can no
longer result in the formation of the same cone stages.
The rods with the swollen inner limbs have been regarded as
“oiant cones,’ although presenting no analogy whatever
with the cones in the frog.
In the Primates, what are usually called the cones are, as
in the fish, merely rods with swollen inner limbs. In the
centre of clear vision, where the pigmentary matter is
70 H. M. BERNARD.
absorbed in large quantities, all the elements are permanently
of this character, but away from the centre only one here
and there has its inner limb enlarged. Borysiekiewitz refers
this to the protrusion of the nucleus, but as the nucleus is
not always protruded, I prefer to refer it to an extrusion of
fluid from the retina. Not only does the early protrusion of
fluid vesicles from the retina in the first stages of rod-
formation make this probable, but also the fact that globules
of fluid are continually escaping from the retina into the
rods, as described and figured in Part II.
The striation of the rods, which has long been known, has
now been traced to its true cause, viz. the existence of
strands, sometimes taking stain, in the walls of the rod
vesicles, while the lumina of these vesicles are occupied by a
staining network in connection with these strands.
The refractive matter which fills the outer limbs
of the rods is absorbed pigment, which is usually,
but not always, clarified during the process of
absorption. ‘he correlation of this with the results of the
classical researches of Boll, Kuhne, Ewald, and others I am
not in a position to work out, for reasons given in Part V. It
must be left to time, on the one hand, to show where we
mutually confirm one another, and, on the other, to eliminate
our respective mistakes. Had I commented on all the results
obtained by previous workers whenever they overlapped the
subject in hand, these papers would have been lengthened
out indefinitely ; as itis, the histological details given in them
have had to be limited to a small selection of those available,
‘he curious zone formation within the retinal syncytium
has been traced largely to the above-mentioned lateral move-
ment of the nuclei of the middle layer from the rim towards
the centre.
The “ Miiller’s fibres,” however startling they may appear at
their highest development, are merely streams of the pigment
matter which have been absorbed by the rods, and which,
with many interesting variations of detail, pass inwards
through the retina, eventually to join the vitreous humour.
STUDIES IN THE RETINA. 71
EXPLANATION OF PLATES 3—5,
Illustrating Parts III, IV, and V of Mr. H. M. Bernard’s
paper on “Studies in the Retina.”
N.B.—The measurements of the different eyes can only be approximate,
because the shape is not always kept in very thin sections. If should be
further noticed that sometimes a slightly older eye may be smaller than one
obviously younger, a fact to be attributed to the accidents of nutrition.
In all the figures m./. = limiting membrane, g./. = ‘‘ ganglionic cell”
layer, a7. = inner reticular layer, m.z. = middle nuclear layer, 0.7. = outer
reticular layer, 0.z. = outer nuclear layer.
Fic. ].—Frog tadpole (Perenyi’s fluid). Eye diameter 0°32 mm. Part
of section showing the spindle-shaped nuclei of the undifferentiated rim of
the retina, attached to either the inner or the outer limiting membrane, and
arranged on the axial side in curves bulging towards the axis of the eye.
The arrow indicates the direction of the nuclear stream. A few of the
nuclei already in the middle layer, selected because attached by trailing cyto-
plasm to the inner reticular layer.
Fig. 2.—Toad tadpole (Lindsay-Johnson’s fluid). Eye diameter 0°528 mm.
Part of section drawn with the camera lucida, to show the attachment of the
inner reticular layer to the membrana limitans interna, this connection being
apparently due to the nuclei trailing their cytoplasmic attachments behind
them as they travel towards the axis of the eye. These nuclear attachments
tend to accumulate on each side of the stream, but persist as an accumulation
only on the inner side (see text, pp. 1O—13).
Fic. 3.—Frog tadpole (picro-sulphuric and iron hematoxylin). Eye dia-
meter 0°24 mm., to show a younger stage in the formation of the inner
reticular layer. The nuclei, which in the central region are loosely arranged
where this layer is beginning to form, are densely crowded, five to six deep, against
the pigment, aud are appareutly pressing inwards from the undifferentiated
rim where nuclear divisions (&.) are taking place. The arrows indicate the
direction of the streaming; 0.7. indicates a line of dark staining matter where
the future outer reticular layer will run.
Fic. 4.—From the other eye of the same animal. In both these eyes yellow
fluid was apparent in the pigment cells, and here and there also apparently in
vesicles which appear among the pigment granules, and were probably pro-
truded from the retina (cf. Fig. 18).
72 H. M. BERNARD.
Fics. 5 anp 6.—Frog tadpole (picro-sulphuric). Still younger eyes showing
earlier stages in the formation of the inner reticular layer as a kind of splitting
of the nuclear ranks into two divisions, those forming the larger division
crowding outwards against the pigment, leaving a loose, matted, and staining
reticulum in the space from which they have moved. In both sections it is
noticeable that nuclei in the very centre have even gone from the innermost
layer. In Fig. 6 the cornea (c.) is seen thinning and clearing of pigment over
the axis of the eye, and a nuclear division (4.) is seen near the centre of the
retina, the two facts together indicating that the eye was only just beginning to
function.
Fic. 7.—Frog tadpole (picro-sulphuric). Eye diameter 0°20 mm. Shows a
still younger stage (i.e. smaller, and with larger cavity in the lens). The
crowding outwards of the nuclei in the optic axis not yet appreciable ; the
beginning of the split among the loosely arranged nuclei is, however, indicated
by an accumulation of vacuolar reticulum along the line occupied later by the
inner reticular layer; nuclei seem also to be breaking away from the innermost
layer, that forming the later so-called “ ganglionic cells.” A slight curving of the
lateral nuclei, like that shown in Fig. 1, is alsoseen. The yolk granules which
obscure the section are not indicated either in this or in the last two figures.
Fie. 8.—Frog tadpole, from Table Mountain (Perenyi). Hye diameter
0°S mm.; cross-sections of rods showing deeply stained internal reticulum ;
this changes its pattern when the focus is changed. The reticulum is some-
times forced to the sides by a refractive greyish mass, which at times may
have a brownish centre of the same colour as the pigment. In these cases
the reticulum frequently comes again into view on changing the focus.
Fic. 9.—From the same retina, showing the distal ends of the cones as
vesicles, often torn, but nearly always leaving ragged proximal ends still
attached to the conical tips of the staining portion; in other cases the vesicles
are complete, and their distal ends are immersed in pigment ; they are shrunken
and often beaded with rows of dots. On comparing the elements marked with
an asterisk and numbered 1 to 7, we can trace the transformation of a cone
into a Schwalbe’s rod.
Fie. 10.—The same, in which the relations are shown more completely.
In both these figures the continuation of the vesicle membrane into that
enveloping the “cone”’ is quite distinct. On the right is a new element with
no staining proximal portion yet visible (cf. Part I, Pl. 3, fig. 2, a).
Fic. 11.—The same, showing the more direct transition of the ‘ cone ” into
the rod, nearer the side of the retina where the elements are shorter (namely,
20 p instead of 45 to 50 p, as they are in Fig. 10). In this case the distal
portion of the cone or new rod was visibly striated, which was not the case on
the very young and still slightly swollen Schwalbe’s rod shown in Fig. 9 on
the extreme right.
Fic. 12.—From the same retina as Fig. 1. A group of elements in the
STUDIES IN THE RETINA. 73
early vesicular stage, the large vesicles not destroyed by the reagents. The
exact relations of the nuclei cannot be made out. The nuclei of the two
youngest vesicles may, perhaps, be those shown in or on the outer reticular
layer.
Fic. 13.—A few elements from the same, selected to show some of the
form-changes from vesicles to rods. Most remarkable are the ** cones’ shown
on the left (a, 6,c). Their tips are quite clearly within vesicles. The pheno-
menon is explained in the next figure (14). The rod on the extreme right
shows one of the bright staining globules referred to in Part II.
Fie. 14.—A diagram to explain the “ cones” shown in Figs. 13 and 15, in
which their tips are thrust into terminal vesicles. The pressure of new pro-
trusions acting in the direction of the arrows converts the vesicle a into d
and ¢ with a progressively lengthening neck. The staining matter which
accumulates in this neck (see the central element in Fig. 12) is then thrust
outwards (1) by the outward movement of the nucleus, and (2) by a fresh
discharge of material (d). These “cone” tips, therefore, have no other
membrane than the vesicle into which they are thrust. They are therefore
sometimes disintegrated, and without defined outline.
Fic. 15.—Frog tadpole (Perenyi). Hye diameter 0°-4 mm. Still shorter
and stouter elements showing the same phenomenon, taken at different dis-
tances from the centre of the retina. The formation of short thick rods out
of vesicles can be easily understood.
Fic. 16.—From the retina of a frog tadpole (picro-sulphuric, safranin). Eye
diameter 0°24 mm., showing early protrusion of vesicles against the pigment.
Deeply staining yolk granules are shown here and there; they are left out of
Figs. 5 and 6. ‘Iwo nuclei are shown in the figure not yet in contact with the
membrana limitans externa. They are selected because they show vacuoles
inside. Intra-nuclear vacuoles and vacuoles extruded within the retina may be
seen in all the sections of young eyes; the earliest phase of the inner reticular
layer looks, indeed, like an aggregation of such intra-retinal vacuoles (cf. Fig. 7).
Fies. 17 anv 18.—From the retina of another frog tadpole of about the same
size (from 0°20 mm. to 0°25 mm.). Portions of sections differently magnified,
showing more vigorous protrusion of vesicles against the pigment. ‘Those
quite in the pigment are often yellowish in colour, while those nearest to the
nuclei are clear white, partially framed round with densely staining matter
(iron-hematoxylin). In Fig. 18 the vesicles marked with asterisks were
yellowish. In other slides the yellow fluid, which here appeared in vesicles,
was certainly inside the pigment cells as well (cf. Fig. 4).
Fic. 19,—Frog tadpole (Perenyi). Eye diameter 0°8 mm.; a group of
nuclei crowded outwards. The membrana limitans externa was not distinguish-
able; its probable position is indicated by the marks of interrogation. Where the
crowding was seen, the nuclei of the middle nuclear layer was diminished from
74 H. M. BERNARD.
four deep to only three (cf. the observation of Borysiekiewitz described in the
text, p. 2).
Fic, 20.—From the viviparous blenny (Bles’ fluid). a. Section of young
retina, showing the early stage, with ordinary cones, Schwalbe’s rods, and two
adult rods forming a double rod. Note the layer of cells at 0.7. and the dark
zones in 7.7. The nucleim.z. were about eight deep. 4. From old specimen of
same. The rods in two kinds only—(1) double rods with enormous inner limbs
(22 p by 144), “ giant cones,” and (2) Schwalbe’srods, ‘The layer of nuclei at
o.r. has disappeared, and most of those from m.z. The 7.7, is coarse-meshed,
and very few strands run radially through. c. From the same, to show the
tight packing of the swollen inner limbs, or “ giant cones.”
Fic. 21.—Section of the retina of a very large old cod (corrosive sublimate).
A“ giant cone,” with inner limb 42 by 22 p, and filled with refractive globules.
Crowds of Schwalbe’s rods packed in between the ‘ giant cones.” The rod
fibres end in terminal conical expansions against the or. The nuclei (m.z.) are
nearly all gone, while the m.v, layer itself is choked up with solid sheets of amor-
phous, finely granular matter, some of which is seen streaming away in a zigzag
through the ¢.7. ‘The nerve-strands (z.s.) are clotted with finely granular matter.
Fic. 22.—a—e. Section of the retina of a small plaice, 6” (Flemming), to
show three parts of the same section, a being nearest the centre. To illustrate
the gradual using up of the ‘‘ ganglionic cells”’ (g/.) and of the nuclei of the
middle layer (z.z.) ; the disappearance of the continuous layer at 0.7. (the nuclei
of Krause’s “ Membrana fenestrata’’), and the increase in the rows of rod
nuclei; the gradual accumulations of granular matter in the m.z., and the
swelling of the inner limbs,
Fie, 23.—a, 6. From a very old plaice. a. Radial. 4. Tangential, showing
the enormous accumulation of amorphous matter, extending in this old fish
almost completely round the retina.
Fic. 24.—a. Sections of retinas of Loch Leven trout, ca. 7’” to 8” (Perenyi).
b. Section of very young trout, var.? To show the change of the ¢.7. from
early “ Punktsubstanz,” with fine zigzag “ Miiller’s fibres’ running radially
through it, to the coarse-meshed older condition, and with the threads thickly
coated with matter in the outer zone. Similar thickened strands are seen just
within the 0.7.
Fie. 25.—From tadpoles from Table Mountain, and characterised by great
abundance of pigment. The inner limbs remain small, aud the “ Miiller’s
fibres’ rise straight from the walls of the rods. In é.d.e., just above the
rod nuclei, are seen the dark-staining bodies which seem to correspond
with those seen within the expansions of the ‘‘ rod fibres ” seen at. 0.7. in some
of the foregoing figures, e.g. 20, 6; 22, a, d (cf. text, p. 32).
Vic. 26.—a, d. From the retina of an old cat (13 years) (lemming), to show
the inner expanded ends of the “ Miiller’s fibres,” containing accumulations
STUDIES IN THE RETINA. 75
of amorphous matter. In 6, exactly similar bodies arise entirely
from the inner edges of the zr. The trumpet shape is due to accidental
compression of the section.
Fic. 27.—From a very old dog, showing the same as Fig. 26, 4, i.e. the
inner expanded ends of the so-called ‘‘ Miiller’s fibres” arising entirely from
the ir. Matter obviously belonging to the vitreous humour is seen flaking off
the membrana limitans interna.
Fig. 28.—From a rat, 3 years old (Flemming). a. To show the flaking off of
matter from the membrana limitans interna; the flakes show a delicate texture ;
the ends of the “* Miiller’s fibres ” filled with amorphous matter, and rising from
various depths in the z.7, 4. To show the thickening of all strands and mem-
branes indifferently, with matter coming from the 2.7. e¢ shows ‘ Miller’s
fibres” both losing themselves and starting again within the ¢.7.
Fic. 29.—From a rabbit (Hermann). a. Froma tangential section, to show
a portion of the membrana limitans interna divided into differently darkened
areas representing the covers of the conical expansions of the ‘* Miiller’s fibres.”
6. Two conical expansions with cross-sections showing the “ fibres ” as solid, and
with the tangential strands of the ¢.7. running into them,
Fic. 30.—From a mouse which had been exposed to an electric arc lamp.
A thick strand of amorphous matter winds its way through the cytoplasmic
reticulum just inside the o.7., eventually giving off, and bending up into
typical ‘ Miller’s fibres.”
Fie. 31.—From a chacma baboon. «@. From an old male, with dense
black pigment, which not only half fills the large inner limbs of the rods, but
streams inward as far as o.r, without changing colour. J. From a young
specimen, 3 months old, with yellowish-brown pigment, which also streams as
far as the 0.7. without changing its colour.
Fie. 32.—From the human retina. @. From a purchased preparation ; appa-
rently of an old retina, showing au enormous development of the ‘ Miller’s
fibres,” a very coarse inner reticular layer, and very few nuclei in the mz.
5. From a “normal healthy ” retina of a man 48 years old, which had appa-
rently been little used for some time prior to excision. Typical ‘‘ Miiller’s
fibres” wholly wanting. Faint streams, often zigzag, and parts of
streams alone occur, many of them processes from the so-called “ ganglionic
cells.” The m.z. was five nuclei deep; compare the number in a. But for
this latter comparison to be of weight we ought to know how far from the
centre of the retina the parts shown were severally taken.
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RELATIONS OF KIDNEYS IN HALIONMS TUBERCULATA. 77
Notes on the Relations of the Kidneys in
Haliotis tuberculata, etc.
By
Hi. J. Fleure, B.Sc.,
U.C.W., Aberystwyth ; Fellow of the University of Wales.
With Plate 6.
Noumerovs accounts of the structure and relations of the
kidneys of Diotocard Gastropods have been written, chiefly
by workers interested in the question of the homology of the
Monotocard kidney, but in the various descriptions occur
several contradictory statements, which naturally lessen the
value of current theories on the subject. The present paper
is the first-fruits of a study of the gastropod kidney, and it
endeavours, by throwing new light on one or two disputed
points, to help on the solution of this difficult problem.
There are also brought forward certain suggestions concern-
ing the kidney and reproductive organs of Monotocards which
I feel convinced should be studied together; these sugges-
tions are naturally extremely tentative, pending further
work. In making this communication I wish to express my
deep indebtedness to Professor Ainsworth Davis, whose
advice and encouragement have alone made this research
possible.
Haliotis possesses structures right and left of the peri-
cardium (8), which, notwithstanding various views concerning
their relations, are generally allowed to be the representatives
of the right (7 x) and left (7 1) kidney of the primitive Dioto-
78 H. J. FLEURE.
card. The structure on the left of the pericardium is a small
sac, whose walls contain, in parts, lymphatic tissue; it is
doubtfully renal in function, and is called the papillated sac.
That on the right side is the functional kidney, and it
possesses various large lobes, including a long anterior
one (A. L.) stretching forward on the left flank of the great
shell muscle. All workers also find two openings at the back
of the mantle cavity : one (1) placed definitely on the left side
of the rectum, the other (2) further towards the right.
Von Jhering (1), Perrier (6), and Weemann (4) consider
that the right opening (2) is the orifice of the functional
kidney (7 R), and the left one (1) that of the reduced left
kidney (71) or papillated sac. They state that the gonaduct
(from 6 R) opens into the right kidney (7 Rr). Haller (9)
found that the right kidney (7x) communicated with the
other (7 1), and that both opened by the orifice (1) of the
left kidney, the right orifice (2) belonging solely to the
gonaduct. My observations on these points agree with those
of Perrier, who also worked with Haliotis tuberculata.
Haller used H. glabra, and it would be necessary to study
that species before rejecting his statements. Perrier, Weg-
mann, and Hrlanger (8) described a pericardial communica-
tion for the papillated sac, but not for the functional kidney ;
while Haller found a funnel opening from the pericardium
into the right kidney, but was unable to discover an internal
orifice to the papillated sac. The former result has been
generally accepted, and it is quoted as evidence that the
right kidney is really degenerating in the Rhipidoglossa, and
that the kidney surviving in Monotocards is therefore the
Diotocard left (71). The methods adopted by these authors
were the usual ones of dissection, injection, and section
cutting, but in this case there seems to be another line of
investigation. It is well known that the Diotocard female
liberates her ova in an irregular fashion a few at a time into
the mantle cavity. Now, if the gonaduct communicates with
the kidney, ova might be found at a certain season in the
cavity of the latter; and, further, if this kidney has a peri-
RELATIONS OF KIDNEYS IN HALIOTIS TUBERCULATA. 79
cardial opening stray ova might even make their way through
it. It therefore seemed to me advisable to examine specimens
of Haliotis taken during the breeding season.
Haliotis tuberculata may be obtained in fair numbers
round the coasts of the Channel Islands, more especially on
the rock-strewn shores of Guernsey and Sark, where it passes
its sluggish life attached to the under-side of large boulders.
It therefore lives with ventral surface uppermost, and is said
by the fishermen often to die if removed from its attachment
and left in the reverse position. It frequents the upper part
of the Laminarian zone, and seems to feed largely on small
aleze. ‘lhe breeding season in this locality I have found to
extend from about the end of December to the middle
of February, and the specimens used for this investigation
were collected in Guernsey during the spring tides of that
period. They were soaked in 5 per cent. formalin, and
mostly examined within a few days of their capture. The
specimens were carefully taken out of their shells, and before
they were placed in water or dissected at all their pericardia
were opened on the left side well away from the kidney wall.
The contents of the pericardial fluid were then examined,
and found to consist mostly of corpuscles, a few epithelial
cells, and sundries. One specimen, however, yielded a
pinkish fluid, in which floated several ova; while two or three
others also yielded each a few ova in the same way. ‘lhe
ova are very different in appearance from the other peri-
cardial contents, and from the components of the various
tissues abutting on the pericardium. ‘They seem to retain
their ovarian covering and a short stalk for a considerable
time, very few having been found without them. The
nucleus is prominent, and there is a small granular accumula-
tion usually near the short stalk (see fig. 4, b).
An interesting feature in the female at the breeding time
is a characteristic pink coloration more or less diffused over
the whole body, but most noticeable on the covering of the
hepatic czecum, on the pericardial wall, on the head above
the tentacles, and on the floor of the mantle cavity.
80 H.f Jt) BEEGRE
After sampling the fluid contents of the pericardium the
contents of the right kidney were examined, and proved in
several cases to be roughly divided into two sorts of material
—darker brown fluid with excreta, and lighter coloured fluid
containing both excreta and ova. Just behind the opening
into it of the oviduct the kidney is partially subdivided by
an internal projection of its right wall. The two parts are
respectively an anterior one containing ova as well as excreta,
and a posterior one containing almost solely the latter
(see fig. 4).
From these finds of ova it seems justifiable to conclude—
a. That the gonaduct opens into the right kidney (6R
into 7R).
b. That the right kidney has a pericardial pore (5).
c. That the anterior part of the right kidney is becoming
connected more particularly with the reproductive system.
Injections of the right kidney were rather unsatisfactory,
as might be expected considering its large size ; but injection
from the pericardium, on the other hand, showed very dis-
tinctly a pericardio-renal communication (5) near the anterior
right-hand corner of the pericardial cavity (8).
By careful dissection of uninjected specimens from the
pericardial side, an opening was found high up on the right
wall of the pericardium, near its anterior right-hand corner.
A fellow-student, Miss A. Ritchie, kindly confirmed this for
me in another specimen. ‘I'he opening as seen in dissection
seemed fairly distinctly lipped, and is possibly imperfectly
valvular; it is situated near the point where the duct-like
portion of the kidney may be said to begin (5).
Further along toward the external opening, the wall of the
functional kidney comes near that of the papillated sac; I
have not, however, been able to find any interrenal communi-
cation such as Haller describes for Haliotis glabra.
Despite numerous attempts, by dissection, by examination
of contents, and by injection, I have not been able to find
evidence of a pericardial communication with the papillated
sac, a conclusion in accord with that of Haller, but not with
RELATIONS OF KIDNEYS IN HALIOTIS TUBERCULATA. 81
that of Perrier and Erlanger. The wall of the sac is very
thin, especially where it overlies the branchial vein. When
it abuts on the pericardium it is also thin-walled over a part
of the area, but the remainder is thickened into a mass of
lymphatic tissue. ‘There are vascular connections taking
blood to the efferent branchial vein and thus to the auricle
direct. Perhaps some one of these is what has been taken
for a pericardial communication of the papillated sac.
To sum up, therefore, I think that Haliotis tuberculata
has two separate kidneys right and left of the pericardium,
opening externally by separate apertures (1 and 2). I find,
also, that the gonaduct opens into the right kidney, which is
the functional excretory organ, while the left kidney is partly
degenerating into lymphatic tissue, and is becoming con-
nected with the efferent branchial vein by direct blood-
channels. So far my results agree with Perrier’s. I find,
further, in opposition to Perrier, and in agreement with
Haller, that the right or functional kidney communicates with
the pericardium (via 5), while the left one does not. The
evidence adduced is, in part, of a different nature from
that brought forward by the authors mentioned.
The foregoing results, if correct, lessen the divergence
hitherto supposed to exist in this respect between Haliotis
and Patella. The limpet has two kidneys right (7 R, fig. 3)
and left of the rectum. The right kidney is very extensive,
and performs most of the excretory work; it has several lobes,
including a subrectal one (s. R. L.), which abuts on the wall of
the pericardium. It serves as an exit channel for the repro-
ductive elements, but evidence I have collected recently
seems to hint at liberation of ova, at any rate, by rupture
as well. This matter, however, needs further investiga-
tion.
The small left kidney is situated between the rectum and
the pericardium, its circulatory system connects it intimately
with the auricle. It is not shown in the diagram.
The right kidney communicates with the pericardium (8),
the opening (2) being in the floor of the kidney’s subrectal
voL. 46, PART 1.—NEW SERIES. F
82 Ho 33 FLEUR:
lobe (s. R. L.), but opinion varies as to a pericardial pore of the
left kidney, the latest statement being that of Mr. E. S.
Goodrich (11), who makes certain he has found it.
Mr. Martin Woodward has recently published a valuable
account (14) of Pleurotomaria Beyrichii, in which he
says that the right (7 R) and left (71) kidneys of that animal
are in many respects comparable with the corresponding parts
in Hahotis. The efferent duct of the right kidney (7 R to 2)
is prolonged forwards, and has thick glandular walls in the
female, so that it is practically an oviduct. Woodward
found a pericardial opening and canal (4) for the left kidney
but not for the right, a result which, if confirmed, makes
Pleurotomaria an exception to the general rule.
The most primitive type of kidney in Diotocards is, how-
ever, that of Cemoria described by Haller (9). Here both
kidneys are well developed and functional, each communi-
cating with the pericardium (vid 4 and 5), and each receiving
genital products from the gonad of its side. If this type is
truly primitive we can, as Haller has said, derive from it the
excretory organs of Pleurotomaria, Haliotis,and the Trochide,
from which series the Docoglossa and the Fissurellidee would
be fairly early offshoots. Throughout, the left kidney and
the left gonad degenerate, whilst the right kidney becomes
both the functional excretory organ and the exit channel for
the sex products. The right kidney retains, in most forms,
its pericardial pore.
Perhaps the most marked contrast between the Diotocards
and their Monotocard descendants is the presence of
numerous accessory genital organs in the latter and their
complete absence from the former group. In the latter, also,
the reproductive and excretory systems are entirely separate,
We must therefore seek out hints of the coming change
among the ancestral forms.
Mr. Woodward has shown that the excretory duct of the
right kidney of Pleurotomaria is practically transformed, in
the female, into an oviduct (vide fig. 2).
In Haliotis the large anterior lobe (A. L., fig. 4) of the right
RELATIONS OF KIDNEYS IN HALIOTIS TUBERCULATA. 83
kidney is, practically, an accessory genital organ in posse.
The external opening (2) of the right kidney is evidently
becoming a genital pore.
Trochus and Turbo vary very much in this respect, but in
some species the “anterior lobe ” 1s very sharply marked off
from the rest of the kidney. There is the same conflict of
opinion between Perrier and Haller about the reno-pericardial
funnels in T'rochus as in Haliotis, and Haller finds also an
interrenal communication in Trochus gibberosus. With
regard to these reno-pericardial funnels it is noteworthy that
in all Teenioglossa, even in the most primitive (figs. 6 and 8),
and in Nerita there is a well-marked communication between
kidney and pericardium on the right side of the latter—a
fact which strongly supports the view here put forward, that
the right reno-pericardial pore is retained, as a rule, in the
Rhipidoglossa.
If the right excretory pore (2) becomes monopolised by the
genital system, the functional kidney must find an exit for its
excretory products; and it seems probable that this exit is
through the external opening (1) of the left kidney, which
would thus be the homologue of the Monotocard excretory
aperture. The probability of this is increased by the fact
that in no Monotocards have traces of a pore or sac been
found to the left of the kidney opening. ‘This view, however,
entails the further supposition that the mght kidney, or
rather its posterior part, comes to communicate with the left
kidney, and Haller claims, as was mentioned above, that such
a communication already exists in Haliotis glabra and
Trochus gibberosus. Perrier contradicts Haller, though
he, too, supposes that the two kidneys come to communicate ;
he, however, almost certainly errs in stating that the Monoto-
card kidney opening is the right one (2) of Diotocards, for
this statement raises a serious difficulty as to the homology
of the genital opening.
Bouvier found such an interrenal communication in Am-
pullaria (5), and Perrier justifiably uses this observation in
support of his views above mentioned. He further supports
84. Hi. J: FLEURE:
his conclusions by observations on the “renal gland.” He
brings forward much evidence in favour of considering this
gland, so generally found in Tznioglossa, as a modified vestige
of the left kidney which has become intimately connected
with the “ pericardial gland.” This gland consists of tubules
which are lined by ciliated epithelium and open into the renal
cavity (see fig. 9, b). The absence of this renal gland from the
primitive Paludina suggests the hope that further work will
reveal traces of the old left kidney in a less modified condi-
tion. The probability that further work will result in the
discovery of the above-mentioned interrenal connection in
other forms is increased by the fact that such connections
are by no means unusual in Mollusca. They exist in several
Lamellibranchs and in Cephalopods, and, without presuming
to suggest that they are homologous throughout, their occur-
rence diminishes the improbability of their occurrence in
Gastropods.
The remaining problem is the derivation of the accessory
reproductive organs of the Tzenioglossa and of their descend-
ants the Opisthobranchs and Pulmonates.
The male has, typically, a large penis (vp) at the right side
of the head ; this organ is retractile in more primitive forms
(Paludina, fig. 8), but permanently extruded in more special-
ised forms (Buccinum, fig. 10). A very similar structure is
found in the hermaphrodite Opisthobranchs, though their
hermaphroditism has been shown by Pelseneer (10) to be due
to the development of a male gonad in the female.
Mr. J. E. S. Moore (12) found an archaic form in Lake
Tanganyika, which he named after its abode—Tangan-
yikia rufofilosa. The female of this animal possesses a
brood pouch (8. P., fig. 5, a) on the left side of the head in the
position of the penis of a male Paludina. He found the same
structure in Melania episcopalis, and both also possessed
a groove connecting this pouch with the genital aperture.
This strongly resembles the spermatic grooves of some
Opisthobranchs, and similar grooves also exist in the females
of some Teenioglossa (figs. 8 and 9), while in the males the
RELATIONS OF KIDNEYS IN HALIOTIS TUBERCULATA. 809
spermatic duct (s.D.) running to the penis along the floor of
the mantle cavity (fig. 10) or situated internally (fig. 8) has
probably been formed by the covering in of such a groove. A
groove of this kind is found in the male T'anganyikia, but the
penis is absent (fig. 5, b).
From these facts Mr. Moore has argued that the common
ancestor of T’enioglossa and Opisthobranchs possessed some
accessory reproductive organ which had probably become
separated from the genital duct and remained connected
with the genital opening by means of a groove, which tended
to become covered over. ‘This accessory reproductive organ
was somewhat variable, as it is lost in all Teenioglossate
females except the two named, though the groove is retained
in a few. In Opisthobranchs, which are originally female,
and in male Teenioglossa this organ becomes the penis, while
the groove is very often covered over and thus transformed
into a duct. The development of such an accessory repro-
ductive organ de novo is a difficulty further enhanced by the
presence of the groove, but it is still premature, perhaps, even
to suggest that possibly its ancestor is the anterior lobe of
the Diotocard right kidney. It is interesting to note that
Typhobia horei has a penis (fig. 7 P) which is extruded
apparently vid the genital aperture, and which is placed as an
anterior dilatation on the reproductive duct.
To sum up, it will be most appropriate to give a brief
statement of the views of previous workers and of the chief
points raised in this paper.
The first theory is that of Professor Ray Lankester. He
believed that the excretory aperture of Monotocards is the left
kidney opening of Diotocards, but he thought also, from its
position with regard to the rectum, that the Monotocard
kidney (7) was the Diotocard left (71). Since these views
were stated, the supposed absence of a pericardial pore of
the right kidney has been used as evidence of the degeneracy
of this organ, and, consequently, in favour of homologising
the Monotocard kidney with the Diotocard left. Hrlanger’s
work on the development of Paludina is also quoted in
86 Hisst PLEURE:
support of this homology, but the structure which he takes
to be the forecast of the Diotocard right kidney is merely a
problematic and very transient vestige. The right kidney, on
Lankester’s view, would become part of the reproductive
system.
Haller thought that the reproductive organs developed a
duct which became continuous with the right kidney duct and
opened through what was previously the right kidney
aperture. The left kidney, he thought, degenerated, but
became connected with the right, so that the Monotocard
excretory organ was mainly right kidney, but opened through
the left kidney’s aperture. According to him, only the right
kidney retained a pericardial pore.
Perrier agrees with Haller’s conclusions except as regards
the pericardial communication, which, he holds, persists only on
the left side. He differs also in a point of great importance,
for he says that the Monotocard kidney opening is the
Diotocard right. His principal contribution to the discussion
is the tracing of the fate of the left kidney. This, he found,
became the renal gland, consisting of tubules lined by
ciliated epithelium, and opening into the kidney cavity. This
gland was, he said, typically associated with that ancient
ce
molluscan feature, the ‘pericardial gland.” The conclusions
supported in this paper are—
1. Lankester’s view that the renal aperture of Monotocardsis
the left one of Diotocards. This opposes Perrier’s conclusion.
2. Perrier’s and Haller’s view that the twokidneys in some
Teenioglossate ancestors came to communicate inter se.
3. Lankester’s and Haller’s view that the rght kidney
opening becomes the genital aperture. ‘This opposes Perrier’s
conclusion.
4, Haller’s view that the right kidney retains its peri-
cardial communication in most Diotocards. This opposes
Perrier’s conclusion. The evidence adduced is partly new.
5. Perrier’s and Haller’s view that the Monotocard kiduey
is composed of the right kidney of Diotocards, together with
the cavity of the left (whose walls form the renal gland).
RELATIONS OF KIDNEYS IN HALIOTIS TUBERCULATA. 87
This I would slightly modify by stating that it is the posterior
part of the Diotocard right kidney which seems to me to
become the functional part of the Monotocard kidney.
6. I would also suggest, in a very tentative fashion, that
perhaps the forecast of the accessory reproductive organ,
which becomes penis or brood pouch in the Monotocards,
was originally a dilatation on the reproductive duct. Perhaps
even an earlier condition of this organ is what has been called
the anterior lobe in the Diotocard kidney.
After outlining these conclusions I saw Mr. Martin Wood-
ward’s paper on Pleurotomaria Beyrichii. Ina short
discussion at the end of his paper he speaks in favour of con-
clusions identical with Nos. 1, 2,3, and 5 above, but from his
observations on Pleurotomaria he sides with Perrier as
regards 4. liven, however, had i not found a reno-pericardial
communication for the right kidney in Haliotis, I think there
would still have been a balance of evidence from Fissurella,
Patella, perhaps from Trochus, and especially from Nerita,
Nassopsis, Paludina, etc., in favour of the view that the
Monotocard reno-pericardial opening is that of the Diotocard
right side.
More work on the Trochide and Neritide is necessary for
any further advance towards certainty in the matters above
discussed, but it is interesting to note that Nerita has a
single kidney with a well-marked pericardial opening on the
right side of the pericardium, while the external aperture is
to the left of the rectum. The genital system, described by
Haller, is quite separate from the excretory organ, and lies to
the right of it. The kidney has no anterior lobe correspond-
ing to that of Halotis. The chief interest of the Trochide, etc.,
arises from the theory put forward by Perrier, Bouvier, and
others that these forms are very near the ancestral stock of
the Monotocards. Mr. Woodward, however, does not seem
to share this opinion,
It has been a difficulty to represent, in diagrams I1—10 the
true relations of the anal, excretory, and genital openings,
and Professor Davis therefore suggested to me the addition
&8 A: cd. REEURE
of schematic cross-sections of the mantle cavities of various
forms, showing the rectum and oviduct or right kidney duct
cut through, and the relative position of the left kidney or
the Monotocard kidney opening.
It will be seen that the openings of right and left kidneys
in Diotocards have the same positions, with reference to one
another, as the genital and excretory apertures in Monoto-
cards. The relation of these openings to the rectum, on the
other hand, varies to some extent in different forms.
The most striking feature of these diagrams (11—15) 1s
the migration of these openings to the animal’s right side,
and we also notice the disappearance of the right ctenidium
and the folding over to the left side of the originally right
leaf of the other ctenidium.
In the primitive Gastropods, as in Cephalopods, the in-
coming streams of water entered the gill cavity on either
side and bathed the gills, after which they made their way out
along the median line of the cavity, taking away the excreta
from the openings on this line (fig. 11). Later on, the left
kidney degenerated, being perhaps partly pressed out of
existence, and the right kidney became the sole functional
excretory organ. ‘This process, already begun in Haliotis,
was correlated with the disappearance of the ctenidium from
the right side, and to the shifting of anus, renal, and
reproductive openings to this side. The respiratory stream in
such forms (figs. 12—15) would now come in along the left
side and go out past the anus, etc., along the right, in-
current and excurrent streams being thus freed from mutual
interference. This more perfect separation, Professor Davis
thinks, was the advantage which led to natural selection of
variations along the lines of the changes just mentioned. This
clockwise shifting of apertures and ducts has in some cases
been continued so far that one or more have become situated
along the extreme edge or even, in some cases, on the floor
of the mantle cavity, so that their original relations appear,
in horizontal plan, to have been reversed. Paludina shows a
further modification, for here the reual opening is situated
RELATIONS OF KLIDNBYS IN HALIOTIS TUBERCULATA. 89
between the anus and the genital aperture (fig. 15), but this
peculiarity may be connected with the development of a ureter.
APPENDIX.
Since the above paper was written I have had an oppor-
tunity of seeing Pelseneer’s recent publication ‘ Les
Mollusques archaiques,’ and as he touches here and
there the questions of kidney homologies, a short discussion
of the work of this eminent scientist will add to the com-
pleteness of this little paper.
The chief new facts which he brings forward all support
the conclusions I have ventured to set forth.
1. He finds that, in the Trochide, the right kidney has a
communication with the pericardium, and, as I have found
the same feature in Haliotis, arguments for the degeneracy
of the right kidney of Rhipidoglossa, based on the absence of
its pericardial communication, are definitely demolished.
Pelseneer also finds the distinction between anterior and
posterior regions of the right kidney of Trochus which has
been mentioned in this paper as regards Haliotis.
2. He finds that Haller’s interpretation of the kidneys of
Cemoria was based upon errors of observation. As he has
carefully examined specimens both from the source to which
Haller had recourse (the Vettor Pisani collection) and
from the White Sea, there seems to be little doubt that
Pelseneer is right. If so, Cemoria, in its excretory and
genital systems, resembles the typical Fissurellid. Its right
kidney is of enormous extent, while the left is quite tiny,
and only the right gonad is found.
This result is of great interest, as it is now possible to say
that in all known Rhipidoglossa the left kidney is either ex-
tremely reduced or has undergone a transformation into
a “papillary sac,’ an alteration which has profoundly
affected its minute structure and its circulatory arrange-
ments. It is therefore still more difficult than before to
90 H. J. PLEDRE:
homologise the left kidney of these forms with the single
kidney of Monotocards.
Towards the end of his paper Pelseneer briefly discusses
this question of the homology of the Monotocard kidney.
He first of all sets aside Haller’s views, as every sub-
sequent worker has differed from that writer regarding the
relation of gonaduct and kidney. Proceeding next to discuss
Perrier’s theory, he points out the following weak points :
1. It necessitates the supposition that the rectum and right
kidney have undergone relative translocation. This he finds
difficult to imagine.
2. If we accept Perrier’s further conclusion that the Mono-
tocard nephridial gland is the remains of the papillary sac,
we are forced to assume that in some type the kidneys came
to communicate inter se. Having examined Ampullaria,
Pelseneer denies that such a connection exists in that type.
Like the late Mr. Woodward, I cannot see that these
objections are really vital. Perrier’s theory of the nephridial
gland seems to have much in favour of it, as also has the
idea that while the Monotocard kidney is that of the Rhipido-
glossan right side the opening is that which formerly be-
longed to the left kidney, but these are not essential to the
theory. If, pending further evidence, we leave aside these
additions, the whole of Pelseneer’s second objection dis-
appears.
As I have said further above, the frequent existence of
the required interrenal communication in the more primitive
molluscan classes seems to me to minimise the difficulty.
The question of translocation of the rectum is raised and
discussed in rather a new light in this paper, and I venture
to think that the conclusions reached very markedly diminish
Pelseneer’s objection. The rectum has undoubtedly shifted
a great deal to the right, such shifting being far more
important for the cleanliness of the ctenidium, and therefore
for the efficiency of the branchial cavity, even than the
rightward shift of the kidney.
On the whole it does not seem too much to say that, not-
RELATIONS OF KIDNEYS IN HALIOTIS TUBERCULATA. 91
withstanding the views he expresses, Pelseneer’s results in this
matter tend to strengthen the theory supported in this paper.
If this theory be adopted it is possible to say, now
Pelseneer has settled the “ Cemoria”’ difficulty, that in all
Anisopleura the (post-torsional) right is par excellence
the excretory side, even in those forms which still have a
median anus. ‘his opens up a possibility of very great and
far-reaching interest as regards gastropod morphology.
Pelseneer’s theory of the gastropod twist is now generally
accepted as amended in the matter of terms by Amaudrut
and others.
According to this the far-off untwisted ancestor had a gut
going straight from front to back. This underwent —
1. A ventral flexure, giving the gut a cephalopod-like
disposition.
2. A lateral torsion through 180° in a counter-clockwise
direction, affecting all the animal except the head and foot.
Asaresult of this torsion the branchial cavity and anus,
previously postero-ventral, became antero-dorsal.
It is generally allowed that the pre-torsional position of
the branchial cavity militated against its efficiency in a form
possessing a creeping foot, for it would be pressed down
against the top of the foot by the weight of the shell.
Natural selection, therefore, led to the upward shifting of
the cavity by survival of upward variations of its position.
As far as I am aware, no one has yet shown how it is that
the twist is counter-clockwise, and this has been an undoubted
weakness in the theory; but the difficulty is, I think,
removed by a consideration of the excretory organs and
ctenidia on the lines suggested in this paper.
Let it be granted that in all Gastropods which have under-
gone the torsion the right is the excretory side of the
branchial cavity, the left being more particularly devoted to
respiration.
Then it is at least possible that this differentiation was
already established during or before the torsion. It is not
sufficient to argue that this is unlikely because the two
92 H. J; FUEEBURE:
ctenidia of Cephalopods are equivalent, for the Cephalopods
have increased the efficiency of this branchial cavity by a
device of their own, which permits the retention of the
kidney and anal openings in their ancient position. The
early Gastropod had to adapt itself to a shore life, where the
branchial cavity was not as easily rinsed as in the more
pelagic ancestor, and where, therefore, the excretory pro-
ducts tended to spread over and interfere with the efficiency
of the ctenidia, a tendency very imperfectly counteracted by
the appearance of the slit in the shell. It was therefore
desirable that any possible separation of incoming and
outgoing currents should be encouraged. The reduction of
its excretory function by the post-torsional left (pre-torsional
right) kidney promoted this kind of separation between the
incoming current of that side and the median outgoing
one, and thus made the ctenidium of this side the more
efficient.
Probably long before this the pre-torsional left side had be-
come more especially connected with the genital function, for
we find such a condition in practically all Cephalopods and
Gastropods. As the genital products in the ancestral Gastro-
pods were expelled through the kidney, the renal opening of
this side was very important. This explaims why it is that
when one kidney diminishes it is that of the pre-torsional
right (post-torsional left) side.
In such a form slight clockwise variations of the position
of the branchial cavity would—
(a) Place the more efficient ctenidium in a less suitable
position nearer the median line where the blockage of the
foot would be most felt.
(b) Place this ctenidium at a lower level than the anus
and excretory openings, and thus make it likely to get soiled
and hampered by fecal and excretory matter falling on it,
there being no such powerful outgoing clearing currents as
in Cephalopods.
Counter-clockwise variations would, on the other hand—
(a) Place the more efficient ctenidium always in a better
RELATIONS OF KIDNEYS IN HALIOTIS TUBERCULATA. 93
position in the sense of being further out of the chink
between foot and shell.
(Lb) Place this ctenidium ata higher level than the anus
and excretory openings, and thus assure it against damage
from the outgoing current.
It may be urged against the suggestion—
1. That, as Boutan says, we must not consider the excre-
tory, etc., organs in discussing the torsion, because in
ontogeny (in Acmiea) the torsion is completed before the
definite appearance of kidney rudiments. The torsion, how-
ever, entails such a serious disturbance of organs that its
appearance in ontogeny is peculiarly liable to be hastened,
for the earlier it appears the smaller is the derangement.
2. That it proves too much ; in other words, that accord-
ing to it the pre-torsional left ctenidium should have dis-
appeared before the torsion was complete. ‘This ctenidium
has disappeared in most Gastropods, but its occasional
persistence is not a serious difficulty. In the first place its
diminution would be retarded by the fact that its possessor
was adapting itself to a life on the shore, where the time
for breathing dissolved oxygen would be limited, thus making
even a less efficient breathing organ temporarily valuable.
In the second place this ctenidium persists mainly in forms
which have evolved on special lines :
(a) Among the Fissurellide, where the deepening of the sht
and further changes have shortened the path of the outgoing
current, thus reducing the possibility of its interference with
the incoming one.
(b) In the Haliotide, which have certainly come off from a
very primitive prosobranch stock. Here, too, a secondary
downward tilting of the other side of the branchial cavity
has given this ctenidium an improved position.
(c) In modern Pleurotomariz. The primitiveness of these
forms is well known, and their very deep-water habitat seems
likely to make respiration more difficult and so encourage
the retention of all available respiratory tissue.
3. The other objection is that it accounts for little more
94, Here ene
than the first 90° of the torsion. This objection is to some
extent valid, but I think the process of completion of the
torsion is correlated with the evolution of the shell-muscle,
which I am at present endeavouring to investigate.
If the supposition that the pre-torsional left side performed
most of the excretory function thus enables us to solve
satisfactorily the mystery of the counter-clockwise torsion,
this is surely a strong argument in its favour. I therefore
venture to hold, even more strongly than before, the view
supported in the earlier part of this paper, though it is not
in harmony with the opinion of a zoologist of M. Pelseneer’s
eminence and insight.
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2. B. R. Lanxesrer.—Article “ Mollusea,” ‘ Encye. Brit.,’ 9th edition.
3. J. T. Cunnincuam.—‘ The Renal Organs of Patella,’ ‘Quart. Journ.
Mier. Sci.,’ 1883.
4. H. Weemann.—“ Contributions a |’Histoire naturelle des Haliolides,”
‘Arch. de Zool. exp.,’ 2me série, t. il, 1884.
5. B. L. Bouvrer.—‘ Etude sur l’Organisation des Ampullaires,’ Mém.
publ. par la Société Philomathique, 1888.
6. Remy Perrirr.— Le Rein des Gastropodes Prosobranches,’ ‘Thése
présentée a la Faculté des Sciences de Paris, 1889.
7. RK. von Ertancer.—‘ Zur Entwickelung von Paludina vivipara,”
‘Morph. Jahrb.,’ Bd. xvii, 1891.
8. R. von Ertancer.—‘‘ On the Paired Nephridia of Prosobranclis,”
‘Quart. Journ. Mier. Sci.,’ June, 1892.
9. Bera Hatter.—‘ Studien tiber Docoglosse und Rhipidoglosse Proso-
branchier,’ Leipzig, 1894.
10. P. Petsenrer.— L’Hermaphroditisme chez les Mollusques,” ‘Arch. de
Biol.,’ t. xiv, 1895.
11. E. 8S. Goopricu.— On the Reno-pericardial Canals in Patella,” ‘ Quart,
Journ. Mier. Sci.,’ vol. xli, 1898.
12. J. BK. S. Moorr.—‘‘ The Molluses of the Great African Lakes,”
‘Quart. Journ. Mier. Sci.,’ vols. xli and xlu, 1898 and 1899.
RELATIONS OF KIDNEYS IN HALIOTIS TUBERCULATA. 95
13. Arnotp Lane.—‘ Lelirbuch, etc.,’ ‘ Mollusea,’ Jena, 1900.
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EXPLANATION OF PLATE 6,
Illustrating H. J. Fleure’s paper ‘ Notes on the Relations of
the Kidneys in Haliotis tuberculata, ete.”
EXPLANATION OF REFERENCE LETTERS, ETC., IN DIAGRAMS.
1. External aperture of left kidney of Diotocards and of kidney of Monoto-
cards. 2. Hxternal aperture of right kidney of Diotoeards and of genital
system of Monotocards. 3. Anus. 4. Left reno-pericardial opening. 5. Right
reno-pericardial opening. 61. Left gonad. 6 R. Right gonad. 7 v. Left
kidney. 7 R. Right kidney. 7. Monotocard kidney. 7 N. Renal gland.
7 7. Tubules of renal gland. 8. Pericardium. a. 1. Anterior lobe of right
kidney. B.P. Brood pouch of Tanganyikia rufofilosa. vp. Penis.
Gr. Groove connecting genital aperture with brood pouch (sometimes vestige
only). Ss. D. Spermatic duct on floor of mantle cavity, probably formed by
covering in of groove. vu. Ureter of Paludina. s.r. L. Subrectal lobe of
right kidney of Patella. mr. c. Renal cavity (Fig. 9, 4). 6. Rectum.
ty. Tissue of pericardial gland, in which renal gland is embedded. ct. Cteni-
dium. ct!. Ctenidium of right side in Haliotis (Fig. 11).
DiorocarDs.
Fic. 1.—lxcretory and genital organs of Cemoria noachina (after
Haller). Contradicted and disproved by Pelseneer.
Fic. 2.—Exeretory and genital organs of Pleurotomaria Beyrichii
(after Woodward).
Fie, 38.—The right kidney of Patella vulgata (after Lankester).
Fie. 4, a—Kidneys, gonad, ete., of Haliotis tuberculata.
Fire. 4, 6.—Ova of Haliotis, much magnified.
Mownorocarps.
Vig. 5, a.—Exeretory and genital organs of the female of Tanganyikia
rufofilosa (after Moore).
96 H. J. FLEURE.
Fic. 5, 4.—Exeretory and genital organs of the male of Tanganyikia
rufofilosa (after Moore).
Fic. 6.—The same organs in Nassopsis nassa (after Moore).
Fic. 7.—Male genital orgaus of Typhobia horei (after Moore).
Fic. 8, a.—Excretory and genital duct in Paludina vivipara—male.
Fic. 8, .—The same—female.
Fic. 9, a.—The samc—female of Littorina litorea.
Fic. 9, .—Section of the hematic gland (pericardial gland and renal gland)
of Littorina (after Perrier).
Fic. 10.—Excretory and genital ducts, ete., in Buccinum undatum.
ScuEmatic Cross-Sections OF MANTLE CAVITIES.
Fie. 1].—Haliotis.
Fie. 12.—Acmeea.
Fic. 13.—Trochus.
Fic. 14.—Littorina female.
Fie. 15.—Paludina female.
In Figs. 5, 8, 9, a, and 10 it is supposed that the roof of the pallial chamber
has been cut along near the middle line and reflexed.
In Figs. 11—15 it is not suggested that the various apertures, ete., occur
in one and the same transverse section.
THE DEVELOPMENT OF PALUDINA VIVIPARA. 97
Notes on the Development of Paludina vivi-
para, with special reference to the Urino-
genital Organs and Theories of Gasteropod
Torsion.
By
Isabelia M. Drummond.
With Plates 7—9.
THESE researches have been conducted in the Laboratory
of Comparative Anatomy at Oxford, under the super-
intendence of Professor Weldon. They were originally
undertaken in order to confirm or correct the account of the
ccelom and its derivatives given by von Erlanger, and the
results obtained are set forth in Part I of this paper. These
then led to a renewed study of the whole course of develop-
ment with a view to obtaining evidence upon the theories of
torsion recently put forward by Pelseneer, Amaudrut, and
Boutan. Von Erlanger’s account deals only with the organo-
geny, and leaves the question of torsion on one side; it is
perhaps for this reason that the development of Paludina
has lately been sometimes regarded as too much modified
and abbreviated to give any clear evidence on this point.
Doubtless great modification has taken place owing to the
loss of the free larval life; nevertheless, bearing this in mind,
it is possible to a large extent so to disentangle the different
processes of development which, owing to abbreviation, here
go on side by side, as to be able to compare the results
VoL. 46, rART 1.—NEW SERIES, G
98 ISABELLA M. DRUMMOND.
obtained with those of the authors above mentioned. In
Part II, therefore, I give a brief account of the development,
aiming not so much at a description of the organogeny,
except in one or two cases where my results differ from those
of von Hrlanger, as at making clear the changes in position
and relative proportions of the organs in successive stages.
Before concluding this introductory note it is conve-
nient to say a few words with regard to the plates. The
outlines of the figures of whole embryos have all been drawn
with a camera lucida from preparations of the whole animal,
which is represented as transparent, the organs being shown
by a dotted line. These have, in fact, been also traced,
where possible, from whole preparations, but the tracings
thus obtained have been added to after a careful study of
sections. Figs. 11 to 17 explain themselves; they are for
the most part transverse sections through the visceral hump,
taken as far as possible through corresponding regions in
successive stages, and all orientated the same way on the
page, that is, as if the creeping sole of the foot were parallel
with the bottom edge, in order to facilitate comparisons.
Allare taken looking from behind forwards,—that is, the left
side of the figure is also the animal’s left side.
Part I.—The Urinogenital Organs.
Von Erlanger (5 and 6), in his account of the developing
coelom and its derivatives in Paludina, made known for the
first time the existence of the rudimentary original left
kidney, and showed conclusively that the existing kidney of
the Prosobranchs corresponds to the definitive left kidney of
other Gasteropods. Moreover, he brought the Prosobranchs
much more closely into line with other Molluscs than had
hitherto been the case, by describing the gonad as a deriva-
tive of the pericardium, and as discharging its products
through a duct which was probably the duct belonging to
the rudimentary kidney. While entirely agreeing to this
extent with his results, | have arrived at conclusions with
THE DEVELOPMENT OF PALUDINA VIVIPARA. 99
regard to the manner of development which differ from those
of von Erlanger in certain important points, and which bring
Paludina even more closely into line with other Molluses in
respect of their ccelom.
According to von Erlanger, the pericardium, while still
showing its two distinct chambers, forms two little evagina-
tions, one on each side, which are the rudiments of a pair of
kidneys. These, fromm the time of their first formation, lie
against the ectoderm, which very soon forms the inward duct-
like prolongations of the mantle cavity. Of these the right
one coalesces with the original right kidney and forms its
duct, while the left is arrested in its growth and the left
kidney disappears. At a later stage a new outgrowth of
the pericardium takes place in the same position as that
which formed the original left kidney, becomes nipped off
from the pericardial epithelium, and forms a little vesicle,
which is the rudiment of the gonad. At the same time
there is an ingrowth of the mantle cavity, which is pre-
sumably the arrested duct of the kidney that has disappeared.
This grows towards the gonad, and finally fuses with it to
form its duct. This account of the origin of the urino-
genital organs has since been confirmed by the more recent
researches of Ténniges (17).!
I have nothing to add to von Erlanger’s description of the
early stages of development of the pericardium and kidneys,
and of their relation to the mantle cavity. The series of
somewhat oblique transverse sections of which one is repre-
sented in fig. 1 shows just such a condition as von Erlanger
describes. In this figure the pericardium is shown with its
two chambers still separated, the right beimg very much the
larger of the two; the first rudiment of the heart, as appear-
ing at h.; and just to the left of this is seen the original right
kidney (r. k.) with its lumen, hardly showing in this section,
communicating with the cavity of the pericardium. In a
' | have, unfortunately, not been able to obtain access to the original paper
by Touniges in the ‘S.B. Ges. Bef. d. ges. Naturw., Marburg,’ for 1899, and
have had to rely upon the abstract in the ‘ Zool, Centralb.’
100 ISABELLA M. DRUMMOND.
corresponding position on the wall of the narrow left-hand
chamber of the pericardium is seen the little left kidney (J. k.),
much less developed than its fellow on the right. In the
next section of the series the solid ends of the two horns of
the mantle cavity are found abutting each against the
kidney of its respective side. It is probably at about this
stage of development that von Erlanger describes the first
appearance of the retrogressive development of the primitive
left kidney. It is, indeed, extremely rudimentary at this
time, and might easily be overlooked, but I cannot find that
it ever wholly disappears; rather it might be said that its
growth is arrested for a time, but at a slightly later stage it
again resumes its development. Still less can I find traces
of real retrogression in the primitive left horn of the mantle
cavity. It ceases to grow, or at least does not grow nearly
as rapidly as the primitive right horn which is to form the
kidney duct, but it always retains its original relation to the
pericardium, with its solid end abutting against the primitive
left ventral corner, and is never, as von Erlanger both
describes and figures, separated from it by a considerable
space (see his pl. xxi, figs. 12 and 13).
As far as can be judged from the relative positions of the
organs, the embryo from which his fig. 12, pl. xxi, is taken
corresponds almost exactly to my fig. F in the drawings of
the whole animal. A section across the visceral hump of an
embryo of this stage is depicted in fig. 15, and shows how
the little left kidney, far from having disappeared, as
von Erlanger describes, is now larger than at the stage when
he believed it to be most fully developed. This figure is
wholly comparable with fig. 1, except for the change in the
position of the organs due to torsion. The heart, now fully
differentiated, is seen at h. in the same position relatively to
the other organs as the similarly marked mass of cells in
fig. 1, and the original right kidney, the definitive kidney of
the adult, is cut across at k., with its duct adjacent to it at
k.d. The original left portion of the pericardium is even
more narrow relatively to the right than is the case in fig. 1,
THE DEVELOPMENT OF PALUDINA VIVIPARA. 101
and in the extreme original left and ventral (now right and
dorsal) corner of it is the left kidney (/.k.), contrasted with
the right in being much less developed, but in every other
respect perfectly comparable to it, and showing exactly the
same relations to both the pericardium and the original left
horn of the mantle cavity as it did before. Dorsally the
pericardium is narrowed to a point beside the liver, and here
a proliferation of cells is just beginning to take place, which
is the rudiment of the gonad (g.).
The same structures are seen further advanced and more
highly magnified in figs. 2, 3, and 4, which are three nearly
adjacent transverse sections through a later stage. The
position of the organs in the body is quite similar to that
already seen, but here only the extreme (original) left-hand
corner of the pericardium (pce.) is cut through. The rudi-
mentary kidney is seen at /.k., now showing a wide lumen,
but having only a narrow communication with the pericardium,
and the solid end of the duct is seen as before at l.m.c.
Fig. 4 shows the rudiment of the gonad (g.), now a well-
developed cord of cells, distinctly connected with the peri-
cardial epithelium; while fig. 3, a section intermediate
between figs. 2 and 4, just cuts through the edge of both
kidney (I. k.) and gonad (g.), and shows their close proximity.
This section is, however, chiefly interesting as showing the
thickening of the coelomic epithelium which connects these
two organs, and very soon becomes indistinguishable from
the gonad. A clear understanding of the position of these
rudiments is important, and will readily be obtained by a
comparison of the above-mentioned figures, especially of
figs. land 15. From these it will be seen that whereas the
kidney is from the first on the original ventral side of the
pericardium, the gonad is a dorsal proliferation, which from
the time of its first formation les close against the liver, the
proximity of gonad and kidney being merely due to the
extreme narrowness of the pericardium in this region. A
comparison of these figures with von Erlanger’s (fig. 5,
pl. xxiii) seems to me to point to the conclusion that his
102 ISABELLA M. DRUMMOND.
gonad (g.) is in reality the rudimentary left kidney, and that
he has missed the true origin of the gonad altogether. I
confess that I cannot fully understand this figure, but as far
as I can make out, the gonad should he in a direction at
right angles to that in which it is shown, if it is to maintain
the relation to the liver shown in fig. 6, and again in fig. 17,
and this would bring it into about the right relation with the
evagination of the pericardium marked g., if we regard this
latter as the rudimentary kidney. This view, moreover,
would account for the discrepancy which exists between
von Erlanger’s account of the origin of the gonad as an
evagination, and my own. ‘The divergence in our descriptions
of the duct is not so easy to explain, but I feel sure that
von Erlanger is not correct when he speaks of the gonad
(the left kidney, according to the present view) growing
towards the ingrowth of the mantle cavity, for these are,
and have been from the first, in the closest connection with
each other (see figs. 2 and 15, l.k. and 1. m.c.).
Fig. 5 shows a further development. The pericardium is
cut across at the extreme right of the figure, from this the
left kidney passes downwards at J. k., and the communication
with the pericardium is still shown at r.p.c.ap. On the left
of the figure is seen the gonad (g.), still solid, but now fused
with the wall of the kidney, so that the little connecting
portion of thickened pericardial epithelium is no longer
distinguishable. The duct is cut at J.m.c., and does not yet
open into the kidney. The exact position of these organs in
the body may be seen in fig. 17, which is a drawing of a
closely adjacent section of the same series. ‘The lettering is
identical, and the gonad, duct, and pericardium are all shown.
It will be seen that essentially the same relations obtain as in
earlier stages, the gonad following the liver, and keeping
always on the inside of the coil.
From this stage onwards very rapid growth of the gonad
takes place, so that it soon reaches the extreme tip of the
visceral hump, and then it takes part in every coil as it is
formed. At the same time it becomes hollowed out, from
THE DEVELOPMENT OF PALUDINA VIVIPARA. 103
the apex downwards, till its lumen is finally put into commu-
nication with that of the kidney, the opening being very
close to the reno-pericardial aperture. A reconstruction of
these organs from a series of transverse sections through an
embryo, with a well-coiled visceral hump, is shown in fig. 6.
The gonad (g.) is a hollow tube widening considerably at the
apex, in reality following the coils of the visceral hump, but
shown here spread out. The left kidney (l.k.) forms, as it
were, merely the proximal extremity of the gonad (g.), from
which it is separated at this time by no sharply marked histo-
logical differentiation. In this particular specimen the reno-
pericardial aperture (7, pc. ap.) is retained even at this late
stage, and I have occasionally found it in other embryos of
about the same age; more often it appears to be closed, but
it is difficult to tell for certain which is the normal condition,
as the opening is small and might become artificially closed
during preservation. In this case, however, the close proxi-
mity of the reno-pericardial and the reno-gonadial apertures
is well seen. Even at this late stage there is as yet no com-
munication between the left kidney and its duct, but the
walls are now even more closely fused than before, and it is
obvious where the exact point of communication will be.
Details are shown in figs. 7, 8, and 9, which represent three
sections through the same embryo from which the reconstruc-
tion was made. Fig. 7, taken across the line a a in fig. 6,
shows the left kidney (/. k.) with its opening into the pericar-
dium (r.pce.ap.), and its blind end lying against the duct
(l.m.c.), and nearly opening into it. Fig. 8 is the next
section, taken across the line } b, and again shows the new
pericardial aperture. Finally, fig. 9 is a section across the
widened extremity of the gonad at ¢ c, showing the position
in the narrow space between the liver and the outer
epithelium of the body.
All the essential relations between the different parts of
the genital apparatus are now established as in the adult,
and I have not followed their development in later stages.
It is interesting, however, in confirmation of the correctness
104, ISABELUA M. DRUMMOND.
of this account, to notice von Erlanger’s description of these
organs when they have more nearly attained their adult
condition, and are beginning to show the development of
the actual genital cells. Such stages, he says, show “ dass
bei beiden Geschlechtern ein wenn auch kurzes Stiick der
Leitungswege der Geschlechtsprodukte aus der Keimdriisen-
anlage selbst hervorgeht;” that is, this small region, ap-
parently belonging to and originating from the genital
organ itself, never, in either sex, gives rise to genital cells.
It is, of course, situated just at the junction of the gonad
and the duct, which, as he himself points out, “‘ findet in der
Gegend statt, wo der Verbindungskanal zwischen Herz-
beutel und Nieren sich findet.” Surely this must be the
original left kidney, still distinguishable in the adult.
To sum up, then, the original left kidney and its duct do
not, as von Hrlanger believed, disappear. Their develop-
ment is arrested for a time, but they are both clearly present
at the time when the gonad is formed as a proliferation
from the original left dorsal extremity of the pericardium,
and from this time increase in importance. ‘The gonad is
for a long time solid, and is connected with the kidney by a
thickening of the pericardial wall on the left side. Ata
later stage the gonad becomes hollowed out, and its lumen
communicates with that of the original left kidney, pre-
sumably by means of the pericardial thickening, which must
also have become hollowed out. The genital products there-
fore pass through the original left kidney, and are ejected
by its duct.
The theoretical bearing of these conclusions is obvious, in
that they show how, even in the adult of one of the most
highly organised of the Rhipidoglossa, an unexpectedly
primitive condition of the ccelom and its derivatives still
obtains. Zoologists have long been agreed that the ancestors
of the Mollusca must have had paired gonads, which shed
their products into the coelom, to be carried thence by the
kidneys; that the ccelom is now represented by the peri-
cardium, and that, though great modification has taken
THE DEVELOPMENT OF PALUDINA VIVIPARA. 105
place, a remnant of the primitive condition is found in the
frequent connection between gonad and kidney in existing
forms.
While there is almost perfect agreement with regard to
the general features of the anatomy of the primitive form,
however, there is considerable divergence of opinion as to
the course which evolution has followed, and consequently
various interpretations are put upon the structures of
existing forms, while hitherto embryology has been almost
silent, and evidence has had to be almost entirely drawn
from the field of comparative anatomy. Pelseneer (12 and
13) and Haller (10 and 11) are among the chief writers who
deal with the ccelom and its derivatives among the Proso-
branch Gasteropoda, and still uphold quite different views
upon many points, though they seem agreed in maintaining
that a gradual loosening of the connection between gonad
and kidney has taken place throughout the group. While
Pelseneer, however, only maintains that the point of com-
munication tends to shift away from its primitive position
by the reno-pericardial aperture nearer to the external
opening of the ureter, as is the case in the Lamellibranchs,
Haller regards the connection between gonad and kidney as
altogether severed among the higher Rhipidoglossa, a
portion of the coelom becoming specialised as the gonaduct.
Connected with this is the different view which these two
authors take of the homology of the existing kidney. Thus
Haller regards the functional kidney as in all cases the
right one (after torsion). Among primitive forms (e.g.
Fissurella) this keeps its connection with the gonad, while
that on the left side of the body loses its connection with the
gonad and kidney, and is fast disappearing. Further stages
of evolution are shown by Haliotis, Trochus, and Paludina,
in all of which the left kidney has entirely disappeared, and
the connection between the gonad and right kidney is lost,
while the latter has more and more passed over to the left
side of the body. Pelseneer, on the other hand, has demon-
strated, though this is still denied by Haller, that in both
106 ISABELLA M. DRUMMOND.
the Haliotidee and Trochide a very small definitive left
kidney is present, and that the large kidney is the definitive
right, and still maintains its connection with the genital
organ.
It might seem that von Hrlanger (5 and 6, see also 7) had
already sufficiently demonstrated from embryology that the
homologies which Pelseneer believes to hold for Haliotis
and 'Trochus are equally true for Paludina. To this, however,
Haller (11) objects that in a highly organised form, such as
Paludina, torsion is very likely abbreviated, and the organs may
be formed in their definitive position. This view is, it seems
to me, quite untenable from von Erlanger’s description, while
a further study of the development of this form shows even
more clearly that a complete rotation of the organs through
180° actually takes place in the course of development,! and
that the adult kidney arises on the right, and ends on the
definitive left side of the body. I have, fortunately, been
able to add further to this evidence by showing how the
gonad still stands in close relation with the definitive right
kidney, though this has altogether lost its excretory character,
and that no such separate duct as Haller describes is ever
formed. It seems, then, that there is every reason for
believing that the definitive right kidney has persisted
throughout the Prosobranchia as the genital duct, in some
cases, as in Haliotis, performing also its renal functions,
while in Paludina these latter are carried on altogether by
the left kidney, the right functioning only as a gonaduct.
With regard to the manner of communication between the
two organs, Pelseneer and Haller are also in disagreement.
In the Docoglossa, at least, Haller describes a ventral
coelomic chamber through which the genital products must
pass in order to reach the kidney ; while Pelseneer regards
this so-called ccelom as merely a portion of the kidney itself,
the gonad being in direct communication with this latter, and
altogether separated from the ccelom, which is only repre-
sented by the pericardium. Whether Haller believes in a
For evidence upon this point, see Part IT of this paper.
THE DEVELOPMENT OF PALUDINA VIVIPARA. 107
coelomic connection between gonad and kidney in the primi-
tive Rhipidoglossa similar to that which he describes for the
Docoglossa is not very clear. Most writers, however, have
described the gonad as having become separated from the
coelom altogether, and having acquired a new opening into
the kidney. To this, and also to Pelseneer’s view that this
opening occurs nearer to the external aperture in the higher
forms than in the lower, von Erlanger’s description of
the course of development in Paludina lent strong support.
This, however, has completely failed, for the communication
between gonad and kidney has been shown to be close to the
reno-pericardial aperture in Paludina, as Pelseneer has de-
scribed in Fissurella and other primitive forms, while traces
of an original coelomic connection between the two are found
in the thickened ridge of pericardial epithelium described
above, which can hardly be otherwise interpreted than as
representing a groove in the ccelomic floor along which, in
more primitive forms, the genital products passed to the
reno-pericardial aperture. That the latter still remains open
even after the communication between gonad and kidney is
established is no real hindrance to such an interpretation, for
the solid nature of the rudiments of both the gonad and the
coelomic connection shows that the ontogeny is abbreviated,
and gives no exact picture of the phylogenetic events. he
opening of this pericardial groove into the kidney must, it is
true, represent at least a portion of the reno-pericardial aper-
ture. Phylogenetically, we may believe, the edges of the
groove drew together and a tube was formed, opening at one
end into the gonad and at the other into pericardium and
kidney at once through the reno-pericardial aperture.
When, by abbreviation, this tube came to be formed in the
course of development as a solid rudiment, it is easy to
understand how the hollowing out and subsequent communi-
cation with the kidney might lead to the appearance of a
rupture of the kidney wall. If this interpretation be correct,
we have in Paludina, which has always been regarded as one
of the most specialised of the Rhipidoglossa, a condition of
108 ISABELLA M. DRUMMOND.
the urinogenital organs in every way comparable to that which
obtains among the Amphinoma.
Finally, with regard to the origin of the single asymmetrical
gonad in the Gasteropoda, both Pelseneer and Haller seem
agreed that this has been formed by the fusion of the
originally separate gonads of both sides. Phylogenetically,
of course, this may have been the case, but ontogenetically
there is no trace of it, the existing gonad being formed
exclusively from the extreme left-hand corner of the original
left division of the pericardium.
Summary.—The conclusions at which we have arrived
are as follows:
(a) The embryology of Paludina demonstrates that the
functional kidney of the adult belongs morphologically to
the definitive left side of the body, as von Erlanger has
already pointed out.
(b) The definitive right kidney is not lost, as von Erlanger
describes, but persists as the genital duct.
(c) An indication of the original ccelomic connection
between gonad and kidney is found in the course of develop-
ment of Paludina as a thickened ridge of pericardial epi-
thelium, which finally becomes indistinguishable from the
gonad, and, after it has acquired a Jumen, communicates with
the definitive left kidney close to the reno-pericardial
aperture.
(d) The gonad arises as a solid proliferation of the
morphologically dorsal wall of the pericardium. It arises
from the original left side only, and shows no sign of a
pared origin.
Part II.—The Development of Paludina viewed in Connection
with Theories of Torsion.
(A) Description of Development.
Stage A (fig. 10).—The youngest stage which I have
examined is a bilaterally symmetrical, oval embryo, with
THE DEVELOPMENT OF PALUDINA VIVIPARA. 109
well-developed velum, and already a slight swelling ventrally,
which is the rudiment of the foot. The chief points in the
anatomy are shown in fig. 10, which is a sagittal section
through an embryo of this stage. The gut (st.) 1s a simple
sac opening posteriorly by the anus (a.), and ending blindly
anteriorly where it abuts against an insinking of the
ectoderm (m.), the rudiment of the stomodeum. No very
clear differentiation of parts is yet visible in the gut, but the
ventral wall begins to show the vacuolated structure charac-
teristic of the liver at a later stage. Dorsally to the stomo-
deeal invagination the velum is seen cut twice (v.), and more
posteriorly is seen the shell gland (s.g.), a deep sac, widely
open to the exterior in other sections of the series. The
mesoderm at this stage is represented simply by scattered
cells.
Stage B (fig. B), shows considerable advance upon the
last. The foot (f.) has grown out to form a prominent pro-
jection on the ventral surface. The shell gland is partially
evaginated, and begins to form the visceral hump (v. h.),
which, however, is still partly surrounded by a groove,
deepest behind, and gradually disappearing anteriorly. The
velar area has increased in size, and the tentacles (¢.) are dis-
appearing. The stomodeum has now broken through into
the archenteron, and considerable differentiation has taken
place in the latter. The middle portion has swollen and
forms the stomach (st.), which lies at the apex of the visceral
hump (v. A.), and from which the rectum runs downwards and
backwards to open in the middle line behind (a.) The liver
(/.) 1s an oval structure, sloping downwards and forwards
from the apex of the visceral hump, where it communicates
mostly with the stomach. The opening into the stomach is
still so wide, and the demarcation between the two organs so
vague in this region, that it is difficult to determine their
exact relations, but the liver appears to lie to the left, and
ventrally behind, while in ventral views of the whole embryo
it can clearly be seen to pass below the cesophagus and to the
right side anteriorly. It seems then to be an outgrowth of
110 ISABELLA M. DRUMMOND.
the left ventral wall of the stomach. Below the rectum, and
lying between it and the liver, is a httle dense mass of
mesoderm cells, which is just beginning to be hollowed out
on either side to form the rudiment of the pericardium. Its
position is shown in the figure at p.c. The otocysts have
appeared on either side at o.¢., and are still widely open to
the surface of the body.
Stage C (figs. C and C,, and fig. 11).—Considerable
growth in length has taken place, and the different regions
of the body are clearly marked out. The foot is now a
prominent organ in the anterior ventral region, and a slight
constriction of the body separates the foot and head from the
now well-developed visceral hump. This latter is surrounded
posteriorly by the mantle folds (m.f.), which form a
prominent ridge dorsally to the anus. At this stage the
first rudiments of the mantle cavity appear as two little
depressions lying one on either side of the anus. These are
best seen in a ventral view of the whole animal, or in section.
Fig. C, is a ventral view of a slightly older embryo than
fig. C, but the essential relations of the organs are precisely
similar. Here the two depressions are seen at c.m.c. and
r.m.c., the right one being considerably in advance of the
left. The same depressions are seen in transverse section in
fig. 11. Von Erlanger describes the first appearance of the
mantle cavity as “eine kleine Grube”’ ventrally and just in
front of the anus, but it is quite clear that at this stage there
is a distinct rudiment on either side, the rectum passing
down a ridge between them (« in fig. 11), to open directly on
to the surface of the body. It is only at a later stage that
the portion of the body immediately in front of the anus
sinks in and unites the two original depressions, thereby in-
cluding the anus within the mantle cavity. I have never
been able to find a stage in which these two original depres-
sions are symmetrical. If this stage closely corresponds, as
I believe it does, with von Erlanger’s fig. 1, plate xxi, he
has overlooked an important point in the external anatomy of
the embryo. It is not, as he says, perfectly symmetrical
THE DEVELOPMENT OF PALUDINA VIVIPARA. tit
externally at this stage, for not only is the symmetry dis-
turbed by the inequality of the rudiments of the mantle
cavity just noticed, but the whole visceral hump appears as if
slightly tilted. The apex lies somewhat to the left of the
vertical plane, which would divide the head and foot
symmetrically, while the mantle fold on the left of the body
is at a lower level from that on the right. This tilting is
difficult to represent in surface views, though by rolling the
whole embryo about it is perfectly easy to see. It is, how-
ever, sufficiently obvious in the transverse section through the
hump (fig. 11), which is orientated on the page as it would be
on the body, the line aa representing the vertical plane
through head and foot.
In the internal organs there is little to add to von Erlanger’s
(5) account. ‘The stomach and liver together form, as before,
the apex of the visceral hump; they are now well-defined,
though still retaining their wide communication with each
other. Posteriorly the liver lies distinctly to the left, while
further forward it gradually becomes almost ventral, passing
over to the right, as before, in front of its opening into the
stomach (fig. C,). From the posterior end of the stomach
the rectum runs almost vertically downwards to open in the
position already noticed between the two rudiments of the
mantle cavity. Just dorsal to these, and anterior to the
rectum, are the two rudiments of the pericardium, still
separated from each other, that on the right being a good
deal the larger of the two, as we saw to be the case also
with the mantle cavity (fig. C,, fig. 11). At the left, in
fig. 11, a slight thickening of the pericardial epithelium is
seen, which must be the rudiment of the left kidney ; the
right kiduey is not yet formed. In the head region the first
appearance of the radular sac is noticeable, and also the
appearance of the ganglia of the central nervous system as
thickenings of the ectoderm, as they have been already
described by von Erlanger.
Stage D (fig. D, fig. 12).—The foot now begins to show
for the first time a tendency towards the formation of the
112 ISABELLA M. DRUMMOND.
characteristic creeping sole. The visceral hump is much
developed and surrounded by a strongly marked mantle fold.
Externally the most noticeable feature is a prominent bulge
on the left side, which appears almost like the first formation
of a coil (fig. 12). A transverse section across the visceral
hump, shows, however, that it is simply a bulging out of the
side of the body where the liver and stomach are located. A
comparison of figs. 11 and 12, in fact, makes clear that this
prominence on the left side corresponds to the original apex
of the visceral hump, which has become still further tilted in
the same manner as before, thus leading to a rearrangement
of the organs when orientated with reference to the head and
foot, while their mutual relations are retained. The liver
now lies ventrally, as well as slightly to the left of the
stomach.
Both liver and stomach are entirely on the animal’s left,
and the mantle cavity lies entirely on the right, while the :
anus, also, is displaced from the middle line and has travelled
towards the right. Though readily seen by a comparison of
transverse sections, this tilting is not so obvious when the
embryo is examined entire, as it was at an earlier stage.
This seems in part due to the great growth of the whole
posterior region of the visceral hump, which has caused the
rectum to bend forward to the anus, and the mantle cavity
to take up sucha position that the two original depressions lie
almost vertically one above the other instead of almost horizon-
tally, as in fig. 11, but chiefly is it the result of the rapid growth
of the mantle downwards on the right-hand side of the animal’s
body, so that the right and left edges are now on about the
same horizontal level, and a kind of false external symmetry
is established in this respect. Rapid growth of the mantle
leads, of course, to rapid extension of the mantle cavity, and
consequently we find this far in advance of the preceding
stage. The two original depressions are now united below
the rectum, and form two horns which abut each against
the kidney of its respective side, and then join and widen
out to form the mantle cavity; a slight extension of the
THE DEVELOPMENT OF PALUDINA VIVIPARA. 13S
mantle cavity to the right of the right horn is first noticeable
in this stage, but will be more fully noticed in the next.
The kidneys have been formed as outpushings of the
(morphological) ventral wall of the pericardium on either
side; the wall of the little left kidney and the adjoining wall
of the left horn of the mantle cavity are cut through in the
“ventral part of the section in fig. 12 (J. k.), while the right
horn is seen more dorsally at r.m.c. The ventral position of
the left horn is also seen in fig. D at l.m.c.! The right
division of the pericardium has become much enlarged, and
now occupies a very considerable portion of the visceral hump,
while the left division remains small, and les in the narrow
region between the liver and the left horn of the mantle
cavity. The rectum bends sharply downwards from the
stomach and then runs forward ventrally on the right
to the anus, which lies just anteriorly to the junction of the
two horns of the mantle cavity, and is now included in the
latter.
Stage E (figs. E, E,, E,; figs. 13 and 14).—The foot
has grown back into its definitive position, and is separated
by a marked constriction from the head. The features of
the visceral hump noticed in the last stage are now accentu-
ated. The bulge on the left side has become much more
prominent, and is a very characteristic feature, giving, even
more than before, the appearance of sinistral coiling when
looked at from above (fig. E,). A comparison between figs.
12 and 13 shows that the essential relations are the same as
in the last stage. The liver, now ventral to the stomach, has
increased much in size; the pericardium, of which only a
portion of the right-hand division is shown in the figures, has
swollen, and is found extending for a considerable distance be-
side the stomach to the right of the original right kidney (see
fig. EK, and fig. 15) ; while close to the kidney on the right is
found the first rudiment of the heart as a little solid ingrowth
of mesoderm cells pushing the pericardial wall before it.
1 For further description of the kidneys and their relation to the mantle
cavity, see Part I of this paper.
von. 46, pArt 1.—NEW SERIES. H
114 ISABELLA M. DRUMMOND.
Corresponding to this extension of the pericardium is a great
development of the mantle cavity to the morphological night
of the original right horn or kidney duct. This is shown in
section with the pericardium lying above it in fig. 14, but is
best seen in a view of the whole animal from the left, fig. K, ;
here the left horn is seen ventrally at J. m. c., the former
roof of the mantle cavity is almost vertical and forms the
posterior boundary, while the kidney duct (k. d.) is dorsal.
Dorsal and anterior to this is now the chief extension of the
mantle cavity, which is already visible in a dorsal view of the
whole embryo (fig. E,), and reaching behind to the mid-dorsal
line, while it narrows slightly in front. Von Hrlanger’s
description of an embryo of this stage differs considerably
in respect of the mantle cavity from the above, but I am
unable to reconcile his fig. 7 on pl. xxi with my own
observations, for he both figures and describes (p. 358) the
kidney duct as arising from the dorsal extremity which is
now advancing over the mid-dorsal line, while a comparison
of the series (of which two sections are shown in figs. 13 and
14) with the living animal (from which the outline of the
mantle cavity in fig. EH, was drawn) seems to me to show quite
conclusively that the kidney duct arises in the position where
I have marked it, and that the dorsal extension of the mantle
cavity is a new development. If this be so, very rapid growth
must have taken place in the region in front of a vertical
line passing through the opening of the two kidney ducts, as
is in fact the case, and this would lead to the increased
ventral flexure which attains its maximum about this
time.
Very little remains to be said of the other organs. ‘The
cesophagus is much lengthened, and has acquired a sharp
downward bend before entering the stomach, which slopes
obliquely upwards and backwards. ‘The rectum bends sharply
downwards and then runs forward to open on the right side
in the mantle cavity. The right kidney has become further
developed and forms a simple sae, still fairly widely open to
the pericardium, while the left remains in much the same
THE DEVELOPMENT OF PALUDINA VIVIPARA. 115
condition as in the previous stage. The pericardial septum
has disappeared.
Stage F (fig. F and fig. 15).—AIl the essential features
may be seen in a view of the left side of the entire animal
(fig. F). Comparing this with the similar view of the pre-
ceding stage we find that there has been very rapid growth
in all parts of the body, especially in the “neck”? region
between the visceral hump and the head. The bulge con-
taining the stomach and liver now lies nearly ventrally,
the mantle cavity has extended over into the left side of
the body, and just posterior to it is seen the pericardium,
with the heart now well developed, and showing the auricle
and ventricle separated from each other by a deep constric-
tion. The rectum lies higher on the right side than in the
last stage, and runs along the roof of the mantle cavity to
open more anteriorly. Fig. 15 is a transverse section across
the visceral hump of an embryo of this stage, and shows
essentially the same relation of the organs to each other as in
previous stages. The pericardium lies dorsally to the liver
and stomach, and contains the heart. Dorsal to the peri-
cardium are the two kidneys, the morphologically right, a
well-developed but still simple sac, being seen at k., its duct
at k.d.; the opening of the kidney into the pericardium on
the one hand and the duct on the other are neither of them
shown in this section. ‘To the right of the section is seen the
rudimentary left kidney (/. k.), which seems to resume its
development about this time. Its opening into the peri-
cardium is well seen, and just above it is the solid end of
the original left horn of the mantle cavity (l. m.c.). At
the extreme right (morphologically left) of the pericardium
the first rudiment of the gonad can just be distinguished
at g.}
A further important feature is the development of the
visceral connectives, which are first visible at this stage.
They arise anteriorly from the pleural ganglia and run back
1 For further description of the gonad and its connection with the rudi-
mentary original left kidney, see Part I of this paper.
116 ISABELLA M. DRUMMOND.
on either side of the cesophagus to about the region where
the anus opens, when they appear to lose themselves in the
epithelium of the floor of the mantle cavity. The morpho-
logically left connective is by far the stronger, and arises
behind, just below the anus, while the morphologically right
appears at the extremity of the right extension of the
mantle cavity. The two connectives are thus separated by a
considerable distance posteriorly, and they are not at present
united by a commissure.
Stage G (fig. G and fig. 16).—The original apex of the
visceral hump now points ventrally, though it is still more
prominent on the left side than on the right, which gives the
appearance, when the animal is looked at from above, of the
visceral hump being set crookedly upon the foot. The
cesophagus is elongated, and bends sharply downwards to
open into the stomach. The stomach itself is much enlarged
and lies chiefly ventrally, but ascends somewhat to open into
the rectum, which then bends dorsalwards and runs forward
in the roof of the mantle cavity’ to the anus. ‘The mantle
cavity now extends far down on the left side, especially
posteriorly, and a portion of the kidney is visible in a view
of the left side of the animal. The rudiments of the
ctenidium are clearly formed as projections from the roof of
the mantle cavity on the left. The kidney duct soon after
leaving the kidney now passes below the rectum and runs
backward on its right side to open into a sort of little pouch
of the mantle cavity together with the genital duct, as the
original left kidney duct may now be called. Von HErlanger’s
fig. 2, pl. xxii, shows very well the disposition of the kidney
duct at this stage. The old relations of the rectum to the
two original horns of the mantle cavity are thus disturbed,
apparently by a drawing together of the edges of the mantle
cavity in this region, the space between the two ducts being
obliterated. The whole, or very nearly the whole, of the
definitive mantle cavity seems, therefore, to be formed by
the great extension of the original right horn, as noticed in
Stage EH. In other respects the mutual relations of peri-
THE DEVELOPMENT OF PALUDINA VIVIPARA. ilu by
cardium, kidneys, and ducts remain as in the last stage, but
the gonad is now clearly formed as a thin cord of cells lying
beside the liver.
The visceral connectives are now completely formed, and
are united by a commissure, which in this stage lies asym-
metrically, that is, wholly below the original right portion of
the mantle cavity. This is, however, a secondary condition,
and probably due to a tendency to place itself in relation
with the symmetry of the external form, for a stage inter-
mediate between this and the last shows that the commissure
is formed in part from the floor of the mantle cavity just at
the entrance of the genital duct.
Fig. 16 is a transverse section across the visceral hump
in the region of the kidneys and heart. Liver and stomach
lie ventrally, and above them is seen the pericardium, with
the heart at the extreme left of the section. Dorsally to the
pericardium is seen the kidney (k.) with its openings into
pericardium (pc.) and duct (k. d.), both cut through. The
kidney duct is beginning to pass ventrally to the rectum (rec.)
as described above. The genital duct is cut through at
1. m. c., and the wall of the left kidney at J. k.
Stage H (figs. 17, 18, 19).—A definite coil is now being
formed on the right side, about one complete turn of the spiral
having been made, and the old crooked setting of the hump
on the foot is nearly lost. In this stage the organs attain very
nearly their adult condition in all essential points. The ali-
mentary canal has increased much in length and become more
coiled. The cesophagus bends down and towards the right to
open into the stomach, which now stretches as a great sac
below the liver, opening into it dorsally, and forming at the
right-hand extremity a blind sac, which shows a tendency to
follow the coiling of the liver (fig. 17). The rectum now
opens out of the stomach quite ventrally and posteriorly,
passes towards the left side of the body, then bends sharply
upwards behind the pericardium, and runs dorsally along the
roof of the mantle cavity, bending suddenly to the right just
before it reaches the anus. The liver is greatly developed
118 ISABELLA M. DRUMMOND.
and lobed, and forms almost the whole of the coil, being
followed only to a very slight extent by the stomach and
gonad. The pericardium is swollen, and the original left
side, which has hitherto been so narrow, widens out con-
siderably. In connection with this we may notice the
advanced condition of the primitive left kidney and the now
well-developed cord of cells (g.) which represents the gonad.
The definitive kidney (k.) is seen on the left side of the
section, and is easily recognisable by its slightly staining
and now folded walls. The kidney duct (k. d.) is cut across
to the right of the kidney, just where it passes below the
rectum (rec.) as described in the last stage. Both kidney
and genital ducts have now lost their primitive condition as
simple specialised portions of the mantle cavity, and run for-
ward in the roof of the latter as well-defined ducts, parallel
to and to the right of the rectum, opening somewhat behind
the anus.
The mantle cavity now extends very low on the left side,
especially posteriorly, so that it is Just cut in the section
represented in fig. 17, the difference between anterior and
posterior regions being much more accentuated than was
formerly the case. The relations of the visceral connectives,
as noticed already, though in a less degree in the last stage,
are deeply affected by the asymmetrical growth of the mantle
cavity, and as posteriorly the mantle cavity appears to lie
wholly on the left side and with its floor almost vertical, so
in the region just anterior to the commissure the two con-
nectives lie almost in one vertical plane. A discussion of
these relations may conveniently be left till the whole
question of torsion is taken into consideration, but the twist
of the connectives now remaius to be described. As is well
known, the two connectives are bilaterally symmetrical when
they leave the pleural ganglia, almost immediately the right
passes below, the left above the cesophagus, each to the
opposite side of the body, after which they again resume
their bilaterally symmetrical disposition, only that now on
the right side is the original left, and that now on the left is
THE DEVELOPMENT OF PALUDINA VIVIPARA. 119
the original right. Following this twist in a series of trans-
verse sections a peculiar relation between the cesophagus and
the connectives is noticeable. In the region of the anterior
ganglia and for some distance behind them the cesophagus is
compressed so as to render its outline oblong in section. At
first bilaterally symmetrical, it soon becomes completely
asymmetrical, the long axis, as seen in section, sloping at
first upwards and to the left, then passing through the hori-
zontal position to slope upwards and to the right; in other
words, the cesophagus apparently follows the connectives in
their twist, as shown in figs. 18 and 19. At the completion
of the twist the cesophagus becomes round in section, and
passes back for some considerable distance lying between the
two connectives, which are now once more in the same hori-
zontal plane; then the connectives appear to become partly
twisted a second time to take up the position with regard
to the mantle cavity posteriorly, which has been already
described. Unfortunately, in this region the cesophagus is
still circular in section, and we have no direct evidence as to
whether or not it follows the connectives in the same way
that it does more anteriorly.
The relations between the connectives and cesophagus are
seen to a greater or less extent in stages previous to this, but
the description of them has been deferred till now as being
easier of comprehension when they are present in such a
marked degree. Already in Stage C, where we first noticed
the tipping of the visceral hump, a slight apparent twist of
the oesophagus was visible; this was more marked in the
following stage, and increased in each succeeding stage up
to the present. It is as well to notice that this condition of
the cesophagus was plainly visible before the visceral con-
nectives appeared at all.
It has already been said that the animal has now attained
in all essential particulars to the anatomy of the adult. A
few points, however, remain to be noticed, being for the
most part only further developments of processes already
begun. Of these the most obvious is the coiling which takes
120 ISABELLA M. DRUMMOND.
place rapidly from this time. The tendency already seen in
tle stomach to grow out into a third sac which follows the
coiling of the liver becomes considerably accentuated, while
tle gonad grows very rapidly and soon passes right up to
the tip of the last coil. At the same time it loses its solid
character, and, becoming hollowed out, acquires an opening
into the duct, as described in the special part of this paper.
Meanwhile the mantle cavity deepens, and the rectum grows
forward to open near the anterior edge. It is during this
growth that it acquires the characteristic disposition of the
adult, passing from mid-dorsally behind, obliquely down-
wards and to the right, a disposition which is doubtless con-
nected with the sharp bend towards the right described in
Stage H. The pericardium alone begins at this late stage to
show new relations, for it widens and becomes very irregular
in shape, spreading amongst the other organs of the body so
as to form a kind of general body-cavity.
Monstrosities.—Whilst collecting material for the study
of the normal course of development a few monstrosities
were found which presented some remarkable features.
Although it seems impossible fully to understand the
meaning of all the abnormal conditions found in_ these
embryos, some of them seem to me to be of sufficient interest
to justify the insertion here of a description of the main
features of their organisation.
1. The simplest of these abnormalities is a small embryo
between stages C and D in degree of development, perfectly
normal in every respect, but wholly reversed. The liver
and stomach form a bulge on the right side, while the mantle
cavity and rectum are on the left, and all the other organs
correspond in every particular. This is, so far as I know,
the first record of a normally dextral Prosobranch so
organised. It is unfortunately too young to show definitely
the manner of coiling.
2. This embryo, shown in fig. M 1, is very remarkable.
A camera tracing was made while it was still alive, and the
organs put in partly from life and partly after preservation.
THE DEVELOPMENT OF PALUDINA VIVIPARA. WI
It was then sectioned in a plane transverse to the long axis
of the visceral hump, and portions of these sections are por-
trayed in figs. 20, 21, and 22. The most noticeable feature is
the greatly developed visceral hump, which was held erect
over its head, bending in a decided manner at the apex, as
though forming the first turn of an exogastric coil. A
further remarkable feature is the perfect bilateral symmetry
of the whole embryo, though it is obviously at an advanced
stage of development, for though it is impossible to compare
it with any given stage in the normal course, the head is
well developed, the foot has found its normal creeping sole,
and a small operculum is already present. The general dis-
position of the organs can be made out from the drawings of
the whole animal. The stomach, it will be seen, forms the
apex of the visceral hump, while just below it an enormously
developed pericardium fills up for some distance the space
between the descending cesophagus and rectum. Paired
kidneys are seen at k and k’, but are better described in
connection with the transverse sections. The same is the
case for the great bulge in the lower posterior region of the
visceral hump behind the rectum, which might be taken for
the mantle cavity. Sections, however, show it to be merely
the continuation of a great space which surrounds all the
organs nearer the apex of the visceral hump, as seen in
fie. 20, which is a transverse section in the region of the
kidneys, and just below the pericardium. The mantle cavity
is shown in figs. 20 and 21 as a narrow and symmetrical
organ lying anterior (morphologically dorsal) to this great
space, and apparently compressed by it. Towards the apex
of the hump it forms two symmetrical horns which run back
on either side towards the kidneys, but never fuse with them.
Here a wholly inexplicable condition obtains. ‘The peri-
cardium, as already mentioned, is greatly developed, and it
is not surprising, in an embryo otherwise symmetrical, to
find out that here also two symmetrical evaginations have
been formed. These, which must be the kidneys, are, on the
one hand, very widely open to the pericardium, while on the
122 ISABELLA M. DRUMMOND.
other they come nearly into contact with the horns of the
mantle cavity, but no communication is formed. Instead,
each kidney communicates with a sort of little vesicle, as
seen in fig. 20, which at the same time shows the close
approximation of the horn of the mantle cavity to the
kidney on either side. Below the kidneys, that is, nearer to
the anus, these two vesicles unite, and run back for a short
distance as a single duct, which opens into the mantle
cavity mid-dorsally, as shown in fig. 21. Shortly afterwards
the anus opens on the same mid-dorsal ridge.
One further feature remains to be noticed, namely, the
absence of liver. The stomach is well developed, but the
only trace to be found of anything which might be interpreted
as liver is a pair of little outgrowths of the alimentary canal
just in the region where the stomach and cesophagus unite.
These are shown in fig. 22. If these may be so interpreted,
then the liver shares also in the symmetry shown by the
mantle cavity and kidneys. The visceral connectives, as
would be expected, run back perfectly symmetrically beside
the cesophagus.
3. Other monstrosities occurred, but of less interest than
the above. They all showed traces of symmetry in a greater
or less degree, and some of them the same tendency to
exogastric coiling, but in most cases the organs were
deformed, and very much less clear of interpretation. Only
one other, therefore, is shown here (fig. M ur). This one is
remarkable in that a greater degree of symmetry than is
usual is combined with a slight sinistral torsion, and a fairly
well-marked development of the mantle cavity to the left of
the original left horn, as seen in the figure. The symmetry
of the body is, however, confused, in that the pericardium is
more developed on the right side than on the left. This
embryo is further remarkable in that it is the only one of
the monstrosities possessed of a clearly defined liver, which
even in this case is very small, and hangs like a sac from the
stomach ventrally, and slightly on the right side of the
body.
THE DEVELOPMENT OF PALUDINA VIVIPARA. 123
(zs) Theoretical Considerations.
I do not mean to attempt to give here anything like a
complete historical summary of the many views which have
been held on the subject of Gasteropod torsion and asym-
metry. This has already been done more or less fully many
times (see especially Simroth [16] and Boutan [2]), and I
shall therefore confine myself merely to a very brief conside-
ration of those views which lend themselves to criticism from
an embryological standpoint, and upon which a study of the
development even of a single form may throw some light.
Such theories, therefore, as that put forward by Lang, which
claims only to be phylogenetic, and for which confirmation is
not sought from the facts of ontogeny, will be passed over
altogether ; while theories of authors who, like Biitschli, seek
to base their conclusions to a greater or less extent upon
embryology may be of some interest in this connection, and
will therefore be considered. At the same time it must be
remembered that the remarks upon these theories are pro-
fessedly based only on embryology, and need not necessarily
invalidate their phylogenetic value, though they may weaken
the author’s argument.
For the sake of convenience the theories under considera-
tion may be placed in two classes. In the first of these are
placed those theories which maintain that the present condi-
tion of the Prosobranchia has been brought about by a simple
process of unequal growth, resulting in the forward movement
of the palleal complex in a horizontal plane ; while the second
comprises those more recent theories of Pelseneer, Amaudrut,
and Boutan, which regard asymmetry primarily as the
concomitant of a twist which causes the palleal complex to
move in a vertical plane.
Biitschli (4) was the first to put forward in an exact and
careful way the point of view which is now common to all
theories of the former class. He puts aside the older view
of Spengel, which obviously runs counter to known embryo-
124. ISABELLA M. DRUMMOND.
logical facts, by pointing out how the anus must have lain at
all stages in the palleal groove, and then proceeds to build
up his own theory of unequal growth, and the resulting
gradual approximation of mouth and palleal complex on the
right side. That such an approximation does take place in
the ontogenetic history is, of course, well known, and the
manner in which it is brought about seemed to Biitschli
equally obvious. According to him, at a time when the anus
lies in the middle line posteriorly, a narrow zone on the
right side of the animal ceases to grow altogether, while the
corresponding zone on the left grows with great vigour, and
thereby the anus appears to be pushed up the right side of
the body, while in reality the distance between it and the
mouth remains always the same. Meanwhile the foot on the
one hand, and the mantle on the other, continue to grow
symmetrically. ‘This process cannot by itself, however, bring
about the crossing of the visceral connectives. For this
Biitschli has to invoke the aid of the mantle cavity, which,
he says, is formed rapidly at a time when the anus hes far
forward on the right side of the body, and, by its growth
backwards and to the left, carries the organs of the original
right side of the body back with it and over the mid-dorsal
line. All this he puts forward as ontogenetic fact, and
therefore probable phylogenetic theory.
Biitschli’s views have been adopted with more or less slight
modification by many authors, and have recently been brought
forward again with some additions by Plate (15). The great
difficulty, to which no one could find a fully satisfactory
solution, was the absence of any known cause of asymmetrical
growth in a perfectly symmetrical body. Plate seeks an
explanation in the asymmetry of the liver. Starting from
the nearly symmetrical liver of the Chitons, and comparing
it with the asymmetrical organ in the Gasteropoda, he
describes how, in the primitive form, a rapid growth of the
left liver must have taken place at the expense of the right,
which would result in the formation of a hernia posteriorly
on the left side. Thus the first rudiment of a coil is formed,
THE DEVELOPMENT OF PALUDINA VIVIPARA. 125
which, for reasons connected with the equilibrium of the
body, lies with its apex pointing towards the right (see his
fies. F to H, pp. 185 and 187). This it is, he believes, which
causes the approximation of the mouth and anus on the right
side. As coiling proceeds this process would be accentuated,
and so, apparently, the condition which obtains in the adult
Gasteropod is reached without the aid of the late develop-
ment of the mantle cavity relied upon by Biitschli. Though
this is put forward merely as a phylogenetic theory, Plate
believes that the facts of development will fully bear it out,
and it is only from this point of view that we can deal with
it here.
Pelseneer (14) was the first to put aside the old point of
view. ‘l’o him it seemed that embryologically two distinct
processes took place, both of which had for their object, as it
were, the approximation of mouth and anus. ‘The first of
these, which he calls “torsion ventrale,”’ leaves the embryo
still symmetrical, but with the alimentary canal bent sharply
so that the anus hes far forward ventrally. he mouth and
anus being prevented from approaching nearer alone this
line on account of the outgrowth of the foot, the second
process comes into play. This isa“ torsion verticale,” which
takes place at right angles to the last, and has the result of
all the organs contained in the shell undergoing a rotation
through 180°, the ventral anus thereby becoming dorsal, the
organs of the original right side being carried over to the
left, and those of the original left to the right.
More recently Amaudrut (1) has approached the same
problems from the point of view of comparative anatomy,
and, from a study of the cesophagus and adjacent organs of
a number of Gasteropods, has come to the conclusion that
the region between the head and the visceral hump has
undergone a twist through 180°. This, of course, would fit
in well with Pelseneer’s observations, for, if the whole visceral
hump has undergone torsion with regard to the head, the
oesophagus must needs be twisted.
Finally, Boutan (2) has brought out a paper on the
126 ISABELLA M. DRUMMOND.
asymmetry of the Gasteropods, which upholds essentially the
same view of torsion for the Prosobranchs as Pelseneer and
Amaudrut had already enunciated, and which derives its
chief value from the author’s claim to have actually observed
the vertical torsion take place in the case of Acmecea.
It is at once evident that the processes which, broadly
speaking, characterise respectively the two classes into which
we divided the theories under discussion, will not have
entirely similar results. Both, indeed, alike have, as their
chief results, the forward dorsal position of the anus and the
crossing of the visceral connectives, for it was to account for
these facts that the theories were originally framed; but, on
the other hand, the twisting of the cesophagus, if true, could
never have arisen from the processes which Biitschli
describes, while the growth of the mantle must be conceived
quite differently, according to which hypothesis is accepted.
Tf Biitschli is correct, what was originally right remains on
the right side throughout; while, according to the view of
vertical torsion, the mantle, and therefore also the shell, share
in the displacement of the palleal organs. The same holds
good for most of the viscera, and is especially clearly illus-
trated in those organs which lie dorsally or ventrally ; for
while on the theory of unequal growth a lateral shifting
might easily take place in the same manner as is the case for
the palleal organs, a dorso-ventral displacement is only
readily understood on such a theory as Pelseneer’s. Thus
Plate has to account for the gonad, which is dorsal in
the Chitons, having a ventral position in the Gasteropoda, by
supposing that a lobe of the great liver of the left side grew
dorsally to it and pressed it against the foot.
Taking these considerations separately, and beginning with
the last, the facts of embryology seem to me to show in a quite
unequivocal manner that actual rotation of the organs has
taken place round an axis coinciding with the cesophagus in
its direction. It is for this purpose that the drawings of
figs. 11 to 17 were made, and a comparison of these with one
another, bearing in mind that they are all orientated on the
THE DEVELOPMENT OF PALUDINA VIVIPARA. 127
page in the same way with regard to head and foot, shows
clearly how stomach, liver, pericardium, kidneys, and mantle
cavity have all rotated in a perfectly definite manner, while
retaining unaltered their relations inter se, and explains
both the original dorsal and later ventral position of the
gonad, without the intervention of any dorsally growing liver
lobe. It is, moreover, striking that the torsion of the visceral
connectives and the apparent twist of the cesophagus noticed
on p. 119, keep pace perfectly with this rotation, so that it is
almost impossible not to connect the two phenomena.
A comparison of figs. 16 and 17, on the other hand, causes
some little difficulty, for although torsion seems to have taken
place through an angle of very nearly 180° in fig. 16, there
seems to be an apparent twist of about 90° further in fig. 17,
and so it may seem that we have proved too much by this
comparison of transverse sections. ‘I'wo facts are, however,
noticeable. In the first place, whereas up to Stage G, a
transverse section of which is shown in fig. 16, the corre-
spondence is perfect between the degree of torsion shown by
the cesophagus and connectives on the one hand, and the
rotation of the organs on the other, this is no longer the
case in Stage H, where the connectives are only twisted
through 180°, while the organs in the posterior region of the
visceral hump have apparently rotated through about 270°.
It is indeed true that an accessory twist has been noticed
for the connectives as well as for the other organsin Stage H,
but this is clearly marked off from the regular twist corre-
sponding to that of earlier stages, which takes place more
anteriorly; and a study of the development of the mantle
cavity, and its disposition in this stage, makes it clear that
the two twists are quite unconnected. The visceral commis-
sure is formed from the floor of the mantle cavity, and, as the
original right portion of the latter is carried far down on the
left side of the body, so also is the origin of the corresponding
half of the visceral commissure; while, as the original left
side of the mantle cavity remains feebly developed, it, and
consequently also the original left portion of the commissure,
128 ISABELLA M. DRUMMOND.
lie more dorsally. The commissure thus comes to lie obliquely
quite independently of any direct connection with the torsion
of the body, and this irregularity is accentuated by a tendency
on the part of the commissure to pass even more over to the
left side of the body, and so place itself symmetrically with
regard to the asymmetrical mantle cavity ; or, in other words,
symmetrically between its two extreme points of origin. This
accessory twist may, therefore, be left out of account for the
present purpose, and we are justified in saying that, as far as
true torsion is concerned, the connectives only show a twist
of 180°.
The second point to be noticed is that, whereas growth in
the circumference of the visceral hump has been hitherto so
slow as to be almost inconsiderable—for instance, between
Stages D and G—though at the same time torsion has
advanced rapidly, an enormous growth has taken place
between Stages G and H. I believe that it is in these two
facts that we find the solution of the difficulty. Between
Stages G and H a kind of accessory or false torsion has taken
place among the organs in the posterior region of the visceral
hump, due merely to unequal growth amongst themselves,
and not having as its concomitant further true torsion in the
anterior region. If we seek further for the cause, I think we
find it in the lenethening of the alimentary canal, and the
great development of the stomach, which have together
brought about the present relation between the stomach and
liver, and also in the sudden rapid widening of the original
left portion of the pericardium, which has hitherto been so
narrow, a process which would have, as a natural result, the
pushing of both kidneys more towards the definitive left side
of the body than they were before. We have some evidence,
then, for believing that the changes which are noticeable in
the position of the organs in the body in successive stages
are at first due to a rotation of the whole visceral hump upon
the head through an angle of 180°, but that after this is
complete a further apparent rotation affecting the posterior
region of the body only is induced by unequal growth and
THE DEVELOPMENT OF PALUDINA VIVIPARA. 129
consequent rearrangement of the organs within the visceral
hump. ‘These two processes are, of course, quite distinct.
It has already been necessary to touch upon the subject of
the torsion of the cesophagus. Amandrut, as noticed above,
has worked out with great care the twisting of the anterior
aorta, the salivary glands, etc., about the cesophagus, and has
come to the conclusion that this is due to an actual torsion of
the region of the body between the head and visceral hump,
but hitherto, so far as I know, no embryological evidence ha
come to hand. I have already described (p. 119) the curious
compression of the cesophagus, and the manner in which, in
transverse sections, the long axis changes its direction in
Paludina embryos. This is not direct proof, but it is difficult
to find any other explanation of the occurrence, except that it
is due to the cesophagus being forced to undergo an actual
twist. In quite old embryos, and in the adult, the cesophagus
is no longer compressed, and this appearance is quite lost.
Now, granted that such a twist does take place, it follows,
as Pelseneer points out, that originally i. e., in the untwisted
forms, the shell, if coiled, must have been coiled exogas-
trically. It is exceedingly difficult to get any direct evidence
upon this point, for,as Plate remarks, we cannot rely on the shell
of any of the primitive Prosobranchs, like Fissurella, as these
have all undergone torsion, and, on either view, an exogastric
shell, if present, must be secondary ; while, on the other hand,
coiling does not begin to take place sufficiently early in the
course of development to give us clear evidence either way.
It is, however, worthy of remark that all the monstrosities
that I came across which, for some reason or other, had
remained untwisted, if they showed any tendency towards coil-
ing of the visceral hump at all, had begun to coil exogas-
trically. Of these the most highly developed is shown in
fig. M u, and has been already described. ‘There were,
however, two others that showed a distinct tendency in the
same direction, while I did not find one with anything like
an endogastric coil. This fact seemsto me highly significant.
The last of these broadly marked points of difference
vou. 46, PART 1.—NEW SERIES. I
13 ISABKLLA M. DRUMMOND.
between the two general theories under consideration relates
to the symmetrical growth of the mantle, and this is the only
one the evidence upon which seems still to point in favour of
Biitschli’s view ; for, if the whole visceral hump has under-
gone a rotation, we should expect to find signs in the
innervation. But the definitive right side belongs to the
original left, and vice versA; whereas it is well known that
the right pleural ganglion gives rise to the mantle nerve of
the definitive right side of the body, and the left pleural
ganglion to that of the definitive left. How this can be
explained upon Pelseneer’s view is not quite clear, unless it
may be that the mantle nerve is embryologically a late out-
growth of the pleural ganglion, and is altogether post-
torsional; but if this be so we shall have to admit a
discrepancy between embryology and phylogeny. ‘The case,
however, is not altogether easy, even for the upholders of
siitschl’s theory of the symmetrical growth of this region
of the body, for the innervation is not wholly symmetrical.
As Bouvier (8) remarks, “‘ la branchie n’est rien autre chose
qu'une formation palléale, et les mémes nerfs qui Pinnervent
se répandent en méme temps dans le manteau,” and, as is
well known, the definitive left ctenidium is innervated from
a ganelion belonging to the original right side of the body.
The question, then, is one of some difficulty whichever view
one takes, and of hardly greater difficulty in the one case
than in the other.
An examination of the broad features of the two great
classes into which we divided the theories of Gasteropod
torsion leaves, then, a balance of embryological evidence in
favour of that class of which Pelseneer was the first exponent.
It will be well now to examine the individual theories more
in detail. Biitschli’s views have already undergone criticism
at the hands of Pelseneer and others as not corresponding to
the facts of ontogeny as one sees them. Amaudrut criticises
him more particularly with regard to the position of the
supra- and sub-intestinal gangha. The supra-intestinal
ganglion, he says, is usually situated further back than its
THE DEVELOPMENT OF PALUDINA VIVIPARA. 131
fellow on the other side of the body. Now, as the former
was originally on the right side of the body, it would fall
into the zone where Biitschli supposes that growth ceases,
while the subintestinal ganglion, on the other hand, would
be originally in a zone of very active growth. This, Amaudrut
argues, should lead to the snbintestinal ganglion being
pushed the further back of the two, while precisely the
opposite is actually the case. If, however, we suppose that
instead of the left it was at first the right side which grew
most actively the existing condition of affairs would be
obtained; and as, after torsion, the zone of active growth
would be transferred from the right to the left, we probably
find an explanation of Biitschli’s error in his having exa-
mined too late stages of development. ‘he argument
concerning the position of the ganglia seems sound so far
as it goes, but the alleged reason of Biitschli’s mistake is
not so easy to accept, seeing that he starts from a form
in which the anus lies in the middle line behind, and in
which even the ventral flexure has apparently not yet
begun.
Biitschli himself relies for his evidence of unequal growth
upon having demonstrated that in different stages of develop-
ment the mouth and anus do, as a fact, remain exactly the
same distance apart while the body is increasing in size and
torsion is taking place. Now, it seems to me that the value
of this argument depends largely upon the view we take of
the development of the mantle cavity. If we regard it as
von Hrlanger did, and as I beleve we must regard it, as
the result of an outgrowth of the mantle rather than as an
invagination of the surface of the body, Biitschli’s argument
is entirely destroyed, for then it is not the mouth and anus
that we must compare so much as the mouth and the back of
the mantle cavity ; and the fact that the anus lies near the
outer edge of the mantle cavity shows only that rapid growth
of the rectum has been taking place in this region, and has
caused a closer approximation of the anus to the mouth than
would otherwise have been the case. Whichever view we
132 ISABELLA M. DRUMMOND.
may take of this matter, however, we are forced to admit
that rapid development of the mantle cavity must mean rapid
erowth in the neighbourhood of its formation ; and we have
already seen in the descriptive part how a rapid formation of
the mantle cavity takes place between Stages D and E to the
original right of the anus, that is, in Biitschli’s zone of
cessation of growth, and this long before torsion is complete.
His view of the late formation of the mantle cavity and its
effect upon the position of the organs seems to me no less in
entire contradiction to embryological fact. The mantle cavity
is not formed by any means so late as Biitschli would put it ;
it takes its origin, in fact, almost simultaneously with the
first appearance of torsion, and is, as we have seen, strongly
developed before we reach Stage F. Finally, it is, of course,
impossible that its formation should have the effect attributed
to it by Biitschli unless we regard it altogether as an in-
vagination, which we have not sufficient evidence for doing.
All the above remarks apply, also, to Plate’s theory, since
he accepted Biitschl’s in its main features. ‘The former’s
suggestion, however, that the liver is the first cause of
asymmetrical growth is an interesting one, and must be
further examined. His working out of the later stages of
development, the formation of the hernia, and the production
of the coil find no support in embryology. A hernia, indeed,
is formed on the left side in the development of Paludina, but
this is of quite a different nature from that described by
Plate, as a comparison between his figures and mine will
show; for whereas in his theoretical form the hernia contains
only the liver of one side, and is at the same time the
beginning of the coil of the visceral hump, the bulging
out of the side of the body in Paludina is, as the figures
show, equivalent to the once symmetrical apex of the visceral
hump, and contains the stomach as well as the liver. It
is, moreover, in no way comparable to the formation of a true
coil, which is formed by a distinct outgrowth of the liver at a
later stage (cf. figs. 13 and 16 with fig. 17). The coil in
Paludina does not begin till torsion is nearly complete, and
THE DEVELOPMENT OF PALUDINA VIVIPARA. 133
therefore it cannot form an ontogenetic cause for the
forward movement of the palleal complex. Phylogenetically
the evidence is only negative, but at least embryology gives
no support to this part of Plate’s theory.
Putting aside, however, the question of the coiling of the
visceral hunip, if we follow the progress of torsion from stage
to stage we can, I think, in no way regard this as dependent
upon the growth of the liver in each stage. Comparing
Stages C and D, for instance, both as complete embryos and
in sections (cf. especially figs. 11 and 12), we find the slight
growth of the liver more than counterbalanced by the great
development of the pericardium and mantle cavity, so that
whereas in fig. 11 a line joining the junction of the stomach
and liver with the ridge «x, which, for our present purposes,
may be taken to represent the position of the rectum, divides
the section into very nearly equal portions, a similar line in
fig. 12 makes that portion which contains the liver considerably
smafler than the other, which contains the great original
right extensions of the pericardium and mantle cavity. In
the next stage (fig. 13) this has been partly rectified by rapid
growth of the liver, but in Stage F the inequality is again
very marked, and, in fact, from Stages EH to G the growth of
the liver is very slight, while torsion is rapid.
While entirely repudiating the idea, however, of the liver
acting, as it were, as the propelling force throughout ontogeny,
it may yet be possible to agree with Plate in regarding it as
the original disturber of symmetry ; and some support is lent
to this view not only by the very early asymmetry of this
organ, but also by the fact that it is not present, or is only
very slightly developed, in the symmetrical monstrosities.
But it should, at the same time, be noticed that the develop-
ment of Paludina gives no more support to this part being
acted by the liver than by the mantle cavity. The latter
organ also is asymmetrical from the time of its first formation
in normal forms ; while in Monstrosity III, which was shghtly
twisted towards the left, the mantle cavity is considerably
developed in a manner to correspond to the torsion, while the
154 ISABELLA M. DRUMMOND.
liver is comparatively insignificant. Moreover, in following
the normal development from stage to stage, it is obvious
that the great original right-hand extension of the mantle
cavity much more nearly keeps pace with the torsion than
is the case with the liver; and, finally, it may be noticed that
this view would harmonise with Amaudrut’s reasoning con-
cerning the supra- and sub-intestinal ganglia. It may well
be, however, that the growth of these organs is to a large
extent dependent the one upon the other, and that equilibrium
is maintained by the asymmetrical growth of the liver on one
side of the body being compensated by asymmetrical growth
of the mantle cavity on the other.
We turn now to the other side of the question, but Pel-
seneer puts forward his theory in such a broad and general
form that it is difficult to enter into any detailed discussion
beyond the general considerations which have already been
adduced in his favour. The chief objection that might be
raised in this case is, perhaps, one which arises from the
difficulty, in a course of development like that of Paludina, of
distinguishing Pelseueer’s two processes of ventral and
lateral torsion. And, indeed, they do go on so closely hand
in hand that as a matter of fact the anus travels in an oblique
and never in a vertical direction. Nevertheless there is
clear evidence of a vertical rotation of the organs contained
in the visceral hump, as has been already pointed out, and
in each stage it is possible to separate from the results of this
process a certain clearer development of the visceral hump
and sharper bend of the alimentary canal from that which
obtained in the previous stage, which must be the result of a
process akin to Pelseneer’s ventral torsion, or, as Amaudrut
has better styled it, ventral flexion. In the early stages the
distinction between the two processes is very clear—as, for
instance, in Stage C, where the ventral flexion is already
strongly marked, while the lateral torsion is but just begun.
With regard to the cause which Pelseneer seeks for these
processes, however, the development of Paludina offers no
conlirmation. The growth of the foot, he says, forms an
THK DEVELOPMEN'’ OF PALUDINA VIVIPARA. 30
obstacle to the close approach of the mouth and anus, and
therefore vertical torsion takes the place of the ventral
flexure. This is really no true explanation at all, for we are
left in the dark as to how the foot brings about this new state
of affairs, and we do not get much nearer if we say, with
Boutan, that there is an antagonism of growth between the
foot and the visceral hump. It seems to me that in the
development of Paludina it is altogether out of place to speak
of such an antagonism, for torsion begins at a time when the
foot is still but a comparatively insignificant ventral pro-
jection, and long before the formation of the creeping sole.
Amaudrut also is unsatisfactory in this respect, for he
attributes torsion ultimately to voluntary effort on the part
of the animal to get its gills into a better situation. With
the main part of Amaudrut’s paper embryology has nothing
to do; there are, however, one or two points in which it seems
to me he is mistaken, owing to a too exclusive regard for the
anterior region of the body. His description of the shell and
visceral hump as aiding the torsion by their weight is wholly
inapplicable to embryology; while his account of the
manner in which the peculiar shape of the mantle cavity and
the disposition of the organs included in it are induced
receives complete contradiction. ‘ A peu prés dans le méme
plan transversal qui passe pas le ganglion viscéral postérieur,”’
he says, ‘‘se trouvent la partie terminal de la région tordue
du tube digestif, le fond de la cavité respiratoire, la partie
postérieure de la branchie et le coeur. Ce plan marquant en
arriére la limite extréme de la torsion, les organes qui s’y
trouvent ont du exécutés un mouvement de rotation d’environ
180° pour se rendre dans leur position définitive.”” This holds
good for all organs behind this position, but in front of it
obviously torsion will be less. It is these facts of which he
makes use to explain the apparent slope of the mantle cavity
from the left towards the right anteriorly, and of the rectum
from mid-dorsal, where theoretically it should be, to the
right side of the body, the characteristic position for the
Prosobranchia. In Stage H, however, the region of the
136 ISABELLA M. DRUMMOND.
twist of the cesophagus and visceral connectives lies alto-
gether in the anterior region of the mantle cavity, and in
front of the anus. All the region behind this, we must
believe, has equally undergone a torsion of 180°, and yet here
we have clearly marked that peculiar disposition of the
mantle cavity which Amaudrut seeks to explain. It seems
to me that this is not to be regarded as due to torsion at all,
but to unequal growth ; already in Stage H, as we have seen,
the main features were present, the mantle cavity reaching
over the mid-dorsal line behind and being much less ad-
vanced in front, while the rectum showed no tendency toa
corresponding disposition. It is not, in fact, till Stage H
that the position of the anus ceases to be where theoretically
it should be according to the degree of torsion, and here the
displacement is due to a sudden bend of the rectum to the
right quite close to the anus. The final disposition of the
rectum is due to a forward growth in the direction indicated
by this bend after torsion is, complete, and may perhaps be
due, phylogenetically, to the advantage gained by the animal
in having the anus in a position as far removed from the gill
as possible.
Boutan’s explanation of the cause of torsion has already
been mentioned, and it has been shown to be hardly
applicable in the case of Paludina. His conception of this
antagonism of growth between visceral hump and foot is,
much more than Pelseneer’s, that of an ontogenetic cause,
for he expresses the opinion most definitely that if this
antagonism could be suppressed torsion would not take place.
Now amongst the monstrosities already referred to it is
certainly true that I did not find one with an abnormally
small foot and yet a visceral hump which had undergone
torsion, but, on the other hand, fig. M ir shows how both
foot and hump may be very highly developed and yet no
torsion take place. Surely the antagonism of growth, if such
exist, must be much greater in this case than, for instance,
in Stage C, where torsion has already begun. ‘This leads
on to Boutan’s view of torsion as the cause of asymmetry.
THE DEVELOPMENT OF PALUDINA VIVIPARA. 137
The normal Gasteropod larva, he says, is perfectly bilaterally
symmetrical, and remains so till torsion takes place; but as
soon as this begins the asymmetrical growth of the internal
organs begins, and, if torsion could be averted, symmetry
would be maintained. Certainly in this connection the sym-
metry of Monstrosity II is striking, but that asymmetry and
torsion are closely connected no one doubts, and whichever
were the cause of the other, or if both were the outcome of
some common cause, the result would be the same. In the
normal course of development it has been repeatedly pointed
out by other writers that asymmetry in some form or other is
found before torsion begins. In Paludina torsion begins so
early that it is difficult to be quite sure of this, but the liver,
at least, is never wholly symmetrical, and the unequal
development of the original rudiments of the mantle cavity
takes place as nearly as possible simultaneously with the
beginning of torsion.
Once more, Boutan turns to antagonism between foot and
visceral hump in order to explain the coiling of the latter.
If, when the creeping sole of the foot is developed, this can
stretch out without lateral displacement of the visceral hump,
then the shell, he believes, will remain symmetrical ; but if
not, then the hump will be pushed to one side or other, and
the sense of the future coil will depend upon which side it is
pushed towards. Such a view, it would seem, would be
quite impossible to accept after even a cursory view of the
facts of development. Jor if, as would seem to be the case,
the coiling of the visceral hump is primarily the result of a
definite process of growth in the liver (cf. figs. 16 and 17),
this is altogether independent of the exact relation in which
this organ finds itself to the foot. As a matter of fact, the
dextral coil of Paludina begins before torsion is quite
complete, and therefore, while the apex of the visceral hump
is still to the left of the foot, by a strong growth of the liver
towards the right, which, when torsion is complete and the
visceral hump nearly symmetrical, points to the right and
upwards. It is altogether inconceivable that any accident of
138 ISABELLA M. DRUMMOND.
erowth, independent of torsion, which should cause the apex
of the visceral hump to remain upon the left side of the foot,
should alter this growth of the liver, and cause the coil to
become sinistral.
With regard to the cause of asymmetry, another view
remains to be mentioned here, namely, that of Grobben (8),
who, while accepting Pelseneer’s main conclusions, finds
himself unable to regard the antagonism between foot and
visceral hump as a true ontogenetic cause of torsion. He
grants Pelseneer’s view that the growth of the foot necessi-
tates a vertical displacement if the anus is to continue to
approach the mouth, and in order to explain how this
is produced he has recourse to Plate’s suggestion of
an unequal growth of the two originally symmetrical
liver lobes. For reasons already stated when Plate’s
theory was under discussion this is not altogether satis-
factory, for the growth of the liver does not keep pace
with torsion, and the chief development of it takes place
after torsion is complete. As a phylogenetic cause it may
have played its part, but probably not quite in the manner
that is described by Plate. _
Thiele (18) also takes up a position somewhat intermediate
between the two extreme points of view on torsion, for,
while agreeing with Pelseneer that before torsion the shell
must have been bent with its apex pointing forwards, as
though forming the beginning of an exogastric coil, he
approaches Lang more nearly than anyone else in his view
of how torsion has been effected. He dismisses Plate’s view
of the liver as the disturber of the original symmetry of the
body, and believes this part to be played by the gonad, in
which he also sees the cause of the coiling of the visceral
hump. Thus, as, in the Gasteropods, the gonad is formed
only on the left, the coil also lies with its apex in the left. The
condition of affairs which is thus reached is shown in his
fig. 3, p. 15, and this be believes to be a position of unstable
equilibrium, consequently a sudden rotation is effected till the
visceral hump comes to rest in the normal adult position.
THE DEVELOPMENT OF PALUDINA VIVIPARA. 139
Thiele seeks very little support for his view in the facts of
development, except in the rapid rotation observed by
Boutan in Acmoea, and it is seen at a glance to be wholly
inapplicable to the development of Paludina where rotation is
gradual, and where both gonad and coil are only formed
when torsion is far advanced. It brings us, therefore, no
nearer to forming a conception of the ontogenetic course of
torsion, and the development of Paludina gives no evidence
to support it as a phylogenetic theory.
Whatever view may be held with regard to phylogeny, in
ontogeny it seems to me that we are ultimately thrown back
upon the problems of heredity, and for the present we must
agree with Guiart (9) when he says, “ Mais 4 ceux qui nous
demanderont la cause mécanique de cette torsion, et qui nous
reprocheront de ne pas lavoir trouvée chez l’embryon, nous
répondrons simplement ceci. Il ne faut pas confondre
ontogénie et phylogénie, la cause w’existe pas chez l’embryon,
mais chez le mollusque primitif.’ From the nature of the
case the evidence which ontogeny can give upon the phylo-
genetic cause 1s merely negative.
SUMMARY.
To sum up, then, theories of Gasteropod torsion may be
divided into two classes :
a. Those which view the present position of the palleal
complex as due to a forward movement along the right side
of the body, which resulted from greater growth of the left
side of the body than of the right.
b. Those which view the present position of the palleal
complex as due to a ventral flexion followed by a vertical
rotation of the whole visceral hump upon the head.
The evidence for the second of these views seems greater
than that for the first, in that—
1. A vertical displacement through 180° of all the organs
contained in the visceral hump takes place in the course of
ontogeny.
14.0 ISABELLA M. DRUMMOND.
2. ‘There is some evidence, both from comparative anatomy
and embryology, for believing that the oesophagus has
undergone an actual twist.
3. Monstrosities which retain the palleal complex in a
ventral position show a tendency to form an exogastric
coil.
The innervation of the mantle was shown to be equally
difficult to explain on either hypothesis.
Also, against the first view was urged the insufficiency of
the evidence upon which Biitschlhi bases his conclusions with
regard to zones of unequal growth.
With regard to the phylogenetic cause of the vertical
twist, embryology can only give negative evidence ; while in
considering the ontogenetic cause we are thrown back upon
unsolved problems of heredity, and must confess our
ignorance,
In conclusion, I wish to offer most hearty thanks to
Professor Weldon, not only for having placed freely at my
disposal all the resources of the laboratory, but also for most
kind personal aid at all stages of the work. My thanks are
also due to Mr. Richard Evans for much help in the
technique.
List or WorKS REFERRED TO IN THE TExt.
1. Amauprut, A.—“ La Partie antérieure du Tube digestif chez les
Mollusques Gastéropodes,” ‘ Ann. des Sci. Nat. Zool. (8), vil.
2. Bouran, L.—‘‘ La Cause principale de l’Asymétrie des Mollusques
Gast éropodes,” ‘Arch. de Zool. exp.’ (3), vil.
3. Bouvier, K. l.—-“ Systéme nerveuse, Morphologie et Classification des
Gast éropodes Prosobranches,” ‘ Ann. des Sci. Nat. Zool.’ (7), iii.
4. Burscnu, O.—* Bemerkungen ueber die wahrscheinliche Herleitung
der Asymmetrie der Gastropoden,” ‘ Morph. Jalirb.,’ xii.
5. Wroancer, von R.—* Zur Entwicklungsgeschichte von Paludina
vivipara,” ¢ Morph. Jahrb.,’ xvii.
6. Extancer, von R.—“*Zur Entwicklungsgeschichte von Paludina
vivipara (vorlaufige Mittheilungen),’’ ‘Zool. Auz.,’ xiv.
7. Wruancer, von R.-—“On the Paired Nephridia of Prosobranchs,”
*Q. Journ, Mier. Sei.,’ xxxiii.
THE DEVELOPMENT OF PALUDINA VIVIPARA. 141
8. Grospen, K —‘ Kinige Betrachtungen ueber die phylogenetische Ent-
stehung der Drehung und der asymmetrischen Aufrollung bei den
Gastropoden,” ‘ Arb. Zool. Inst. Wien,’ xii.
9. Gurart, J.—‘‘ Gast éropodes Opisthobranches,” ‘Mém. de la Soc. Zool. de
France,’ xiv.
10. Hater, B.—“* Betrachtungen ueber die Phylogenese der Gonade und
deren Miindungsverhaltnisse bei niederen Prosobranchiern,” ‘ Zool.
Anz.,’ XXxill.
11. Hatter, B.—‘ Studien ueber Docoglosse, ete.,’ Leipzig, 1894.
12. PrtsenerrR, P.—“ Les Reins, les Glandes génitales et leurs Conduits
dans les Mollusques,” ‘Zool. Anz.,’ xix.
13. PELSENEER, P.—‘ Recherches morphologigues et phylogénétiques sur les
Mollusques archaiques,’’ ‘Mém. cour. et Mém. des Savants étrangers
de Acad. R. de Belg.,” lvii.
14. Petsenerr, P.— Recherches sur divers Opisthobranches,” ‘Mém. cour.
et Mém. des Savants étrangers de l’Acad. R. de Belg.,’ lili.
15. Prare, L.— Bemerkungen ueber die Phylogenie und die Entstehung der
Asymmetrie der Mollusken,” ‘ Zool. Jahrb.,’ ix.
16. Simrotn, H.—‘ Bronn’s Klassen und Ordnungen des Thierreichs,’’ B. iii.
5 >
17. Tonnices, C.—“Zur Organbildung von Paludina vivipara,” ‘S. B. Ges.
Bef. d. ges. Naturw., Marburg,’ 1899. (Abstr. in ‘Zool. Centralb.,’ vi.)
18. ‘TureLe, J.—‘‘ Ueber die Ausbildung der Korperform der Gastropoden,”
‘Arch. f. Naturgeschichte,’ Ixvii, 1901.
DESCRIPTION OF PLATES 7—9,
Illustrating Isabella M. Drummond’s paper, ‘Notes on the
Development of Paludina vivipara, with special
reference to the Urinogenital Organs and Theories of
Gasteropod Torsion.”
Significance of Reference Letters.
a. Anus. aa. Line representing a median ventral plane through head and
foot. ect. Outer epithelium of the body. £ Foot. yg. Gonad. 4%. Heart.
k. Kidney. 7. &, Original right kidney. 7. 4. Original left kidney. 4%. d.
Kidney duct. /, Liver. m. Mouth. m.c. Mantle cavity: 7. m. e. Original
right horn; Z. m.c. Original left horn. m./. Mantle fold. a@s. (Esophagus,
142 ISABELLA M. DRUMMOND.
op. Operculum, of. Otocyst. pe. Pericardium: 7. p. e. Original right
division ; 2. p. ¢. Original left division. p. g. Pedal ganglion. p. x. Pedal
nerves. 7. ap. Renal opening into the mantle cavity. vec. Rectum. 7.4. ap.
Reno-gonadial aperture. 7. pe. ap. Reno-pericardial aperture of the original
left kidney. 7.8, Radula sac. s.g. Shell gland. st. Stomach. sad. e. Sub-
cesophageal connective. sep. c. Supra-cesophageal connective. 7. Tentacle.
v. Velum. ves. and ves’. Vesicles attached to kidneys. v. 2. Visceral hump.
a. Ridge between the two original horns of the mantle cavity on which the
anus opens.
PLATE 7.
Fic. 1.—Slightly oblique transverse section through the visceral hump of
an embryo between the age of that shown in Fig. C and that in Fig. D.
Both kidneys and the first. rudiment of the heart are shown. xX 330.
l'te. 2.—Oblique section through the extreme (original) left portion of the
pericardium of an embryo rather older than that shown in Fig. G. The
sharp curve of the surface of the body shown at ecé. indicates that the
visceral hump is just beginning to coil, x 380.
ie. 3.—Another section of the same series, and showing the same region,
but rather more posterior. Two sections intervene between Figs. 2 and 3.
x 330.
Fig. 4.—The next section posterior to Fig. 3, and showing the same region.
x 330. The figure should be rotated about 9 degrees to the left to compare
with Fig. 3.
Fic. 5.—A transverse section through the same region as the above, but of
an older embryo in which about one complete turn of the spiral coil of the
visceral hump is complete. It is one of the same series as Fig. 17. x 3880.
Fira. 6.—An ideal longitudinal section of the whole genital apparatus of an
advanced embryo, with about two turns of the spiral complete, reconstructed
from a series of tiansverse sections. The gonad is represented spread out
instead of coiled. x 140.
l'ie. 7. —A transverse section across the region aa. of Fig. 6. x 380.
Fig. 8.—A transverse section across the region 6d. of Fig. 6, showing also
the close proximity of the genital organs to the liver. x 330.
lic. 9.—A transverse section across the region cc. of Fig.6. x 380.
Fie. 10.—Sagittal section through an embryo belonging to Stage A. x
330.
Fie. 11.—Transverse section through the posterior region of the visceral
hump of an embryo belonging to Stage C. x 119.
THE DEVELOPMENT OF PALUPINA VIVIPARA. 148
Fig. 12.—Transverse section through the posterior region of the visceral
hump of an embryo belonging to Stage D. x 119.
Vie. 13.—Trans verse section through the posterior region of the viscera}
hump of an embryo belonging to Stage E. x 119,
Fic. 14.—Another section from the same series but passing through the
anterior region of the visceral hump, and showing the creat anterior or right
extension of the mantle cavity. x 119. The section of the overhanging
foot, /, is introduced here in its relative position, but is omitted in Fig. 13.
Fie. 15.—'Transverse section through the posterior region of the visceral
hump of an embryo belonging to Stage F. x 119.
Fre, 16.—Transverse section through the posterior region of the visceral
hump and foot of an embryo belonging to Stage G. x 119.
Fie. 17.—Transverse section through the posterior region of the visceral
hump and foot of an embryo belonging to Stage H. x 119.
Fies. 18 and 19.—Transverse section through the “neck” region of the
same stage, showing the twist of the visceral connectives. x 119.
Fic. 20.—Transverse section through the visceral hump of the monstrosit
to)
shown in Fig. 10, Plate 8, in the region of the kidneys. x 87.
Fic. 21.—Another section of the same series passing through the opening
of the kidney duct into the mantle cavity. x 87.
ie. 22.—Transverse section through the stomach of the same embryo just
behind the opening into it of the cesophagus. x 80.
I'ic. B.—View of an embryo belonging to Stage B from the right side. x
Fie. C.—View of the right side of an embryo belonging to Stage C. x
70.
Fig. C,.—View of the ventral surface of an embryo slightly older than the
last: x70:
Fic. D.—View of the left side of an embryo belonging to Stage D. x 70.
Fie. f.—View of the left side of an embryo belonging to Stage NH. x 70.
Fic. f,.—View of the right side of the same. x 70.
Fre. Kj.—Dorsal view of the same. x 70.
Fic. F.—View of the left side of an embryo belonging to Stage F. x 70.
Fie. G.—View of the left side of an embryo belonging to Stage G. x 70.
Fie. M 1r.—View of the left side of Monstrosity Il. x 70.
Fic. M 111.—View of the left side of Monstrosity II. x 70.
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FERTILISATION OF THE EGGS OF ANIMALS. 145
Is Chemotaxis a Factor in the Fertilisation of
the Eggs of Animals?
By
A. H. Reginald Buller, B.Se., Ph.D.,
Lecturer in Botany at the University of Birmingham.
ContTENTS.
PAGE
I. IntRopucTION . : : : : ; . 145
Il. Some Frnrrintisation Prorgiems : ; ‘ 5 Us)
WT. Marertan : 3 : : ; : loi!
1V. Remarks wpon THE FEoecs aNnbD SPERMATOZOA OF THE
EcHINOIDEA . E ; : : 5 alte,
V. Tue Cuemoractic QursTIon . : ‘ ; = los
VI. Tue Movements or SPERMATOZOA UPON SURFACES . 7 59
VII. Tue Direction or PENETRATION OF THE GELATINOUS Coat. 167
VILL. Toe ArracuMENT oF Spermatozoa To THE Eee 4 . 174
IX. Summary or THE Cuter Resutts : : ; 5 YS
I. IntRopuction.
Te well-known researches of Pfeffer! have demonstrated
the importance of the part played by chemotactic stimuli in
causing the spermatozoa of liverworts, mosses, ferns, etc., to
approach the oospheres. Among the yet higher plants—
Gymnosperms and Angiosperms—the chemotropism* of
1 TLocomotorische Richtungsbewegungen durch chemische Reize,”
* Untersuchungen aus d. Bot. Inst. zu Tubingen,’ 1884, Bd. i, p. 363.
2 Molisch, “* Ueber die Ursachen der Wachstumsrichtungen bei Pollen-
schliiuchen,” ‘Sitzungsber. der Kais. Acad. d. Wiss. in Wicn,’ 1889 and 1893.
Also Lidforss, “ Ueber den Chemotropismus der Pollenschlauche,”’ ‘ Ber. d.
D. Bot. Gesell.’ 1895, Bd. xvii, p. 236.
VoL, 46, part 1.—NEW SERIES. K
146 A. H. REGINALD BULLER.
pollen-tubes takes the place of the chemotaxis of spermatozoa.
We may therefore say that actual contact of the sexual
elements of all plants from the liverworts onwards is brought
about by chemical stimuli.
Tn all the above-mentioned groups of plants the oospheres
ure fertilised in their place of origin without being set free.
The chemical stimulus, so far as is known, does not arise
directly from the oospheres. The spermatozoa of the vas-
cular cryptogams are attracted into the archegonia by a
substance liberated from the cell-sap of the neck-canal-cells.
The pollen-tubes are guided on their tortuous way to the
oospheres by substances excreted by various tissues of the
ovary and ovules.
On the other hand, the ova of animals are fertilised after
being set free from their place of origin, namely, the ovary.
Fertilisation takes place in the case of terrestrial animals,
e.@. mammals, reptiles, birds, and insects, in the oviduct, or,
as happens with many aquatic animals, e.g. Echinoderms,
many fishes, and amphibia, after the eggs have been deposited
in water. If, therefore, chemotaxis plays a role in bringing
the spermatozoa of animals into contact with the ova, the
source of stimulation must be looked for in a substance
excreted from the eggs.
It appears to be the general opinion among zoologists that
chemotaxis is actually a factor in the fertilisation of animal
egos. Thus Bergh! says that during an artificial fertilisation
experiment, e.g. in the case of the sea-urchin, “ the sperma-
tozoa collect around the ripe eggs, probably attracted by a
special substance.”’
Wilson,” in his latest edition of ‘The Cell,’ in dealing
with the union of the germ-cells, remarks: “ There is clear
evidence of a definite attraction between the germ-cells,
which is in some cases so marked (for example, in the polyp
Renilla) that when spermatozoa and ova are mixed in a small
1 Bergh, * Vorlesungen uber allgemeine Embryologie,’ 1895, p. 43.
2 Wilson, ‘The Cell in Development and Inheritance,’ 2nd ed., 1900,
p. 196.
FERTILISATION OF THE EGGS OF ANIMALS. 147
vessel, each ovum becomes in a few moments surrounded by
a dense fringe of spermatozoa, attached to its periphery by
their heads, and by their movements actually causing the
ovum to move about. The nature of the attraction is not
positively known, but Pfeffer’s researches on the spermatozoa
of plants leave little doubt that it is of a chemical nature.
‘The experiments indicate that the specific attrac-
tion between the germ-cells of the same species is owing to
the presence of specific chemical substances in each case.”
Here it may be at once remarked that the collection of
spermatozoa attached by their heads to the eggs in artificial
fertilisation experiments is no proof whatever that the
spermatozoa have been attracted from a distance to the ege
by a substance excreted from the latter. All that we can say
in such a case, without further observation, is that the eggs
retain the spermatozoa after these have come in contact with
them.
Verworn! goes so far as to say : “The splendid and methodi-
cal researches of Pfeffer upon chemotropism had their origin
in observations upon the spermatozoa of forms in which
chemotropic relations to the egg-cell were discovered. Such
relations, as we now know, have analogies in almost the
whole of living nature and for the fertilisation of the eggs of
animals by spermatozoa, just as for the eggs of plants, form
an indispensable condition. The spermatozoon seeks the egg
and is guided on the right course everywhere in the living
world by a chemotropic action, which the metabolic products
of the eggs exercise upon the free-swimming spermatozoa.
That from the innumerable hosts of spermatozoa of the most
diverse animals which in many places cloud the sea, each
species finds its right and specific ege, a phenomenon which
would otherwise excite astonishment, is in the great majority
of cases a direct result of chemotropism, and easily explains
itself on the ground that each spermatozoon is chemotropi-
cally attracted by the specific substances which characterise
the eggs of the species concerned.” It is one of the objects
1 Verworn, ‘ Physiologie,’ 1895, p. 425.
148 A. H. REGINALD BULLER.
of this paper to show that such sweeping generalisations with
regard to animals are so far entirely without experimental
justification.
With mosses, ferns, etc., there is as yet no proof that the
egos attract the spermatozoa to them in the manner in which
Wilson, Verworn, and others believe to be the case with
animals. As was pointed out, the eggs of these plants are
fertilised in their place of origin. ‘This permits of the
surrounding cells, neck-canal-cells, and ventral canal-cell
taking upon themselves the function of chemically attracting!
the male sexual element to the female. ‘The eggs may not
do more than simply retain the spermatozoa after contact has
taken place. Since the eggs of animals are fertilised after
liberation from their place of origin, there 1s no chance of
such a division of labour as occurs with plants. In the
analogy made by zoologists between ferns and animals there
is thus a weak point. Credit is given to the reproductive
egg of animals for an excretory function, which has not been
demonstrated in the case of plants.
There is one group of Algee—the Fucaceee—which are
unique among plants in that their eggs, like those of the
Echinoidea, are fertilised after extrusion into water. The
egos of the Fucacez differ, however, from those of most
animals, in being perfectly naked during fertilisation, and
in containing chlorophyll which assimilates? in the light.
1 The neck-canal-cells and ventral canal-cell secrete in the cell-sap of their
vacuoles an attractive substance or substances (probably a salt or salts of
malic acid). When the archegonium bursts these cells burst too, and die,
thus liberating their cell-sap, which diffuses slowly out of, and from, the
mouth of the archegonial tube. Pfeffer, loc. cit.
2 This fact I was able to prove by means of Engelmann’s method, using,
however, the spermatozoa of a sea-urchin instead of bacteria. A vast number
of spermatozoa were added to a preparation containing a few eggs of
Cystocyra barbata (one of the Fucacee). ‘The spermatozoa not in the
neighbourhood of the eggs came to rest in five minutes. Those around the
eggs continued in motion for more than an hour. ‘The movement also took
place around non-nucleated fragments of eggs. When the light was cut off
from the eggs the movement quickly ceased, to return again when light was
once more admitted,
FERTILISATION OF THE EGGS OF ANIMALS. 149
According to Strasburger! they excrete a substance which
attracts the spermatozoa from a distance equal to two
diameters of an ege. On the other hand, the observations
and experiments by Bordet? upon the fertilisation of the eggs
of several species of Fucus led him to entirely negative
conclusions as regards a chemotactic attraction, while he
found that the spermatozoa were highly sensitive to contact.
According to this observer it is simply the ability of the
spermatozoa to adhere to surfaces by the tip of one of their
two cilia, which leads to their collection upon an egg, while
their meeting with it is simply a matter of chance. A few obser-
vations of my own at Naples upon the fertilisation of Cysto-
cyra barbata (one of the Fucacez) did not reveal to me any
certain attraction of the spermatozoa from a distance, but the
collection of the spermatozoa upon the eggs in consequence of
their ability to cling to surfaces was clearly seen. Neverthe-
less, in view of the positive statement of Strasburger, a careful
reinvestigation of the question seems to me desirable.
The other cases® of supposed attraction of spermatozoa to
the egg-cells of plants all await a critical study.
In the cases of Clamydomonas and of Ulothrix,' Pfeffer
has observed that the meeting of the swarm-spores, which
afterwards copulate, is purely a matter of chance. He also
found that the spermatozoa of a bull? were not attracted by
meat extract.
At present, to the best of my knowledge, not a single case
is known where chemotaxis plays a role inthe fertilisation of
the eges of animals.
Dewitz® has shown that the spermatozoa of certain insects
1 Strasburger, ‘ Das bot. Prakticum,’ 2 Aufl., 1887, p. 402.
2 Bordet, “ Contribution al’ Etude de l’Irritabilité des Spermatozoides chez
les Fucacées,” ‘ Bull. de l’Acad. Belgique,’ 3e sér., tome xxvii, 1894, p. 889.
3 See Pfeffer, loc. cit., pp. 446—449.
4 Loe. cit., p. 447.
5 Loc. cit., p. 449.
6 Dewitz, ‘‘ Ueber Gesetzmiassigkeit in der Ortsverinderung der Sperma-
tozoen und in der Vereinigung derselben mit dem Ki,” ‘Arch. f. die gesammte
Physiologie,” Bd. xxxviil, 1886, p. 558.
150 A. H. REGINALD BULLER.
find their way to and through the micropyles of the eggs
owing to the remarkable fact that on coming to a surface
they remain in contact with it and continue to move in circles.
This characteristic, which will be discussed more fully after-
wards, I have found also shared by the spermatozoa of repre-
sentatives of every group of the Echinodermata.
Massart! made a careful investigation of the fertilisation of
frogs’ eges. He came to the conclusion that the spermatozoa
come in contact with the gelatinous coat by accident, and
cling to it owing to a special sensibility to contact. He found
that they bore through it radially. He believed that this is
explained on the supposition that the spermatozoa seek to
bore from the more watery outer layers to the less watery
inner layers in consequence of a sensibility to the differences
of saturation.
My own investigations, undertaken at Naples, were made
to determine the nature of the forces which bring the
spermatozoa and eggs of the Hchimoidea in contact, especial
attention being paid to the chemotactic question. ‘lhe work
was taken up after a fairly extended study of the chemotaxis?
of the spermatozoa of ferns.
II. Somn Ferrinisation PROBLEMS.
In the case of such eggs as those of the Echinoidea, which
are surrounded by a thick gelatinous coat, some of the
physiological questions that may be asked in regard to the
manner in which the spermatozoa meet and fuse with them
are as follows:
1. Does a spermatozoon meet the gelatinous coat (zona
pellucida) by accident, or is it attracted to it by some
' Massart, (1) “Sur VIrritabilité des Spermatozoides de la Grenouille,”
‘ Bull. de Acad. roy. de Belgique,’ 3me sér., t. xv, No.5, 1888; (2) “Sur la
Pénétration des Spermatozoides dans uf de la Grenouille,” ‘ Bull. de PAcad,
roy. de Belgique,’ 3me, sér., t. xvii, No. 8, 1889.
* Buller, “Contributions to our Knowledge of the Physiology of the Sper-
matozoa of Ferns,” ‘ Ann. of Botany,’ vol. xiv, 1900, p, 543.
FERTILISATION OF THE EGGS OF ANIMALS. boil
chemotactic substance which is excreted by the hving egg
and diffuses through the gelatinous coat into the surrounding
water ?
2. After a spermatozoon has come in contact with the outer
surface of the gelatinous coat, is it retained there mechani-
eally or in consequence of a tactile stimulus exerted upon it
by the surface ?
3. Does the spermatozoon bore through the gelatinous coat
radially ? Ifso, why?
4, After reaching the outer surface of the hving egg (1. e.
the protoplasm), what is the nature of the forces which lead
the spermatozoon to unite with it ?
5. Closely connected with the latter is the further question:
by what means is the progress of a spermatozoon from the
surface to the interior of an egg brought about ?
TIT. Marerrat.
The following species of Echinodermata were made use of :
(Echinus microtuberculatus, Bly.
Class Hchinoidea |Spherechinus granularis, Ag.
Reeulares parbaieke pustulosa, Gray.
\Strongylocentrotus lividus, Brdt.
Irregulares Hchinocardium cordatus, Gray.
Asterias glacialis, O.F.M.
ate ais sepositus, Mill. Tr.
Ophioderma longicauda, Mill. Tr.
(Ophioglypha lacertosa, Lyman.
Class Holothuroidea Holothuria Stellate, D.Ch.
Class Crinoidea . Antedon rosacea, Norman. -
Class Asteroidea
Class Ophinroidea
Observations upon the motility, especially in regard to
surfaces, were made upon the spermatozoa of all. the above
species. The experiments and observations upon fertilisa-
tion were restricted to the first three Echinoidea, namely,
Echinus, Spherechinus, and Arbacia.
152 A. H. REGINALD BULLER.
TV. RemarKs UPON THE EGGs AND SPERMATOZOA OF 'THE
ECHINOIDEA.
The eggs of the Echinoidea (as is also the case with all
the Hchinodermata) are surrounded by a thick, very trans-
parent, gelatinous coat, the zona pellucida, through which
the spermatozoa have to make their way before they reach
the living protoplasm of the egg.
The following measurements from Echinus will give some
idea of the relative sizes of the living eggs, the gelatinous
Fic. 1.—Eeg of Echinus microtubereculatus. x 170. a.
Outline of protoplasm. 4. Outline of zona pellucida after five
minutes in sea-water. ce. Outline of zona pellucida after several
hours in sea-water. s. Spermatozoon.
coat, and the spermatozoa :—Diameter of living egg alone
O-ll mm.; diameter of living egg and gelatinous coat
0:18 mm.; thickness of gelatinous coat 0°036 mm.; length
of a spermatozoon 0°051 mm.
Kach of the measurements just given is the average of ten
measurements. The eggs were measured almost directly
after being placed in water.
The jelly increases in thickness after deposition of the
egg in sea-water. After twenty-four hours it is found to
have nearly doubled in thickness, and to have become
0°057 mm. wide. The width of the jelly, which in the fresh
FERTILISATION OF THE EGGS OF ANIMALS. 5
ege is less, is, then, in the eggs which have stood twenty-
four hours in water, slightly more than the length of the
spermatozoa (Fig. 1).
The presence of the gelatinous coat 1s quite unessential to
the union of spermatozoa and eggs, for if by shaking it be
removed, fertilisation will still take place with the greatest
ease.
~ When liberated at the top, the eggs gradually sink to the
bottom of a beaker containing still sea-water. ‘They thus
appear to be heavier than their normal medium. Very small
currents are, however, sufficient to keep the eggs floating.
Probably in the sea, where the eggs are liberated, such
currents are always present. In that case, too, the currents
are of considerable importance in mixing the eggs and
spermatozoa.
V. Tae Cremoractic Question.
After repeated trials with unilateral illumination, I was
unable to detect the least sensitiveness of the spermatozoa of
Arbacia or Echinus for heliotactic stimuli. It was there-
fore not possible by application of such a stimulus to allow
the spermatozoa to stream in the direction of the eggs (which
may be done in the case of Fucus), and to observe whether,
when passing, they deviate toward them.
A large number of artificial fertilisation experiments were
undertaken. ‘l’o one side of a drop, either open or under a
raised coverglass, a small drop bearing spermatozoa was
added. The spermatozoa spread quickly in all directions,
and in the course of their wanderings came in contact with
the eggs, bringing about fertilisation. Within a few minutes
this was made evident by the raising of the vitelline mem-
brane. I failed, however, to observe any attraction of the
spermatozoa toward the eggs from a distance, or any collec-
tion of the spermatozoa around the eggs outside the gela-
tinous coat. On the other hand, spermatozoa were frequently
154 A. H. REGINALD BULLER.
seen to pass by an egg so as almost to touch it, apparently
without being in any way influenced by its presence.
Nothing was seen which in any way reminded me of the
chemotactic phenomena either of bacteria or of the sperma-
tozoa of ferns.
It is undoubtedly true that the spermatozoa collect rapidly
in the gelatinous coat of an egg. This is, however, due to
the fact that the spermatozoa which strike the outer surface
immediately bore into the interior. It will subsequently be
shown more fully that the phenomenon takes place equally
well when the jelly encloses (1) a ripe egg; (2) an egg not
having undergone maturation; and (3) an egg which has
been killed with osmic acid, and then washed. There is thus
not the shghtest necessity to account for the collection of the
spermatozoa in the gelatinous coat by any chemotactic sub-
stance which diffuses through the jelly into the sea-water,
and so attracts spermatozoa towards the ege.
If a substance causing attraction is really excreted by the
eges one should be able to collect it. On this assumption
the following experiments were made.
A freshly obtained female Arbacia was cut open. The
egos, which were then extruded by the animal in dense
masses from the oviducts, were collected in about 100 c.c. sea-
water contained in a crystallising dish, As soon as the eggs
had settled to the bottom, for the purpose of washing them,
the water was nearly all removed by means of a pipette.
Another 100 ¢.c. was then added, and so much again removed
after the eggs had settled that the latter, very thickly placed
together, formed a layer in about 1—5 mm. of the sea-water.
Snfficient oxygen could thus be obtained for respiration.
The eges were left in the water from two to twelve hours,
usnally about six. At the end of this period the water was
filtered, and the ege@s thus removed. Capillary glass tubes,
about 12 mm, long and 0:1—0°3 mm. internal diameter, and
closed at one end, were then half filled with the water by
means of an air-pump. The tubes were then introduced into a
large open drop of sea-water, in which fresh, highly motile
FERTILISATION OF THE EGGS OF ANIMALS. 155
spermatozoa were swimming. If the eggs excrete an
attracting substance it was argued that it should be present
in the tubes, and the spermatozoa should collect there.
In order to make certain that the eggs had remained
normal during their stay in the sea-water, just before filtra-
tion some of the eggs were tested by artificial fertilisation.
In all the experiments upon which reliance has been placed
for results this took place in the normal manner, 1. e. the
vitellne membrane became raised. The first stages of
segmentation were also often watched, and found in the
majority of cases to follow the usual plan. The eggs were
also tested soon after they had been placed in water. If, as
rarely happened, they did not become fertilised readily they
were rejected. ‘he experiments were repeated with four
sets (g¢ and 9?) of Arbacia, three of Spherechinus, and
two of Hchinus.
No attraction of the spermatozoa into a tube could be
observed. Except for a surface-contact phenomenon to be
further discussed, they went in and out with indifference.
Apparently, therefore, the water which had contained the
eggs exercised no directive stimulus on the spermatozoa
whatever.
I then attempted to find some substance which would give
a chemotactic stimulus to the spermatozoa. The substances
tested were such as are known to give a directive chemical
stimulus to many protozoa, the spermatozoa of ferns, pollen-
tubes, ete. The following solutions were tried by the capillary
tube method: distilled water; meat extract 1 per cent. ;
potassium nitrate 10 per cent., 2 per cent.; sodium chloride
5:8 per cent., 2°9 per cent., 0°58 per cent.; potassium malate
1 per cent., O'l per cent.; asparagin | per cent.; glycerine
5 c.c. per cent.; grape sugar 18 per cent., 9 per cent., 4°5
per cent., 2°25 per cent. ; peptone | per cent.; alcohol 50 per
cent., 25 per cent., 10 per cent.; diastase (from Merk) 1 per
cent. ; oxalicacid 0°9 per cent., 0:09 per cent., 0:009 per cent. ;
nitric acid (concentrated) 1 per cent., 0-1 per cent., 0°01 per
cent.
156 A. H. REGINALD BULLER.
No definite chemotactic reaction—neither attraction nor
repulsion—was observed in any case. Into tubes containing
the weaker solutions the spermatozoa went in and out with
apparent indifference. On coming into contact with highly
concentrated neutral substances (potassium nitrate 10 per
cent., sodium chloride 5°8 per cent., grape sugar 18 per cent.)
the spermatozoa came to rest from loss of water. They are
evidently not able to avoid solutions by a negative tonotactic
reaction. On coming into contact with strong acid solutions
(oxalic acid 0°9 per cent., 0°09 per cent., nitric acid 1 per
cent., 0°1 per cent.) the spermatozoa were killed, and thus
formed slight collections. They were thus not able to avoid
acids by means of a negative chemotactic reaction.
Having obtained, so far as chemotaxis is concerned, only
negative results by means of the capillary tube method,
another method was employed, which Massart ! found effective
in determining the tonotactic sensibility of a number of
marine micro-organisms.
Two large and equal drops were made in a moist chamber ;
one was of sea-water containing spermatozoa, the other
distilled water. The drops were then joined by a narrow
bridge, so that diffusion between them could take place.
The experiment was watched between one and two hours.
Some spermatozoa gradually entered the fresh water. No
collection, however, took place in either drop. There were
thus no signs of attraction or repulsion. The spermatozoa
that entered the too diluted water were killed.
In another experiment a small drop of sea-water was dried
upon a glass slide. A large oval drop of sea-water contain-
ing motile spermatozoa was then placed near and spread so
that one end just covered the crystals from the dried drop.
The crystals began to dissolve rapidly, locally concentrating
the sea-water. As diffusion from the concentrated end of
the drop took place the spermatozoa in the neighbourhood
1 Massart, ‘* Recherches sur les Organismes Inférieurs. IT. La Sensibilité
a la Concentration chez les Etres Unicellulaires Marins,” ‘ Bull. del Acad.
roy. de Belgique,’ 3me sér., tome xxii, No. 8, 1891, p. 148.
FERTILISATION OF THE EGGS OF ANIMALS. 157
were killed or brought to rest. They were thus not repelled
by concentrated sea-water.
When to one end of a large oval drop containing sperma-
tozoa some crystals of sodium chloride or potassium nitrate
were added, similar results were obtained. ‘The sperma-
tozoa allowed themselves to be surprised by the advancing
salt and were accordingly killed.
The drop experiments were, then, not more successful than
those with capillary tubes. No evidence of chemotactic
reaction could be obtained by either method.
Massart! found by experiment that two species of
Spirillum A and C, the flagellate Heteromita rostrata
and two species of Ciliata fled from solutions both more or
less concentrated than sea-water, i.e. they always sought a
zone with the concentration equal to their normal medium.
Oxytricha gibba also fled from solutions more highly
coucentrated than sea-water, but failed to avoid those less
concentrated. In the case of his Spirillum B he obtained
an organism which did not flee from solutions either more or
less concentrated than sea-water, and which suffered in the
experiments accordingly. Both the drop and capillary tube
experiments described above appear to indicate that the sper-
matozoa of the Echinoidea, like Massart’s Spirillum B, are
quite insensible to tonotactic stimuli.
Finally, I attempted to determine whether the spermatozoa
are attracted or repelled by oxygen. Fresh spermatozoa
were removed from a testis and placed fairly thickly together
in a drop, so that the latter was very slightly milky. A cover-
glass 0°15 mm. thick and 18 mm. square, supported on pieces
of another cover-glass also 0°15 mm. thick, was then placed
upon the drop in such fashion as to include a small bubble
of air near the middle. Under these circumstances one sees
neither attraction nor repulsion from the bubble, even when
the experiment has continued some time and when the
oxygen supply must be getting low.
When, however, the drop is made very milky by spreading a
' Massart, loc. cit., pp. 151—154.
158 A. H. REGINALD BULLER.
little of the thick white sperm-fluid in it by means of a pipette,
a peculiar effect may be observed as a result of the presence
of an air-bubble. The spermatozoa, in incredible numbers
and constantly colliding, first swarm equally well all over
the preparation. After about five minutes one sees macro-
scopically, when looking at the slide upon the microscope
stage, a black zone arise about 1 mm. from the edge of the
air-bubble. On examination with the microscope one sees
that there are fewer spermatozoa there than anywhere else.
Three zones (Fig. 2) may then be made out around the air-
bubble 2: a,an inner zone crowded with actively motile sper-
matozoa; b, a much thinner zone (that appearing macro-
Figs. 2 and 3.
scopically black) in which there are comparatively very few
spermatozoa; and c, the zone outside b (which extends over
the rest of the preparation nearly to the edge of the cover-
glass) where the spermatozoa are crowded, so far as I could
judge, about as thickly as in zone a, but have all come to rest
from want of oxygen. As one watches the preparation one
sees (fig. 3) that the spermatozoa gradually leave the zone a
and collect on the inner edge of the zone c, upon reaching
which they cease to move. Jennings, “On the Movements and Motor Reflexes of the Flagellata and
Ciliata,” ‘Amer. Journ. of Physiology,’ vol. iil, Jan., 1900, p. 229.
166 A. H. REGINALD BULLER.
often stop in their rotation, and that the head becomes fixed
in the jelly. Sometimes the spermatozoon then succeeds in
boring its way through, and may then reach the living
protoplasm. In most cases, however, the head of a sper-
matozoon which has rotated a number of times gets stuck
in the outer layer of the jelly, and no successful penetration
occurs.
The gelatinous coat of an egg which has only been in
water a few minutes is much more difficult for the spermatozoa
to penetrate than that of an egg which has been in water
several hours. Comparative experiments easily demonstrated
this point, the difference being really striking. The jelly, as
already mentioned, swells in water, and gradually nearly
doubles its original breadth. At the same time it becomes
softer. When spermatozoa are added after the swelling has
taken place, scarcely a single spermatozoon is seen to rotate
upon the eggs; on the contrary, they nearly all succeed
in fixing their heads in the jelly, and the majority penetrate
almost up to the living egg.
Dewitz! believes that the rotation of the spermatozoa of
Blatta upon surfaces is of prime importance in enabling the
spermatozoa to find their way into the micropyles of the eggs.
This may well be the fact. In the case of the Echinoidea,
however, there are no micropyles, and the gelatinous coat is
everywhere penetrable. Further, rotation upon the eggs
appears not to be the rule. It seems to me, there-
fore, that the fact that the spermatozoa will rotate upon
resistant surfaces has no special biological significance in
respect to fertilisation. On the other hand, the ability of the
spermatozoa to cling to surfaces and to get stuck to them by
the pointed end of their heads is of great importance in
causing them not to leave the gelatinous coat of an egg after
having come in contact with it, and in penetrating the same.
! Dewitz, loc. cit.
FERTILISATION OF THE EGGS OF ANIMALS. 167
VII. Tse Drrecrion or PENETRATION OF THE GELATINOUS Coat,
Fol! observed for Asterias that when a spermatozoon had
come in contact with the gelatinous coat it placed itself
perpendicular to it, and then penetrated radially to the egg,
meeting and fusing when half-way through with a curious
“cone Wexudation.”’ Fol concluded that the gelatinous
coat is an apparatus for catching the spermatozoa when they
come in contact with it, and attributed the radial structure to
lines of more or less resistance, which serve to guide the
spermatozoa directly to the egg.
Selenka* investigated the development of the gelatinous
coat, and found that at first it is penetrated by fine radial
protoplasmic filaments, each in its own canal. Later the fila-
ments become withdrawn, but the canals remain until after
fertilisation. Selenka also observed that the spermatozoa
penetrate the jelly “always ina radial direction,’ and stated
that this is due to the spermatozoa making their way through
the canals. This explanation appears to be very plausible.
Before, however, accepting it as being sufficient, we shall do
well to bear in mind the observations of Kupffer and Benecke
upon the fertilisation of the eggs of Petromyzon.
According to the last-named authors the spermatozoa of
Petromyzon penetrate a thick gelatinous dome covering
one end of an egg in a radial direction. This observation,
upon which special stress was laid, confirmed the statements
previously made by August Miller. Although the inner
shell-layer was found to contain radial canals, Kupffer and
Benecke could discover no trace of such in the outer shell-
layer, while they described and figured the dome as being
1 Fol, ‘ Recherches sur la Fécondation, ete.,’ 1879.
* Selenka, ‘ Zoologische Studien, Befruchtung des Kies von Toxopneustes
variegatus,’ 1878, p. 2.
= ioe: cit., p. 5.
* Kupffer and Benecke, ‘Der Vorgang der Befruchtung am Wi der Neu-
naugen,’ Kouigsberg, 1878, p. 11.
168 A. H. REGINALD BULLER.
quite hyaline. The radial path of the spermatozoon is so
striking that the authors believed it necessary to postulate
some attraction! of the egg for the spermatozoon from a
distance. Concerning the nature of the forces, however, no
suggestion was made.
In my own investigations special attention was paid to the
direction of penetration of the spermatozoa through the gela-
tinous coat of the eggs of Echinus. In this case, at least,
it cannot be stated that penetration is always in a radial
direction. A great many spermatozoa penetrate obliquely.
It appeared to me, however, after having made a large
number of observations for determining the point, that on
the whole there is a tendency for the spermatozoa to make
their way from the outside to the inside of the gelatinous
coat. his tendency is best seen after the eggs have been
from three to six hours in sea-water and the jelly has become
considerably swollen. One then observes, upon adding
spermatozoa, that on the whole, although many penetrate
obliquely, the spermatozoa pass in a radial manner through
the jelly to the egg. It is easy to observe spermatozoa which
take an almost perfectly radial course. The path of many of
them is seen to incline to a radius by an angle equal to
between 10° and 30°. Others may be observed to start
fairly radially, soon turn aside, and continue obliquely
striking the eggs thus obliquely, or occasionally even making
their way out again in a tangential direction. A considerable
number of spermatozoa, after entering, stick fast in the jelly.
The heads of these are then seen to be very variously oriented
with respect to a radius.
Having come to the conclusion that the spermatozoa do
pass more or less radially through the gelatinous coat, my
next inquiry was concerning the cause. It was found that
the radial penetration could be equally well observed in (1)
a ripe egg; (2) a full-sized egg which had not undergone
maturation, the nucleus being still very large and uncon-
tracted ; and in (5) a ripe egg which had been killed with
1 Kupffer and Benecke, loc. cit., figs. 1, 7, and 8.
FERTILISATION OF THE EGGS OF ANIMALS. 169
osmic acid and then washed. In the last case the osmic acid
turned the eggs brown. ‘The eggs so killed were put in
100 c.c. sea-water for half an hour and stirred round at
intervals. They were then caught in a pipette, placed in a
drop on a slide, and spermatozoa added. he radial penetra-
tion was quite as clear as in the living eggs.
From the foregoing observations it seems evident that the
radial penetration is not brought about by any special attrac-
tion by the living egg, for it takes place equally well with a
dead egg. Nor do the facts point to any chemotactic attract-
ing substance as causing the phenomenon, for from a dead
egg no excretion can take place. Selenka’s suggestion that
the spermatozoa take a radial course because they make their
way through canals, which during the development of the
egg contained protoplasmic connections, also does not seem
to me a satisfactory explanation. ‘I'he radial structure of the
gelatinous coat after an egg has been a few hours in water
is extremely faint, and, so far as one can directly observe,
absent at the periphery where the spermatozoa start on their
course. Selenka' admitted that the canals were finer than
the width of the head of a spermatozoon. Surely with the
swelling of the jelly these canals must be practically filled up.
I have, as already stated, very frequently seen spermatozoa
penetrate the gelatinous coat obliquely, often very obliquely.
In these cases the spermatozoa could not be making their
way through Selenka’s canals. Hence we may conclude that
the canals, if such there are, are not necessary for penetra-
tion. The thick gelatinous dome of a Petromyzon eg,
and also, according to Massart,? the jelly around the ovum of
the frog, are penetrated radially without the presence of any
canals whatever. These various facts point to the conclusion
that the penetration of the gelatinous coat in a more or less
radial direction by the spermatozoa is not due to canals, but
to some other cause.
The above reflections led me to make experiments to find
1 Selenka, loc. cit., p. 5.
* Massart, “Sur la Pénétration, ete.,” loc. cit., p. 217.
170 A. H. REGINALD BULLER.
out how the spermatozoa behave toward jelly from other
sources than that from the eggs of the Echinoidea.
When the oosporangia of Cystocyra barbata (one of
the Fucacez) are liberated into sea-water, the outer coat
rapidly swells and gelatinises. Spermatozoa from Arbacia
were added to a preparation containing some of the oospor-
angia, Ata certain stage in the gelatinisation the sperma-
tozoa entered the jelly in large numbers, thus becoming
densely crowded together in it.
A similar gathering was observed when the seed-coat of
Linum usatissimum was placed in water contaiming
spermatozoa. The outer cell-walls rapidly swell and become
gelatinous. The spermatozoa, when the jelly had reached a
certain consistency, collected in it in large numbers.
It was also found that if the gelatinous coat of an
Hchinus egg be separated by shaking, and spermatozoa be
allowed access to the coat after several hours’ isolation, the
number of spermatozoa which will gather in it is very
considerable.
The conclusion that is to be drawn from the above experi-
ments appears to be that the spermatozoa are so constructed
that they will bore their way into any jelly of a certain
consistency without any aid from canals, chemotactic sub-
stances, or influences from living protoplasm.
Massart, as already mentioned, explains the radial pene-
tration in the frog by supposing that the spermatozoa seek
to pass from the more watery to the less watery layers of
jelly, owing to a sensibility to these differences in saturation.
Although this theory is plausible, it does not appear to me
to be convincing. It does not sufficiently explain why the
head of a spermatozoon is at first pushed into the jelly ina
radial direction. After the head has been pushed in, whether
this be radially or somewhat obliquely, the spermatozoon of
Wchinus usually takes a fairly straight course with respect
to the axis of the head. Evidence of picking and choosing
between the gelatinous layers thus appears to be wanting.
After an egg of Echinus has been in water for several hours
FERTILISATION OF THE EGGS OF ANIMALS. RAL
it is doubtful whether the outer layers of jelly are the more
watery and the inner the less so. In fact, from the ease with
which the spermatozoa rotate the egg inside the gelatinous
coat (vide infra), one might well suppose that the innermost
layers are the more watery. ‘The view that the resistance of
the jelly decreases inwards has, indeed, already been upheld
by Selenka! for the eggs of Asterias. For the Echinoidea
and Asteroidea, therefore, the necessary basis of fact for an
application of Massart’s theory seems to be wanting.
There appear to me to be yet two possible explanations of
the penetration: (1) It is due to reaction to a stereotactic
stimulus ; (2) it is purely mechanical.
1. Stereotropism has long been observed. Very many
organisms, both animals and plants, in sea- and fresh-water,
grow perpendicularly to their substratum, owing to the
influence which the position of the latter has upon their
direction of growth. In the same manner as for geotropism,
heliotropism, chemotropism, etc., we have a corresponding
tactic phenomenon, so also may it be with stereotropism. It
is possible to imagine a free-swimming organism which, upon
coming in contact with a surface, receives from it a stimulus
which causes it to alter its movements in such a manner as to
attempt to make its way more or less perpendicularly to the
same, and through the substance concerned. Although such
a stereotactic sensitiveness would neatly explain the radial
penetration for the Echinoidea, Petromyzon, and the frog,
yet conclusive observations in its support appear to me to be
lacking.
2. Owing to the extreme difficulty or impossibility of seeing
exactly what the movements of a spermatozoon upon a gela-
tinous surface are, the mechanical explanation must at present
remain tentative and almost purely hypothetical. When a
spermatozoon, swimming spirally, comes in contact with the
outer surface of the gelatinous coat, the tip of the conical
head, which reaches it first, possibly owing to the force of
contact, possibly to adhesiveness, may well be supposed to
' Selenka, loc. cit., IL ‘ Die Befruchtung, Das Spermatozoon.’
172 A. H. REGINALD BULLER.
immediately fix itself in the jelly. This is, indeed, what
appears to take place under the microscope. The tail of the
spermatozoon then probably adheres to the outer surface of
the egg-coat, and is dragged round and round on it about
the conical head, which is gradually pushed forward through
the jelly. It may well be these revolutions (the modified
spiral of the usual mode of swimming) which cause a
spermatozoon to bore through the jelly more or less perpen-
dicularly to the surface. The fact that the head is of such a
shape that when once embedded in the gelatinous coat it can
be easily pushed forward, but offers considerable resistance
to moving either backwards or sideways, together with the
particular consistency of the jelly, may well account for the
steady progress forward of the whole spermatozoon in one
direction. It seems to me probable that some such explana-
tion as the foregoing will be sufficient to explain all that
takes place during the penetration of the gelatinous coat.
It may here be remarked that since the presence of a
gelatinous coat doubles the diameter of an egg its presence
multiplies the chances of contact with its exterior surface
by a spermatozoon four times. Since the more active sper-
matozoa, after coming in contact with the jelly, are con-
ducted by it to the living protoplasm of the egg, the chances
of fertilisation by them is, by the presence of the gelatinous
coat, also increased four times. Since, however, a consider-
able percentage of the weaker spermatozoa get stuck in the
gelatinous coat after entrance, thus not reaching the living
ego, our estimate of the increased chances of fertilisation
must undergo a large reduction. The first function of the
jelly, which surrounds so many eggs, appears to be that of
protection, making them distasteful to larger, and unassail-
able by smaller, enemies. For the purpose of fertilisation its
consistency must be such as to allow easy penetration by the
spermatozoa.
The ease with which spermatozoa enter and become fixed
in gelatinous substances will explain a phenomenon which at
first puzzled me. It was observed that, when a capillary
FERTILISATION OF THE EGGS OF ANIMALS. 175
tube containing sea water, in which eggs had previously
been deposited, was placed in a drop contaiming sper-
matozoa, the spermatozoa were not attracted into the tube.
On the other hand, it frequently happened that the sper-
matozoa gathered very thickly into small balls just inside
and outside of a tube. The balls were sometimes 0°01 to
0:05 mm. in diameter. It was apparent that, since the balls
were only formed at the mouth of a tube, the cause of their
formation was to be sought in the filtered sea-water. The
phenomenon was found to take place after six successive filtra-
tions. A drop of sea-water in which eggs had been deposited
was placed upon a slide and a drop containing spermatozoa
near it. On joining the drops a large number of small balls
were formed in a very few seconds. When very numerous
spermatozoa were present the balls became 0:1 mm. in
diameter, containing many thousands of spermatozoa packed
together in a dense mass. The following appears to be the
explanation of the phenomenon:—From the ovary there
come out with the eggs a large number of very small bits of
jelly, which are so small that they will (like spermatozoa)
pass through ordinary filter paper, and so transparent that
one cannot directly see them. A few spermatozoa become
attached to each piece of jelly, the presence of which may be
inferred from the manner in which the small group of sper-
matozoa move about. Owing to the length of a spermatozoon,
although its head, may be embedded in a jelly particle, the
tail may remain partly free. The little collections of sper-
matozoa thus move about hither and thither in no particular
direction. When two such groups come by accident into
contact they fuse. Certain of the spermatozoa adhere to
both little masses of jelly and lock them together. The
fused mass combines with other simple and fused masses, and
soon. It is by this curious synthetic process that, in a very
few seconds, there may be formed a ball as large or larger
than an Echinus egg and containing thousands and
thousands of spermatozoa, looking black under the micro-
scope, and easily seen in a drop of water with the unaided eye.
174 A. H. REGINALD BULLER.
VII. Tae ATTACHMENT OF SPERMATOZOA TO THE Kaa.
As soon as a spermatozoon has penetrated the gelatinous
coat it usually becomes fixed by the head to the periphery of
the living egg. Sometimes it executes circles for a while
upon the protoplasm, and occasionally even re-enters the
jelly and makes its way through this in a radial direction,
thus leaving the egg entirely.
When a great number of spermatozoa are allowed access
to an egg which has been some hours in sea-water, so many
immediately penetrate and become attached by their heads
that they set the egg in rotation. The rotation may be in
any direction,! and often continues for about a minute,
ceasing with the formation of the vitellne membrane. The
rate of revolution varies according to the number of sper-
matozoa attached to the egg. A rapidly moving egg of
Arbacia was observed to make ten revolutions in thirty
seconds. ‘The gelatinous coat during rotation scarcely moves
at all, the living egg revolving quite independently within it. |
The spermatozoa often move the egg with such force as to’
separate it from its gelatinous coat. One then observes that,
except for those attached by their heads to the egg, there is
no collection of spermatozoa around the latter. This fact is
in accordance with the supposition that no chemotactic sub-
stance is excreted by the egg. Numerous spermatozoa enter
the isolated gelatinous coat.
The spermatozoon attaches itself to the egg by its most
adhesive part, i. e. the tip of the head. The question arises
whether the attachment is purely mechanical. It may be
that the outer surface of the protoplasm is such as to be
best adapted for retaining a spermatozoon by adhesion as
' For the eggs of the Fucaces the rotation appears to be constantly in a
clockwise direction. Thus Farmer and Williams (‘ Phil. Trans. Roy. Soe.,’
vol. 190, 1898, p. 633) state for Halidrys “the movement is always in a
clockwise direction.” I have also found this true for Cystocyra barbata.
The fact as yet has not been explained.
FERTILISATION OF THE EGGS OF ANIMALS. 175
soon as this comes in contact with it by the tip of its head.
On the other hand, it is possible, and even probable, that un-
known stimuli here play a part. The advance of the sper-
matozoon into the egg after leaving the periphery is, like the
formation of the vitelline membrane, doubtless due to a
stimulus given the egg by the spermatozoon. With regard
to the exact nature of the stimulus and of the protoplasmic
movements which appear to be its reaction we are as yet
without any explanation.
TX. Summary oF THE Cuter Resottrs.
The chief conclusions arrived at during the research upon
the fertilisation of the eggs of the HEchinoidea were as
follows :
1. The meeting of the spermatozoa with the outer surface
of the gelatinous coat (zona pellucida) is a matter of chance,
and not due to chemotaxis.
2. The passage of the spermatozoa through the gelatinous
coat (observed chiefly in Hchinus) is more or less ina radial
direction as regards the egg. The direction taken is not due
to any chemotactic substance being excreted from the egg.
The phenomenon is possibly due to stereotaxis, but a purely
mechanical explanation seems to the author more probable.
3. The spermatozoa are probably not chemotactically
sensitive. They do not respond to tonotactic or heliotactic
stimuli.
4, On coming in contact with a surface bounding their
medium the spermatozoa cling to it, and usually continue for a
time to revolve upon it in (from their point of view) a counter-
clockwise direction. ‘l'his statement applies to every group
of the Echinodermata.
5. The spermatozoa easily become attached to glass and
other surfaces by the tips of their conical heads. This
phenomenon doubtless plays a réle in causing the spermatozoa
to bore through the gelatinous coat after having come in con-
176 A. H. REGINALD BULLER.
tact with its outer surface, and also in their becoming
attached to the living egg.
6. The vast number of eggs, and still vaster number of
spermatozoa produced, together with the motility of the
latter and the action of sea-currents, quite suffices to bring
the male sexual cells into contact with the zona pellucida.
7. Many writers have supposed that chemotaxis is a
constant factor in the fertilisation of animal eggs. This
generalisation, which has been made by arguing from the
attraction of the spermatozoa to the eggs of certain plants,
is as yet entirely without experimental justification. From
my own results with the Echinoidea, which are in accordance
with those obtained by Massart in the case of the frog, and
with the work of Dewitz upon the fertilisation of the eggs of
certain insects, I have been led to suppose that chemotaxis,
at least for a great number of animal species, plays no
role whatever in bringing the sexual elements together.
The work for the above paper was done at the Stazione
Zoologica, Naples, during the months of March and April of
each of the years 1900 and 1901. It gives me much pleasure
to thank the Committee of the British Association for grant-
ing me the use of the table, and also to acknowledge my
indebtedness to the staff of the Stazione Zoologica for
supplying me with material and apparatus during the
research.
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CONTENTS OF No. 182.—New Series.
MEMOIRS:
PAGE
Maturation of the Ovum in Echinus esculentus. By THomas
| H. Bryce, M.A., M.D. (With Plates 10—12) . : : ee Is
Studies on the Arachnid Entosternite. ai R. I, Pocock. (With
Plates 13 and 14). : : : : ; P F . 225
On the Morphology of the Cheilostomata. By Stpyry F. Harmer,
Se.D., F.R.S. (With Plates 15—] Byiow : : ‘ - 263
On the Development of Sagitta; with Notes on the ae of the
| Adult. By L. Doncaster. (With Plates 19—21) , Peet)!
OC 1902
MATORATION OF OVUM IN RCHINUS ESCULENTUS. bias
Maturation of the Ovum in Echinus
esculentus.
By
Thomas H. Bryce, M.A., M.D.
With Plates 10—12.
INTRODUCTION.
Tur subject of the maturation of the sexual cells is a
thorny terrain. It can be attacked only by the highest
powers of the microscope, and the facts can only be
reached by a process of patient mental reconstruction of the
various phases. Historically the subject has been overlaid
by some brilliant but premature hypotheses, which, how-
ever much they may have stimulated research, have also
tended to foster prepossessions. The necessary stimulus
for research has been supplied by the hypothesis that
the chromatin of the nucleus is the hereditary sub-
stance, or, at least, the bearer from one generation to
another of hereditary qualities. But apart from the interest
connected with problems of heredity, and the meaning of
fertilisation, the study of the intricate details of the process
of maturation goes to the bottom of all our knowledge of
cellular morphology. The study of the maturation phe-
nomena in Kchinus was, in the first place, taken up merely
with the motive of seeing some of the actual phases in the
most readily obtainable material. But it was soon dis-
covered that although the outward phases had been fre-
quently studied, most of the finer details of the process, as
seen in Kchinus, were undescribed, and therefore it was con-
sidered worth while to make a study of the whole process.
yon, 46, PART 2.—NEW SERIES. M
178 THOMAS H. BRYCE.
In view of the hopelessly diverging results for different
forms obtained by different observers, an interest in the
behaviour of the chromatin during maturation has declined
of recent years, and the question of the centrosome has
occupied more attention. Results which came out led me to
certain conclusions, which, to my mind, tended to clear up in
some measure the confusion at present prevailing. As
the research was proceeding, Strasburger’s work, ‘ Reduk-
tionstheilung, Spindelbildung, Centrosomen, und Cilien-
bilden in Pflanzenreich’ (1900), came into my hands. In
that work conclusions in the matter of the reducing divisions
identical with my own, and foreshadowed in several
previous botanical memoirs, are brought, by new com-
parative investigations, to a focus, and are made a means
of harmonising the apparently contradictory results in the
case of plants. This obviously increased the importance of
my own results, and inspired me to follow out, in spite of the
large amount of labour involved, the whole series of
phenomena, in order to obtain as complete a demonstration
of the facts as possible.
Maturation IN ECHINUS ESCULENTUS, L.
Previous Observations on Maturation in
Echinoderms.
The Echinoderm ovum has been the classical material
for all observations on the hving egg. The earliest
observations on the maturation of the sea-urchin ege
were made by Derbes in 1847. Agassiz, in 1864, described
the polar bodies in both Toxopneustes and Asteracan-
thion. Between 1872 and 1882 Van Beneden examined
the phenomena in Asterias, Hertwig in Toxopneustus
lividus and Asteracanthion, Giard in Psammechinus,
Fol in Asterias glacialis, Greeff in Asterias rubens,
and Flemming in Sperechinus brevispinosus, Echinus
miliaris, and Toxopneustes. Since then the favourite
material for the examination of the phenomena in the
MATURATION OF OVUM IN ECHINUS ESCULENTUS. 179
living egg has been Asterias. Notwithstanding this,
little is known as to the finer details of maturation.
Hartmann (1902) has, since this paper was written, pub-
lished an account of the changes in this egg up to the forma-
tion of the first polar spindle. The early observations were
made on the entire egg—either in the living state, or
fixed and cleared. The polar bodies in Hchinus are
normally thrown off within the ovary, and when the naked
eggs are shed into the sea water they remain entangled
in the connective tissue of that organ. Sometimes it may
happen that a partially immature ovary may be manipu-
lated and some ova caught in the maturation stages. In
the starfish, on the other hand, the eggs commence to
show the phases when placed in sea water, and they can be
watched. Again, by shaking immature sea-urchin eggs
the stages can be induced artificially. Boveri (1890) has
figured a few stages after the formation of the first polar
spindle in Echinus microtuberculatus, but either the
chromosomes, which are very minute in Hchinus sphera,
are still more minute in Echinus microtuberculatus, and
cannot be further analysed, or he has not seen the figures
which I have made out by my methods. Further, the
number of chromosomes is different. Matthews (1895)
examined maturation in Asterias Forbesii. He was able
to obtain only one ovary showing the stages up to the forma-
tion of the first polar spindle, but supplemented his observa-
tions by stages obtained by shaking the eggs. He describes
the behaviour of the centrosomes, but gives no details as to
the chromatin. Wilson, in his atlas of ‘ Fertilisation and
Karyokinesis,’ shows a single photograph of a second polar
spindle in Toxopneustes, and Boveri has drawn a single
figure of the second polar spindle in his recent work pub-
lished in 1901. Haecker (1893) also gives a diagrammatic
drawing of the first polar spindle, but gives no description
of maturation. In none of these figures is the finer constitu-
tion of the chromatin elements represented. Cuénot and
other observers have written on oogenesis in Echinoderms,
180 THOMAS H. BRYCE.
but their observations were confined strictly to the ovary
and the formation of its epithelium, and to certain points in
the characters of the nucleolus. Various observers have
treated specially of Echinoderm spermatogenesis
(Jensen, Pictet, and others),and many have studied the
morphology of the spermatozoon, but of these Field (1895)
brings the latest account. Owing to the excessive minute-
ness of the chromosomes he seems to have confined himself
to counting them in the different phases. Haecker (1893)
published observations carried out on the living egg on the
germinal vesicle and nucleolus of Echinoderms, but does not
give any detail regarding maturation.
Personal Observations.
Methods.—My material was obtained from animals
freshly out of the water! Small pieces of close on
seventy ovaries were fixed, embedded in paraffin, and a
few dozen sections cut from each. These were all care-
fully examined, and when maturation was found to be
proceeding some hundreds of sections were cut and gone
over, and the details built up from these. The fixative fluids
used were Flemming’s strong solution, and Hermann’s platinic
chloride and osmic acid mixture ; almost identical results
were obtained by both, and the small pieces of ovary—about
a cubic centimetre or a little more—were well fixed through-
out. At a later stage of the research, by way of control,
pieces of ovary were fixed in Boveri’s picric and acetic acid
mixture, and sublimo-acetic acid, as well as Lindsay John-
stone’s fluid. ‘he picro-acetic material was unsatisfactory,
but the sublimate gave good results in some respects. The
chromatin was, however, much better differentiated by the
osmic acid mixtures, especially by Hermann’s fluid; while in
1 The material was obtained at the Marine Biological Station at Millport
in late March and early April. At the end of April and beginning of May
the ovaries are mature throughout. In January maturation has already
begun, and from that time onward the relative proportion of mature to
immature ova gradually increases,
MATURATION OF OVUM IN ECHINUS ESCULENTUS. 181
the sublimate material the centrosome gave quite a different
picture, as will be seen in the sequel.
Staining.—Osmic acid preparations being proverbially
refractory to most staining reagents I have confined myself
almost entirely to Heidenhain’s iron hematoxylin method,
but have used by way of control other stains. To facilitate
staining I have always allowed my preparations to stand for
some time in old turpentine to remove the osmic acid. ‘The
best results were obtained by iron hematoxylin alone, the
picture presenting the vivid black chromosomes on a blue-
grey field. Heidenhain’s preliminary stain with Bordeaux
red rather confuses the picture of the chromatin, and the
only other contrast stain used was a very weak coloration
by alcoholic solution of fuchsin 8.
It is necessary here to refer to the recent criticism by
Boveri (1901) of the iron hematoxylin stain. He shows, as
every one knows who has used the method, that different
degrees of washing out yield different results, and refers to
the fact that structures may appear which owe their existence
to a purely mechanical cause and not to any difference in
chemical composition. Thus a part which is not readily
accessible, on account of its position, to the differentiating
fluid retains the stain while the parts in the neighbourhood
are decolourised; further, the fluid having a concentric
effect in washing out, the superficial parts are decolourised
while the central parts retain the black stain. ‘Thus he
explains the different accounts given of the structure of the
centrosome, and points out that by strong extraction of the
colour even the chromosomes may be apparently diminished
in size owing to their peripheral parts being decolourised.
This is weighty criticism in view of a number of the
appearances I shall have to describe, for he combats the
generally accepted view that the true appearances are
obtained by strong washing out, and believes that in regard
to the centrosomes the opposite is true. As to the chromo-
somes | may forestall criticism of my results by stating, first,
that I have obtained similar appearances both with the osmic
182 THOMAS H. BRYCE.
acid and the sublimate mixtures, and by other stains besides
the iron hematoxylin, though the greatest vividness of
differentiation has been obtained by a combination of Her-
mann’s or Flemming’s fluid with iron hematoxylin, and,
second, that the proof that I am dealing with realities and
not illusions is to be found in the fact that the appearances
described for the chromosomes represent a complete and
unbroken series of the steps or stages of a process that can
be explained only by reference to the completed story.
The drawings were made by aid of the Abbe drawing
apparatus of Zeiss, the finer detail being filled in free-
hand. ‘The combination used was in every case Zeiss
2 mm. 1:40 numerical aperture, apochromatic objective,
with either eight or twelve compensating eye-piece. The
illuminating apparatus employed was a Zeiss 1 mm.
numerical aperture, achromatic condenser. ‘The sections
were cut in paraffin, and were of varying thickness. ‘he
object in most cases being to obtain the masses of chromatin
entire, comparatively thick sections were taken, six to seven
microns. Thinner sections down to three microns were
employed to determine certain points regarding the achro-
matic structures.
In dealing with the subject I shall first describe the
changes in the ovum leading up to the disappearance of the
germinal vesicle, and after that treat in separate sections of the
behaviour of the achromatic and of the chromatic structures.
My earliest preparations are from the growth period. Out
of a large number of young oocytes of the first order I have
only seen two or three in mitotic division, and these only in
the spireme stage, so that I cannot speak as to the number
of chromosomes in these divisions. ‘The young ovum shows
a delicate reticular protoplasmic structure (fig. 1). The
nucleus is already large and vesicular, with a distinct
nuclear membrane, and a deeply staining eccentric nucleolus.
This being intensely black, contrasts strongly with the
granular and irregular nuclear network, which refuses to
take on the chromatin stain, and remains pink in prepara-
MATURATION OF OVUM IN ECHINUS ESCULENTUS. 188
tions stained either with “fuchsin S” or “ Bordeaux red.”
There is frequently a second smaller deeply staining circular
body in the nucleus, but it has no regularity in position and
is not invariably present. I cannot in any of my prepara-
tions see the double nature of the threads described by
Haecker. Close to the nucleus, very frequently on the side
of that body towards which the nucleolus lies, there is some-
times at this stage a body which presents much the appear-
ance of the centrosome of a resting cell. It consists of
either a single granule or pair of granules, sometimes a
group of smaller granules enclosed in a circular area. While
this may represent a centrosome it is impossible to dis-
tinguish it from similar bodies with central granules that
may be found in other parts of the cell, which are un-
doubtedly cell inclusions, and therefore no structure can with
certainty be identified as a centrosome.
Structure of the Protoplasm.—Wilson (1899) has
shown that in the young ovum the protoplasm is granular,
and that as the ovum grows in size an alveolar structure 1s
assumed. In the youngest ova of my fixed material the
protoplasm presents a granular appearance which is certainly
not alveolar, and can hardly be termed reticular (fig. 1). In
the fully grown egg the appearances vary according to the
stain. In fig. 3 the cytoplasm is represented as showing a
reticulum which is composed of separate minute granules ;
the meshes of this reticulum bound alveolar spaces. ‘These
alveoli are on the whole rounded, and in this particular
specimen, from which the iron hematoxylin was very
thoroughly washed out and{replaced by a slight counter-
stain by fuchsin, they were faintly red with a slightly darker
periphery. In fig. 2, on the other hand, the appearances are
different. The iron hematoxylin has not been so com-
pletely washed out, and the alveoli have retained the dark
stain, showing up as rounded dark points separated by an
unstained reticulum. Sometimes the centre of the alveolus
is occupied by a black dot, as if the centre had not been de-
colourised. hus my preparations fully bear out Wilson’s
184. THOMAS H. BRYCK.
latest conclusion (1899) regarding the structure of the sea-
urchin egg—namely, that the condition of the cytoplasm
conforms to Biitschli’s description. It has the same physical
characters as an emulsion; that is, there is a fluid framework
in which the microsomes are suspended, and the alveoli are
filled with a fluid of different physical characters. When
the alveoli are wholly destained all that is seen is the micro-
somic network, whereas when they are stained the alveoli
stand out as the yolk granules embedded in the cytoplasm.
Wilson shows, however, that the cytoplasm at certain periods
may have a fibrillar structure, but to this point I shall return
later.
The changes which the nucleus undergoes during the
vrowth of the oocyte, until it becomes the fully developed
germinal vesicle, are very complicated and uncertain.
Many irregular figures suggest that the germinal vesicle
may undergo changes of shape. ‘They may well be arte-
facts. I shall only refer to certain facts regarding the
chemical reaction of the nucleus, which seem to be fully
vouched for in my preparations. It has been shown by
a number of observers that the staining reactions of the
uucleus vary at different times. At one time the chromatin
network will take the specific stains deeply, while at other
times it remains unstained (Riickert, 1892). My experience
tends to support these statements, though one must
admit that very different effects are produced by different
degrees of coloration with iron hematoxylin. ‘The effect
depends on the degree of extraction of the colour, but it is
quite certain that at certain stages of the nucleus the
network very readily parts with the black stain, and is left
as an irregular granular reticulum of a blue-grey colour, or
of a red tint, in preparations stained for contrast with rubin.
The nucleolus, on the other hand, is exceedingly tenacious of
the stain, and appears as an intensely black spot (fig. 2).
Again, at a stage I consider to be of later date, the network
shows a basis of delicate linin threads, with deeply stained
chromatin particles arranged on the thread, giving it a very
MATURATION OF OVUM IN ECHINUS KSCULENTUS. 185
irregular or feathery structure, while the nucleolus generally
is less deeply stained and vacuolated (fig. 2). Finally, when
the nucleus is fully grown and maturation imminent, we find
the contrast is exactly the opposite of that described for the
young nucleus. The network is intensely black, consisting
of particles of chromatin arranged in a very intricate and
irregular fashion, while the nucleolus parts with the stain
very readily, and is left as an almost colourless, apparently
empty vesicle. Soon after the resolution of the nuclear
membrane it disappears from view. Very similar changes
are described in many other forms,—for instance, in the
Turbellarians, according to Francotte (1897) ; in Polycheerus,
according to Gardiner (1898) ; and according to Gathy (1900),
in Tubifex (an Annelid) the nucleolus loses its capacity for
staining with iron hematoxylin at the end of the growth period.
It is difficult to resist the conclusion that the chromatin
substance is at first confined to the nucleolus, and later leaves
it to form the chromatic basis of the nuclear network as a
whole, and therefore also of the future chromosomes. The
fate of the nucleolus in Hchinoderm ege’s has been variously
interpreted. Derbes (in 1847) thought it was directly
converted into the pronucleus of the mature egg, and
Hertwig (1877) took the same view. Fol (1877) and
Flemming (1882), however, proved that the chromatin
itself became the future nucleus, after it was provided
with a new nuclear membrane. Recently Carnoy and
Le Brun (1899) have maintained the view that in the
amphibian egg where there is no chief nucleolus, but a large
number of smaller ones, certain of these become converted
into the future chromosomes, thus reverting to the older
view of Schultze (1887). It seems certainly true, as said
above, that the chemical substance which is lodged in the
nucleolus in the early ovum becomes later distributed into
the germinal vesicle, and so indirectly goes to form the
chromosomes. Hartmann (1902), for Asterias glacialis,
describes the chromosomes passing directly out of the nucleo-
lus, the remainder of the nuclear reticulum being rejected.
186 THOMAS H. BRYCE.
Strasburger regards the body as a storehouse of reserve
substances, which pass into the cell during division to
form the “kinoplasm,”’ which goes to form the spindle,
the Hautschicht, membranes, and cilia. We shall see later
that the phenomena observed in the sea-urchin egg
may combine these two views. But in contradiction to
both is Haecker’s view. His observations on the living
ege@ of the sea-urchin reveal to him the nucleolus as a
pulsating organ in which, periodically through the whole
growth period, small vacuoles appear; these run into a
single central vacuole, which increases and then diminishes
in size. When the largest central vacuole appears the
nucleolus removes itself to the periphery of the nucleus, and
meantime the vacuole comes into relation with the outer
layers of the nucleolus, as if to bring its contents into
relation with the nuclear sap; and further, an indrawing of
the wall of the germinal vesicle itself suggested that there
was a communication between the cytoplasm and nucleolus.
From these and other observations Haecker regards the
nucleolus as a secretory organ, collecting the by-products of
nuclear activity—not as a storehouse, or “ nuclein labora-
torium ” (Fick, 1899). So far as my observations go, they
tend to support the idea of the nucleolus being a storehouse
or laboratory of nuclein.
Centrosome.—There has been a great deal of discussion
as to this enigmatical structure in the sea-urchinegg. Vary-
ing accounts have emanated from Boveri, Wilson, Fol,
Biitschli, Reinke, Hill, Kostanecki, and Erlanger. Boveri
says, “Das Seeigel-Ei ist von allen objecten die von mir
bekannt sind, dasjenige, welches einer sicheren Darstellung
der Centrosomen die grossten Schwierigkeiten bereitet.”
This quotation is taken from his recent work, ‘On the
Nature of Centrosomes.’ He reconciles more or less the
different accounts, and suggests a nomenclature which I
shall adopt as being the latest and most authoritative.
The centrosome is composed of a special and peculiar
substance, the centroplasma, which, according to the perfec-
MATURATION OF OVUM IN ECHINUS ESCULENTUS. 187
tion of fixation and the manner of staining, presents different
appearances. ‘his accounts for the different forms under
which the body has appeared. It stains best with iron
hematoxylin, and destains concentrically. When destaining
has been carried far, it shows as a discoidal area surrounded
by a clear halo, and has a very fine alveolar structure. This
is the form in which I have observed it in all my osmic acid
preparations, except that I do not see the halo, and when
counterstained with rubin it has a red colour, which in-
definitely fades away into the bluish-grey astral rays and
spindle fibres. This rounded body, as division proceeds,
becomes enlarged, then lens-shaped, and ultimately flattens
into a plate which lies along the side of the nucleus. In
polar view this is dumbbell-shaped; the enlarged ends are
the daughter centrosomes which become surrounded with new
radiations. In the maturation stages my preparations are
not numerous enough to enable me to follow in detail the
behaviour of the centrosomes, and I have not been fortunate
enough to see the division of the body in the first maturation
spindle. The centrosome of Boveri corresponds to the
centrosphere of Wilson. In another set of preparations less
destained, Boveri described the centrosome as a smaller body,
showing in its centre a darkly staining particle, the centriole,
which corresponds to Wilson’s centrosome. ‘his, as division
proceeds, divides into two, and goes through the usually
described evolutions. In_ picro-acetic and sublimo-acetic
preparations I have seen such a centriole, but have been un-
able to trace its division. Again, when destaining has been
stopped early the whole centroplasm is black. This I have
also seen in picro-acetic and sublimo-acetic material. The
rays, according to Boveri, stop at the margin of his centro-
some, and do not enter it so as to be inserted into the centriole.
This seems to be the case, and in my cleavage preparations
fixed with Lindsay Johnstone’s fluid the central reticular
body is sometimes seen to have completely dropped out, so
that the astral rays are seen to end abruptly, leaving an
absolutely round empty space occupied in the other eggs by
188 THOMAS H. BRYCE.
the alveolar or reticular centroplasm. I do not presume to
give an opinion on the much vexed question of the persistence
of the centrosome as a special cell organ, but one thing seems
clear, that the centroplasm is a focus of protoplasmic
activity, and is ultimately to be explained on physiological
and not on mechanical grounds.
Changes in the Germinal Vesicle Preparatory to
Division.—When the germinal vesicle has reached its full
growth the nucleolus loses its staining capacity to chromatin
stains, the nuclear network takes an intense stain, and the
cytoplasm to its very outer edge is seen to have an alveolar
structure. In many cases, presumably in stages close to the
onset of maturation, the nuclear membrane is puckered. The
germinal vesicle then moves towards the surface, and, as long
ago described by Hertwig (1877) for Asteracanthion, at the
spot nearest the surface a protoplasmic process projects into its
interior (fig. 4). In osmic acid preparations the nuclear mem-
brane is seen to be indented and folded before the process ;
this, as it projects inwards, spreads out in every direction
from the neck, so that at the margins of the process are seen
sections of peninsule andislands. In sublimate material the
nuclear membrane is not so sharply differentiated, and the
inward folding of it is not so clearly seen. I have no doubt
from my sections, such as shown in fig. 4, that this is a true
invagination of the germinal vesicle by the cytoplasm. At
the neck of the invagination the alveolar walls of the cyto-
plasm are drawn inwards towards the centre, but in the pro-
cess itself no very distinct fibrillar structure is at first to be
seen.
Hartmann represents at this stage a very distinct aster
between the invaginated wall of the vesicle and the surface
of the ovum. I have not seen such an aster in my sections ;
the wall of the vesicle is always very close to the surface of
the egg, leaving no room for such a formation, and the aster
seems to form within the process. Sometimes the radiations
from the neck of the invagination are much better marked
than in the ovum represented, and in the process itself there
MATURATION OF OVUM IN ECHINUS ESCULENTUS. 189
is a distinct suggestion of radiation, which is, however, very
difficult actually to define. The appearances suggest that
the centrosome or kinetic centre les in the neck of the
invagination ; but at this stage there is not, so far as the
study from sections can determine, any wide-spreading astral
formation as in Asterias. Soon it is seen that the whole mass is
made up of what seem to be looping fibres. Possibly the folded
and puckered nuclear membrane contributes to this appear-
ance. In fig. 5 is represented a stage in which the chromatic
reticulum has become finer, and at the neck of the process is
an irregular mass which is destined to form the future chromo-
somes. All this time the germinal vesicle remains close to
the periphery of the egg.
The next stage I can determine is the one represented in
fig. 6. The nuclear membrane has now entirely disappeared,
and in the irregular mass of looping fibres there are seen two
asters. In each is a circular finely reticular area, the centro-
plasm, and from the periphery pass out in every direction
very delicate interdigitating fibres. Between the two asters
the fibres are drawn out to form an irregular spindle arrange-
ment. Round this area the greater part of the nuclear
reticulum, which does not form chromosomes, but was related
to the vegetative stage of the germinal vesicle, is seen merg-
ing with the cytoplasm, but still retaining its reticular
character. Betwéen the spindle and the surface the chromo-
somal chromatin mass is seen.
This description corresponds with Hertwig’s original
account, and also in the main with Hartmann’s recent repre-
sentation of the facts, but differs from Fol’s in that he describes
no process projecting into the vesicle. It also differs from
Mathews’ description of what occurs in Asterias Forbesii.
He describes the two centrosomes probably passing out of the
germinal vesicle at the nearest point to the surface of the
egg by the rupture of the nuclear membrane at that point.
They then pass some distance from the nucleus, and are seen
to have round them a faint halo of ‘archoplasm.’’? This
latter becomes distinct, radiations are developed, the whole
190 THOMAS H. BRYCE.
archoplasmic area divides, and the two parts being drawn
asunder, a spindle is spun out between them, which moves
tangentially over the nucleus. As it grows the spindle-fibres
project into the vesicle, the nuclear membrane is dissolved,
and the spindle then rotates to become the first polar
amphiaster. In Haecker’s text-book (p. 123) the process in
the living egg is described in much the same fashion. A
clear area is developed between the remains of the germinal
vesicle and the surface, surrounded by a radiation which soon
forms a double star, which is the beginning of the amphiaster.
The invagination of the germinal vesicle in the egg of
Echinus is probably secondary. It may be related to the fact
that the wall of the vesicle comes exceedingly close to the sur-
face of the egg. The mounting of the vesicle to the surface
is a fact which, so far as I know, has not been satisfactorily
explained. Haecker (1893) suggested that it is due to the
action of gravity causing a movement in the elements of the
ego after the force connected with the exchange of material
between nucleus and cytoplasm, which keeps the vesicle in
the centre of the egg, ceases with full growth. My prepara-
tions do not throw any light on the point.
Fig. 8 represents a somewhat oblique section of the
germinal vesicle at a later stage. It shows the two asters
arranged tangentially to the surface of the egg, but between
them, and extending towards the surface, is a finely reticular
mass, out of which the delicate wavy and interdigitating rays
of the asters are evidently spun. Hmbedded in this reticulum
are seen the chromatin segments. At a later stage (fig. 9)
all these are drawn into the area between the asters, which is
seen now as a finely alveolar or reticular plate. Round this
central plate is a complicated reticulum of fibres crossing and
intercrossing, but on the whole radiating from the central
plate. In this reticular zone is also seen, at a little distance
from the plate, one centrosome surrounded by rays, obviously
part of the general reticulum. In the adjoining section a
second aster was present on the side of the plate removed
from the surface ofthe ovum. At this stage one hardly ever
MATURATION OF OVUM IN ECHINUS ESCULENTUS. 191
sees a preparation in which both asters are cut in the same
section. Griffin, in Thalassema, has described a disappear-
ance of the spindle spun out between the centrosomes, and
the development of a central mass very hke that which I
have described between the asters. In most instances,
however, the conditions are more like those described by
Matthews in Asterias Forbesii.
It is evident that when the nuclear membrane disappears,
and the rejected chromatin passes into the cytoplasm, a pro-
found effect is produced on the organisation of the ege.
Whereas, with the germinal vesicle still intact, the alveolar
structure can be traced to the surface of the egg, we now
find that round the transformed nucleus, and projecting into
the centre of the egg, is a fibrillar mass, which is sharply
differentiated from the alveolar yolk. From this central
area there also extends round the surface of the egg a layer
of differentiated protoplasm. The central mass and surface
layer have each a definite fate. The one is differentiated
into the spindle and asters, while the surface layer is, I
believe, associated with the formation of the membrane
thrown off by the egg at the moment of fertilisation. The
central mass of the yolk is unchanged in appearance, and the
question is whether this reticular mass of protoplasm is
differentiated from the cytoplasm, or is derived from the
rejected nuclear reticulum. I am inclined to think that it
is in large measure formed from, or under the influence
of, the discarded nuclear material. This would be in
harmony with the results of Carnoy and Le Brun (1899)
in Triton. Another evidence of the excitement produced in
the egg at this stage may perhaps be seen in the accessory
asters formed, which, so far as I can see, have no relation to
the formation of the definite asters of the spindle.
From experiments by R. Hertwig (1896) and Morgan
(1896) it seems that under special artificial chemical stimulus
the cytoplasm may be excited to form asters, and even, in
Hertwig’s experiments, amphiasters. Reinke (1894) also
found that in the peritoneal cells of the larval salamander
192 THOMAS H. BRYCE.
three grades of asters are formed—primary, secondary, and
tertiary. The last contribute to the secondary, and these
again to the primary or definitive asters. Carnoy described
accessory asters during the formation of the second polar
body in Ascaris, and Meade (1897) showed that a great
number of such asters were formed before the formation of
the first polar spindle in Cheetopterus (an Annelid), which he
thought contributed to the formation of the spindle asters.
Watase (in Macrobdella) found as many as thirteen asters in
the cytoplasm, with centres varying in size from the smallest
microsome to the true centrosome. Griffin (1899) also de-
scribes the formation of accessory asters in Thalassema.
These experiments and observations are held to afford strong
evidence of the free formation of the centrosome, in which
case both that body and its aster would be the expression
rather than the cause of cell activities.
The secondary asters in Hchinus at this stage are pos-
sibly produced in the cytoplasm under the influence of the
nuclear material let loose on the disappearance of the nuclear
membrane.
All this tallies better with Strasburger’s views of the kino-
plasm than with any other theory. He thinks of protoplasm
as of two kinds, trophoplasm and kinoplasm: the former is
vegetative in function and alveolar in structure ; the latter
presides over the activities of the cell, forms centrosomes,
mid-bodies, asters, and spindles, constitutes a peripheral layer
from which membranes and cilia are derived, and is fibrillar
in structure. This differentiation of the protoplasm takes
place when mitosis sets in. Further, he thinks the nucleolus
is a storehouse of reserve material, out of which, on need, the
substance of the kinoplasm is drawn.
I have shown that the nucleolus at first seems to contain
all the chromatin substance which later is found in the
nuclear reticulum, the larger portion of which is rejected, to
form in turn, if I be right, directly or indirectly a
reticular zone, out of which the asters and spindle are
spu n,
MATURATION OF OVUM IN ECHINUS ESCULENTUS. 193
My conception of the meaning of the changes in the ovum
does not, however, involve an acceptance of either Stras-
burger’s kinoplasmic or of Boveri’s archoplasmic theory. It
inclines rather to the view that the same ground substance,
under the influence of the chemical changes underlying vital
activities, may take on different forms in response to varying
physiological needs, and further, that whereas, during the
vegetative period, the main centre of these chemical activities
hes in the nucieolus, in the division period that centre is trans-
ferred to the centrosome, which is the expression of activities
resulting in the vital phenomena of division.
Fig. 15 represents a later stage. The spindle is not yet
complete, but the two asters are situated radially, and the
reticular mass, though still showing in some parts a radial
distribution from the central plate, is becoming more and
more focussed on the centrosomes. The spindle, in most
forms, is said to be fully formed before this radial position is
assumed, and the whole spindle is said to rotate through
90 degrees. A very good example of this is seen in the egg
of the mouse, as described by Sobotta (1895).
In my preparations the spindle, as is the case also in
Thalassema (Griffin), is late in being completed, and the asters
seem to move independently through the cytoplasm, the fibres
arranging themselves round the centre of activity until the
definitive position is reached.
The conditions described for the formation of the polar
spindle are not unlike those accompanying the formation of
the multipolar spindles described in the pollen and the spore-
mother cells in many plants by Farmer, Belajeff, Osterhout,
Mottier, Nemec, and Byxbee. According to the description
of these authors there is a filar zone round the nucleus, out of
which the multipolar figure is spun, the poles of which draw
together to form the definitive bipolar spindle. I have seen
one or two four-poled first-maturation spindles, but I cannot
make out that in the reticular zone there are more than two
asters which have any relation to the future spindles, and
such four-poled spindles would thus merely indicate the
vou. 46, part 2,—NEW SERIES, N
194 THOMAS H. BRYCE.
tendency to the formation of multiple centres of activity, or
putting it in terms of the centrosome, to the formation of four
centrosomes instead of two.
The whole process leading up to the formation of the first
polar amphiaster is very complicated in Echinus, and
extremely difficult to trace in sections. It is as difficult
to be sure of the phases in the living Hchinus egg, which
are moreover difficult to get as the process normally
takes place within the ovary. ‘The process does not
seem to me to be so simple as it has been described for
Asterias ; indeed, it is in many respects like what Mead has
described for Cheetopterus. Dr. Teacher has recently studied
the phases in the living egg, and has kindly let me see his
results, which supplement my own. He has seen frequently
a stage which I have described as follows—“ near the surface
of the ovum is a clear granular area having the appearance
of ground glass, surrounded by a darker ring merging into
the alveolar-looking cytoplasm. This ring, at its circumfer-
ence, is distinctly irregular, and suggests delicate radiations
from the central granular area.” This is obviously the
stage represented by the section depicted in fig. 9, and
corresponds to the transformed germinal vesicle. At this
stage I could not make out distinct asters in the living egg,
and in the sections the astral rays which lie within this area
are of great delicacy. Dr. Teacher has seen in this phase
many specimens with a number of asters in the cytoplasm
around the transformed germinal vesicle, and has made out
at the same time, within the area itself, astral formations
which he believed to be the definitive asters of the spindle.
While, therefore, I have in the foregoing description traced
the centrosomes as if they were persistent centres travelling
through the cytoplasm, I cannot exclude the possibility of
their free formation as described by Mead.
Fig. 16 represents the spindle now completed. The
chromosomes are being drawn into the equatorial plate.
Fig. 17 shows the now completed spindle in metaphase.
It is relatively bulky, with blunt and rounded ends, and
MATURATION OF OVUM IN ECHINUS ESCULENTUS. 195
frequently, when the section is through the side of it, the
centrosome is not cut at either end. There is no central
spindle. The chromosomes extend through the whole
equatorial plate, and the peripheral rays of the asters are
seen interdigitating opposite the equator. The fibres of the
spindle itself are somewhat uneven, and in relation to the
chromosomes there are darker bundles apparently of several
fibres spun together. As metakinesis proceeds the waviness
of the fibres becomes more distinct. The central centrosome
becomes flattened, but I cannot determine the manner in
which a division takes place. The outer centrosome
diminishes in size. Its central astral rays shorten, fig. 19,
and ultimately disappear as the apex of the spindle is
protruded. The lateral rays are obliterated progressively
until the point of the spindle stands clear. The first appear-
ance of the protrusion of the polar body is a tiny elevation
into which the end of the spindle is directed. Later, when
the spindle has risen to the height of the equatorial plane,
there is seen a depression on the surface of the egg where the
constriction takes place, and in which afterwards the polar
body lies. The rise of the spindle and its protrusion are very
difficult to explain. It remains approximately of the same
length throughout, and I do not see any special development
of the central aster over the polar one. Wilson (1900) finds
evidence in the protrusion of the spindle in favour of
Diiner’s (1895) theory that the divergence of the poles of the
spindle in mitosis is due to the progressive elongation of the
central spindle. In Hchinoderm ova, neither before nor after
fertilisation, is there a central spindle spun out between the
centrosomes, but it is probable, according to Wilson, that the
difference is only a secondary one, and that the spindle
consists in part of continuous fibres, and the waviness of the
spindle-fibres in the metakinesis would speak for the pushing
hypothesis. In any event, I cannot see how any hypothesis
founded on mechanical principles, such as illustrated in
Heidenhain’s model, can explain the peculiar circumstances
of the polar mitosis,
196 THOMAS H. BRYCE.
Fig. 23 shows the earliest phase of the second division
which I have had the opportunity of observing. The asters
are already separate, and a bunch of fibres from each is pro-
jected towards the chromosomes, which are immediately
drawn into the equator of the spindle. Thus no resting stage
intervenes between the two divisions. The whole figure is
still surrounded by the remains of the reticular or kino-
plasmic zone. The spindle when fully formed is slighter than
the first polar spindle. The central centrosome and aster
progressively increase in size until the condition is found as
in fig. 30. The astral rays are thick and fairly straight and
widely spreading. The behaviour of the outer centrosome
and the manner of protrusion of the polar body is exactly as
I have described for the first polar body (figs. 27—29).
Fig. 33 shows the condition of the nucleus long ago de-
scribed by Hertwig after the extrusion of the second polar
body. The first stage in the reconstitution of the nucleus is
the formation of several small vesicles, which run together to
form a single vesicle which is the mature nucleus. ‘The
description given of the process in the living egg is, that
several small vesicles appear approximately in the middle of
the radiations remaining in the egg. In the sections this is
clearly seen not to be the case, but the vesicles surround the
centrosome, and the astral rays are broken up into bundles
passing out between them. Later these all disappear, and a
single vesicle is left without any trace of centrosome or radia-
tion in its neighbourhood.
Fig. 34 shows an interesting abnormality of the second
polar body. It is here very distinctly a small cell, and pre-
cisely the same phenomena are seen in the reconstruction of
the nucleus as in the egg.
I must now refer to a series of figures which accompany
the constriction of the spindle in both maturation divisions.
Associated with the disappearance of the spindle is formed
the body called by Flemming the “ zwischenkorper.” This
plays a considerable rdle in spermatogenesis, but is figured
also in a considerable number of the descriptions of polar-
MATURATION OF OVUM IN ECHINUS ESCULENTUS. 197
body extrusion. I have seen it in various forms. In fig. 30
are seen round the constricting spindle a series of points
which afterwards, as seen in fig. 23, condense to form a ring
round the remains of the spindle. It seems to persist for
some time, fig. 35, and then disappears.
A final point still remains to be described. When the
matured nucleus retires towards the centre of the egg all
remains of the reticular or kinoplasmic zone have dis-
appeared, and the nucleus lies surrounded by the alveolar
yolk, while round the periphery of the egg the kinoplasmic
girdle has narrowed down into a delicate layer of differenti-
ated protoplasm. In sublimate material this is seen as a
distinct layer, in which large microsomes are arranged
regularly side by side. In the osmic-acid material the dis-
tinction is less sharp, but there is generally a difference in
the characters of the surface layer. I think that possibly
this layer has to do with the formation of the membrane
thrown off when the selected spermatozoon enters the egg,
and, as has been said, I refer it to the kinoplasmic zone
which is differentiated on the breaking down of the germinal
vesicle.
History of the Chromatin.—As has been described
the greater part of the nuclear reticulum is rejected, and
gives rise probably to the reticular zone round the trans-
formed germinal vesicle. Close to the base of the neck of
the invading cytoplasm is found an irregular mass of chro-
matin, just as Matthews describes for Asterias, which is pre-
sumably the chromatin destined to form the future chromo-
somes, figs. 5—7. ‘This condensation of chromatin at one point
perhaps corresponds to Moore’s (1895) synaptic phase, though
only a part, not the whole of the chromatin, as in spermato-
genesis, is involved in the condensation. Hmerging from this
condensed mass are seen in figs. 5 and 6 a series of separate
elements as to the number of which I am not certain, but I do
not think there are more than at a later stage. ‘The following
stages, figs. 8 and 9, involve the collection of this mass of
chromatin elements into the central plate before described.
198 THOMAS BH. BRYCE.
Often one sees the chromatin collected to one side of this
plate; sometimes the separate elements are widely scattered ;
in many instances, as in fig. 12, there are chain-like clusters,
which suggest that a thread is being broken up into
segments, and in practically every ovum at this stage one
sees compound masses which are breaking down into the
separate elements which enter the equatorial plate of the
spindle. Hartmann, as already mentioned, has quite recently
described the chromosomes as arising directly from the
nucleolus. They arise as isolated rods, clumps, or threads
having the chromatin particles arranged in series in them.
The nature of my material makes it impossible for me either
to deny or affirm the direct origin of the chromosomes from
the nucleolus, but the appearances I have described are
not otherwise at variance with those described by Hart-
mann.
I have between fifty and sixty sections of this stage, and
the relatively large number indicate that the prophase is
protracted. From the very first these always present the
same form. Fig. 14 shows a fragment of the thread com-
posed of spheres, united by a less deeply staining subtance.
When separation is complete sixteen tetradal chromosomes
of nearly uniform appearance are found. When seen from
the side they have a dumb-bell shape, when seen en face
they are obviously the tetradal groups of authors. I have
counted the chromosomes at the various stages again and
again, and have always reached the number fifteen or sixteen.
Fifteen is an improbable number, and I feel sure that the
proper figure is sixteen. I have never succeeded in making
the number eighteen, which would be double the number
(nine) found by Boveri (1890) in Echinus microtubercu-
latus. R. Hertwig (1896) made the number of chromosomes
emerging from the germ nucleus, in his experiments on the
development of unfertilised sea urchin eggs, sixteen or
eighteen, which would agree with my results. Field
(1893) in EKchinoderm spermatogenesis counted twenty-six to
thirty-two chromosomes in the spermatogonia, sixteen to
MATURATION OF OVUM IN ECHINUS ESCULENTUS. 199
eighteen in the spermatocytes, and eight or nine in the
spermatids. He confesses to great uncertainty in regard to
these figures on account of the minuteness of the chromo-
somes, and the last figure is quite out of harmony both with
Rh. Hertwig’s counts and my own.
Careful analysis of this tetrad body shows that it is com-
posed of two short stout rods placed side by side (figs. 8
—10, 12, and 23). The ends have the form of little spheres,
and looking back to fig. 14 one may conclude that they corre-
spond to the spheres seen in that thread united in pairs, but
there is no transverse cleavage of the thread between the
four spheres. A complete tetrad, consisting of four indepen-
dent round bodies as figured for Ascaris, or the mole-cricket,
does not occur in Hehinus. Further, one cannot regard the
two rods as separate and independent at this stage; they
are bound together closely, and the figure is really a com-
pound chromosome.
According to the above interpretation the tetrads thus arise
by a single longitudinal split of an original thread or threads.
At no time are there any ring or other irregular figures, as
described in so many other cases. The possibility. is not
excluded, that the groups might result from conjugation of
the dyadal bodies in pairs, as described by Wilcox (1895)
in the grasshopper, and Calkins (1895) in the earthworm.
In two instances only out of a large number of prophase
stages have I seen a figure other than those described. In
one section, just before the spindle is formed (fig. 15) there is
a double comma form, which appears in all other sections at
a later stage.
As the compound chromosomes are gathered into the
equatorial plate they lie irregularly, and in the metakinesis
they do not seem to be resolved simultaneously, for in all my
sections of this stage, about sixty in number, figures in
different phases are seen, and as the chromosomes lie
throughout the whole equatorial plate, and not only round
the periphery of the spindle, various irregular bodies are
seen which are portions only of whole chromosomes. ‘lhe
200 THOMAS H. BRYCE.
relatively large number of sections obtained in this stage
indicates that it is of long duration.
The varied figures drawn in figs. 18 and 21 are capable of
only one satisfactory explanation, keeping in view that the
end result is always the same. The little rods come to be
placed radially on the spindle. Their central ends move
apart to form a T-shaped figure. The cross-piece of the T
representing the separating limbs opening out on the
spindle, the stem of the T the outward directed, and still
united portions of the chromosomes. As separation proceeds
the stem of the T is pulled down until the figure is like two
commas placed end to end. It is obvious that this evolution
will open out the chromosome along the plane of the original
longitudinal split from within outwards, as is seen in a series
of drawings (fig. 21) of the chromosomes in profile view, but
when observed en face (same figure) it is equally clear that a
second longitudinal split has simultaneously been effected
along a new plane, from without inwards, giving the double
V-shaped figures represented in figs. 18, 19 and 21.
If we describe the appearance in terms of the minute
terminal spheres of each rod, we see that the spheres come to
lie in a row exactly as Wheeler (1897) describes in Myzo-
stoma glabrum. The equatoria] bodies then divide (figs.
16—18 and 21), but the terminal spheres of each rod remain
undivided, and are drawn away from the equatorial spheres,
so that the whole chromosome is lengthened out very greatly,
and the apical spheres are carried far away from the equa-
torial, delicate, less deeply staining threads uniting them
together. The equatorial spheres, after remaining long in
contact in the equator, then part, and give rise to V-shaped
figures with a single apical and two equatorial spheres, one
at the end of each limb. These figures then shorten up by
the contraction of the elongated thread, and in the final
anaphase condense into short stumpy masses (figs. 19 and 21).
These, when analysed, show that the apical sphere has also
divided, and we have produced small tetradal bodies exactly
like those in the prophases of the division, but of smaller
MATURATION OF OVUM IN ECHINUS ESCULENTUS. 201
size. In reality, just like the earlier bodies, they are short,
somewhat curved rods, with dilated extremities placed side
by side. Those at the outer pole pass into the first polar
body, and those remaining in the egg persist, enlarge
somewhat, and pass otherwise unchanged into the second
polar spindle. Sometimes during the metakinesis the second
longitudinal split is not so evident, and then long drawn
out threads are seen, the double nature of which is difficult
to make out. Ultimately, however, the two halves separate
in the anaphase exactly as in other cases. Exactly similar
figures have been described by quite a number of observers
in other forms, for instance, and especially distinctly, by
Klinckowstrém (1897), Francotte (1897), Van der Stricht
(1898), Griffin (1899), Gathy (1900), but, as I shall describe
in the sequel, their interpretation has been different, and
leads to very ditferent theoretical conclusions. The transi-
tion between the first polar andthe second polar spindle is very
rapid, so that the number of sections found in this stage is
relatively few.
The little compound chromosomes are drawn into the
equatorial plate of the second spindle (figs. 23 and 24),
and there different appearances are seen, according to the
plane of the section. In fig. 24 we have apparently little
tetrads, which are really the lobed ends of the small, slightly
curved chromosomes. In fig. 25 again the rods are seen
lying back to back. These rods I have every reason to
believe, from the various figures I have drawn, open out just
as in the first spindle, only there is no second longitudinal
split, and therefore the division is homotypical. A single
preparation rather suggests that the rods may sometimes
be simply separated along the plane of cleavage. It may
well be that both methods are adopted, according to whether
the body lies radially or tangentially to the spindle. The
result 1s the same; the separation is effected in the plane
of cleavage established in the anaphase of the first division.
Similar figures in the second division have been described
by the authors above mentioned, and in other instances,
202 THOMAS H. BRYCE.
also, the short, slightly curved rods have somewhat the
appearance of tetradal groups. When the daughter chromo-
somes have separated they pass to the poles of the spindle.
Those at the external pole pass out with the second polar
body, and remain as short, stout, distinctly bilobed bodies in
many instances, after the second polar body is cut off
(fig. 31). Those remaining in the ovum, however, at once
begin to lengthen, and in the telophase are seen (fig.
30) as long, bent rods. ‘These are gathered into the
series of vesicles already described. Within each of these
vesicles are seen elongated, curved rods, and round the walls
there are tiny particles of chromatin, forming an incomplete
membrane (fig. 33). Later, when the vesicles are fused, the
nucleus is seen to be bounded nearly all round by semicir-
cular loops of chromatin, and in the centre the reticulum is
becoming restored (fig. 31). At a later stage (fig. 32) the
reticulum takes on the form of irregular feathery strands,
beset with chromatin granules of varying size, accumulated
here and there to form irregular net-knots of chromatin.
All trace of the separate chromosomes is absolutely lost in
this network.
The phenomena attending fertilisation and cleavage are so
well known that I do not intend to enter on that subject, but
I wish to refer to the behaviour of the chromatin threads in
the metaphase of the cleavage division. ‘he primary rods
segment into about thirty-two chromosomes. I have counted
them in cross sections of the spindle a good many times, and
generally reach that figure, which would make my count of
the chromosomes in the maturation stages fall in exactly
with the general law.
Each chromosome when divided forms first a V-shaped
figure. This mounts on the spindle so that a loop is formed
with its apex directed outwards, and the ends of this loop
are drawn out to the poles of the spindle, the threads
lengthening as they go. Finally, the daughter chromosomes
separate by the breaking apart of the thread at the point
which corresponded to the apex of the loop.
MATURATION OF OVUM IN ECHINUS ESCULENTUS. 208
This is exactly the manner in which I have described the
short, stout chromosomes of both maturation divisions as
opening out on the spindle. The difference between the two
types consists only in the stoutness of the chromatin rods in
the polar mitoses, the occurrence of a second longitudinal
spht in the first division, and consequently the absence of
the usual longitudinal cleavage in the second division.
Summary of Results (Text-figs. 2 and 3, pp. 218, 214). —
The chromatin thread or threads, derived only from a portion
of the mass of chromatin in the germinal vesicle, are found
split longitudinally and segmented into sixteen bodies—half
the number of the chromatin rods in the nuclei of the
cleavage divisions. These bodies consist of two short rods
placed side by side, and each rod is composed of two spheres
united by a less deeply stained portion of the thread. The
two rods are intimately associated so as to form a tetrad-
like mass, and the whole figure is to be considered a com-
pound chromosome.
After a relatively long prophase each of these is resolved
in the first polar metaphase, in such a manner that while the
body is opened up along the original cleavage plane, another
longitudinal cleft is effected, which is completed in the
anaphase, and the final result is another compound chromo-
some exactly like the original from which it sprang except in
size. Hach of the sixteen double rods which remain in the
ovum after the extrusion of the first polar body is resolved
in the second polar spindle into its two elements without
further cleavage taking place.
In the telophase of the second division the elements which
remain in the ovum after the extrusion of the second polar
body elongate into rods which become bent on themselves,
while those in the second polar body remain condensed as
small bilobed rods.
The maturation phases differ from the ordinary cleavage
mitoses in respect of (a) the thickening and condensation of
the chromatin rods, (b) the second longitudinal splitting
which occurs in the first metakinesis, and (c) the absence of
204 THOMAS H. BRYCE.
longitudinal cleavage in the second metakinesis. The second
mitosis thus merely distributes the granddaughter chromo-
somes formed by the second longitudinal splitting in the first
mitosis.
There is thus no “ reducing division.” The only reduction
which occurs is effected in the germinal vesicle, and the
chromatin destined to form the chromosomes of the polar
divisions is diminished in bulk merely.
Critical Analysis of Results and Comparison with
those of other Observers.—In describing the achromatic
structures I have sufficiently indicated how the appearances
I have described in my material are to be compared with
those described by other observers. With regard to the
chromatin elements I may now give a further analysis.
Glancing over the whole field of research on the subject
the first thing that strikes an observer is the remarkable
unity of the process, even in detail, over a very large range of
forms. ‘he figures represented for the great majority of
both the higher plants and the Metazoa show resemblances
so close that one cannot imagine they are produced in one
way in one form and in another way in another form.
Interpretation and theoretical conclusions may differ, the
process is identical throughout.
It has been insisted that the solution of the problem of
reduction lies in the determination of the origin of the
tetrads, but as these in typical form occur in a relatively
small number of cases, it seems that the solution rather lies
in a closer analysis of the heterotypical division, such as has
lately been done for plants by Strasburger.
Heterotypical division was first described by Flemming,
in 1887, as a form of mitosis occurring in the spermatocytes
of the salamander, and in all cases in which tetrads are not
formed a heterotypical division in some sort ushers in the
first maturation division with its reduced number of chromo-
somes, and this is true of plants as well as animals. The
distinctive features of this division as originally stated are:
1. The spireme stage is not so compact as in other kinds
P
MATURATION OF OVUM IN ECHINUS ESCULENTUS, 205
of cells. 2. Thesister threads round which the segments split
are fused by their ends up to the metakinesis. 3. The
monaster stage is short lived, and shows a radial arrange-
ment only indistinctly on account of the twisted position of
the threads. 4. The end stage of the metakinesis is very
prolonged, and has a very special character, in consequence
of the fusion of the ends of the threads. 5. A temporary
and not understood second longitudinal cleavage of the
threads appears in the anaphase. The outstanding feature
of the heterotype was considered at first as being the in-
complete separation of the two halves of the longitudinally
split rods resulting in the formation of ring chromosomes,
but the figures may assume very various forms according as
the loop is bent, or drawn out so as to obliterate the hollow
of it. Again, the rings or their derivatives may be attached
to the spindle in different fashion, so that in their resolution
different irregular figures emerge. This is shown in the
series of diagrams given in the paper of Farmer and Moore,
who first clearly pointed out the essential resemblance of
the heterotype in plants and animals. The feature described
by Flemming, namely, the second longitudinal cleavage
found in the anaphase, seems until recently to have had very
little significance attributed to it.
The simplest idea of heterotypical division is that the two
halves of the ring-shaped chromosomes are drawn out into
U- or V-shaped daughter loops. This simple explanation will
not explain many of the figures observed. Farmer (1895),
in a study of the phenomena in the lilies, described a
double cleavage taking place simultaneously in different
planes as the compound chromosome is resolved into its
daughter elements. In 1896, along with Moore, he gave an
explanation of the phases, which only involved one split, the
second being merely apparent. ‘The idea elaborated was
that the elliptical ring was bent on itself, applied to the
spindle at its apex, and then drawn out to the poles from
the point of bending. ‘The original ends were ultimately
broken across at the equator. Moore, in his work on
206 THOMAS H. BRYCE.
Elasmobranch spermatogenesis, adopted this explanation
of his figures. Gregoire (1899), describing the stages
in lilies, and Strasburger in his recent work, from a
careful examination of the prophase in a large number of
plant forms, absolutely decide against the idea of the bend-
ing up of the ring. In Kchinus, where no ring is seen at
any time, the explanation is easier and more direct, and the
facts decide conclusively against such an interpretation.
Strasburger, in addition to examining a large series of cases,
reviews the results of other observers, and comes to the
general result that all the processes can be referred to one
type, namely, (1) As the result of the primary longitudinal
cleavage of the chromatin thread, two rods, wavy or
curved, are formed. These straighten and_ ultimately
shorten down into stout rods. In shortening, various adhe-
sions and twistings may take place, so as to form rings or
twisted threads. (2) According to the position assumed by
these various figures on the spindle, the character of the
resulting metaphase figures depends. (a) If the chromo-
somesare placed radially in the form of two rods side by side,
they are drawn apart in the plane of the first cleavage, and
at the same time a second slit is effected from the free end
inwards. The result is the formation of V-shaped daughter
chromosomes, which in the anaphase break apart at the apex
to complete the second longitudinal cleft. (b) If placed
tangentially the result depends on the point of attachment
of the “ zugfasern,” but invariably as the limbs are drawn
apart, a second longitudinal cleavage reveals itself, and two
daughter V’s are formed. ‘The first type (a) is exactly what
I have described in Hchinus; the second form (b) is exactly
that described in amphibians.
Flemming (in 1887), and Meves (1896) in Salamander,
McGregor (1899) in Amphiuma, Kingsbury (1899) in Desmo-
gnathus, give us conclusions in the main identical. All
describe and figure a second longitudinal cleavage of
the chromosomes in the dyaster stage, and this cleavage
is preparatory to the second division, ‘The nucleus is
MATURATION OF OVUM IN ECHINUS ESCULENTUS. 207
partly reconstructed between the divisions, and the longi-
tudinal cleft is lost sight of, to reappear in the second
division either by re-establishment of the old or by a
new longitudinal splitting. Kingsbury was able to trace
the longitudinal cleft directly into the second division,
owing to the fact that the nucleus is not so far recon-
structed. More recently (July, 1900) Janssens described
the phases in Triton, and took the further step of inter-
preting the process in exactly the same terms as
Gregoire in lilies. Flemming (1887), in his first paper,
described tetrads, but regarded them as abnormal. Vom
Rath (1893) redescribed these bodies as normal appearances,
but his results were not maintained by Meves (1896), who
failed to find the least evidence of tetrads in amphibian
spermatogenesis, though he, in a short paper, described
tetradal figures as an abnormality in the early oocytes.
Amphibian oogenesis has been attacked by Fick (1893)
and Born (1894), and Carnoy and Le Brun (1899). Practi-
cally identical figures are given by all three, but the later
authors give much more complete details, and offer a new
interpretation. They describe the chromosomes as condens-
ing after some intermediate phases into short rods. These
are complex structures formed by the fusion of a considerable
number of separate elements. ‘These short rods, or rather
blocks, place themselves in the equatorial plane of the spindle
in a circle round its periphery, and orient themselves so as to
be placed with their thicker and larger ends on the spindle,
the other end being directed outwards. Once installed in
this position, the chromosomes go through varied movements,
during which they submit to a double longitudinal splitting.
The one is effected in the equatorial plane, the other in the
axis of the spindle and perpendicular to the first. The
equatorial division shows itself first and begins in the large
part attached to the spindle, rising insensibly into the stalk.
The second occurs later, and begins at the summit of the
stalk, descending by degrees till a kind of tetrad is formed.
Further complicated changes are described, which result in
208 THOMAS H. BRYCE.
double V’s originating in the wings of the “ equatorial
crown.” These are separated and carried to the poles. A
partial reconstruction takes place in the anaphase, but V’s
again appear in the telophase, and the double V’s remaining
in the ovum are separated from one another in the second
polar spindle. The whole description shows a very compli-
cated process, and exactly what I have found in Echinus in
a much simpler form, because as there are no V’s or twisted
threads to complicate the picture, I may say that only the
initial stages described by Carnoy and Le Brun are found in
Kchinus.
This is the only positive evidence of the occurrence of a
simultaneous double split of the compound chromosome of
the heterotypical division in animals. It will be seen that
my interpretation agrees in the main with that of Carnoy and
Le Brun, and Janssens; and further, that the same idea has
enabled Strasburger to reduce the heterotype in the higher
plants to one common plan.
Now Kchinus falls exactly into line in every essential
respect with another considerable series of cases recently
described.
1. Prostheecrzeus, Klinckowstrém, 1897.
2. Various Polyclads, Francotte, 1897.
3. Thysanozoon, Van der Stricht, 1898.
4. Thalassema, Griffin, 1899.
5. Zirphea, Griffin, 1899.
6. Tubifex and Clepsine, Gathy, 1900.
In every one of these the figures belong to the same type,
except that Van der Stricht and Griffin describe rings in the
prophases. The last observer has not given details, because
the chromosomes were too minute for analysis.
The first four authors all explain their results according to
the diagram given below (‘Text-fig. 1). ‘The double rods
resulting from the compression of the ring are placed with
the longitudinal cleft in the plane of the equator of the
spindle, and are drawn apart by their middle points to
form U-shaped or V-shaped figures, and the breaking
MATURATION OF OVUM IN ECHINUS ESCULENTUS. 209
apart of the U’s or V’s at the apex in the anaphase is
held to be a transverse division of the original long
chromosomes. Griffin alone says that the possibility of a
second longitudinal cleavage is not absolutely excluded,
but as the T-shaped figure is rare, he held to the other
explanation. It is obvious that it is extremely unlikely
that such exactly similar appearances should arise in different
ways. I believe that the demonstration I have given of the
nature of the process as it is seen in Hchinus might, if
applied to them, reconcile all these instances with what is
known to occur in Amphibia and the higher plants. A certain
part of the contradiction in results would thus be removed,
thy
E 4. 5. 6.
TEXT-FIG. facet Fee showing ie suczessive stages in the resolution
of the chromosomes in the ‘‘ heterotypical divisions ” according to
an interpretation which makes the apical break of the V atrans-
verse cleavage. (After Wilson.)
and instead of these cases being held to prove a reducing
division in Weissmanu’s sense, they would, as does Hchinus,
disprove it.
The figures given by Linville (1899) for certain pulmonate
Gastropods are very similar to those described in this group,
but the origin of the figures is not completely worked out.
He decided for a longitudinal division in the first division,
and the elements are doubled in the anaphase, while these
again are distributed in the second division.
On the other hand, in Helix pomatia, Bolles Lee (1897)
describes appearances which lead him to conclusions different
from most other observers. He finds transverse divisions in
VOL. 46, PART 2.—NEW SERIES. O
210 THOMAS H. BRYCE.
both mitoses, but no reduction in the number of chromosomes.
He holds that there is both quantitative and qualitative
reduction. His figures have a strong family resemblance to
those in Hchinus, but the chromosomes are very lumpy and
solid, and do not show the compound character of their
prototypes which I have described. Bonin and Collin, in a
recent paper on the ‘‘ Mitoses in the Spermatogenesis of
Geophilus linearis (Koch),” also interpret the appear-
ances as due to two successive transverse divisions.
This is a very good example of the extraordinary variety
in the manner of interpretation of closely similar appearances,
which is evidence of the great difficulty of reaching any
degree of certainty in cases where the chromosomes are
small and numerous.
I come now to another series of cases in which the so-called
tetrads play a large part.
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CONTENTS OF No. 183.—New Series.
MEMOIRS:
PAGE
On a Cestode from Cestracion, By Wixitam A. HasweE.t, M.A.,
D.Sc., F.R.S. (With Plates 22— —24) . : : ; . 399
The Development of Lepidosiren paradoxa.—Part III. t Develo
ment of the Skin and its Derivatives. By J. Granam Kerr.
(With: Plates 85—98) cs) of. ag). Pee ea ee
The Metamorphosis of Corystes Cassivelaunus (Pennant). By
Rogert Guryey, B.A.(Oxon.), F.Z.S. (With Plates “39="31)- ea
Artificial Parthenogenesis and Fertilisation: a Review. By ‘'Homas
H. Bryce . : : : : : : : Ars
ON A CESTODE FROM CESTRACION. 399
On a Cestode from Cestracion.
By
William A. Haswell, M.A., D.Sc., F.R.S.,
Professor of Biology, University of Sydney.
With Plates 22—924.
General Features.
Tue Cestode, the results of a study of which are embodied
in the present paper, occurs, usually in abundance, in the large
intestine of the Port Jackson shark. It is one of these re-
markable forms to which attention appears to have been first
specially directed by P. J. van Beneden (1 and 2), in which
the proglottides are set free from the posterior end of the
strobila long before full maturity has been reached, and only
attain a stage corresponding to that of the “ripe” proglot-
tides of a Tenia after having pursued an independent
existence for some considerable time.
The strobila is actively locomotive, and appears to use the
suckers more in connection with progression than as organs
of permanent attachment. It is only 9 or 10 cm. long in the
preserved condition. There is an elongated neck-region with
a breadth, in the preserved specimens, of half a millimétre.
The four sessile bothridia (fig. 1) are somewhat spoon-shaped,
the anterior end being the narrower. The margin of the
bothridium is very prominent, finely crenulate, and in the
livmg condition extremely extensile, so that the shape is
undergoing constant modification. In preserved specimens
VoL. 46, pARY 3,—NEW SERIES. BB
400 WILLIAM A. HASWELL.
they are about 1 mm. in length. Each bothridium is so
directed that a line running along the floor of its cavity in
the direction of the long axis, and prolonged forwards, would
meet the median axis of the neck at an angle of about 45°.
The cavity is not divided or reticulated. At its anterior
narrower end, where its margin is lowest, each bothridium
bears a small circular accessory sucker.
The last segment (fig. 2) is 5 mm. long and 2 mm. in
breadth in the preserved specimens; relatively narrower in
the extended living condition.
Separated segments are to be found in abundance along
with the entire strobile, moving actively through the intes-
tinal contents. In the course of these movements the shape
undergoes constant alteration, the phases through which it
passes being comparable to those of a Ligula in its most active
condition. ‘The anterior end becomes thrust sharply forwards
until the “ head” becomes long and narrow and pointed, and
the “neck” constriction becomes more or less completely
obliterated. Then suddenly the anterior end becomes drawn
together and thickened to form a distinct rounded knob,
constricted off from the rest (fig. 3). The part behind this
“head” now becomes drawn forwards, the region imme-
diately following on the head gradually becoming thickened,
while the head itself becomes gradually retracted until it
nearly completely disappears, to become again thrust for-
wards as before. The effect of these movements is clear
enough. By the thrusting forwards of the narrowed head
end, the thick matter contained in the intestine is readily
penetrated, the subsequently formed knob at the anterior end
then forming a point d’appui, towards which the rest of
the proglottis becomes drawn forwards.
These independent proglottides attain a relatively con-
siderable size, the largest being about 11 mm. in length and
1:75 mm. in greatest breadth.
Attention has been recently directed by Liihe (18) to
isolated proglottides from Acanthias,in which there is a
distinct mobile “ head” similar to that above described, but
ON A CESTODE FROM CESTRACTON. 401
covered with spinules; and a similar case had previously
been observed by Pintner.
The early separation of the proglottides in this and other
species is obviously correlated with the free locomotive habits
of the strobila. With a much longer train of connected
proglottides, the posterior loaded with eggs, such movements
would be rendered difficult or impossible. The spiral valve
in the intestine of the Elasmobranch renders it possible for
the separated proglottides, without definite organs of adhesion,
yet with an adaptation for creeping movement, to remain
within their host until such time as the uterus has become
fully charged with eges.
This Cestode is to be referred to the genus Phylloboth-
rium of P. J. van Beneden. In the definition given by that
author! the bothridia are described as notched externally,
but the notch is not present in one of van Beneden’s own
species (P. auricula), and cannot be looked upon as of
generic importance. I propose the name of P. vagans for
the Cestracion parasite, which appears to be distinct from all
the species described hitherto.”
The only species of Phyllobothrium, of the structure of
which a detailed account has been published, are P. thridax
and P. Dohrnii. These have both been pretty fully described
by Zschokke (20, p. 327 et seq.) ; but, as mature segments
were not met with by that author, many features of import-
ance, more particularly in the reproductive apparatus, were
overlooked.
Integument and Nervous System.
The cuticle (fig. 4, ew.) is homogeneous and not divided into
layers. Immediately beneath it are the usual external longi-
tudinal (e.l.m.) and circular (e.c.on.) layers of muscular
fibres. The subcuticular cellular layer is much _ better
developed in the strobila than in the free proglottides, in
1-1;,p.. 120; and 2, p:.123.
2 | have not seen the original description of P. gracile, Wedl., from
Torpedo marmorata, but only the brief definition given by Lonnberg (11).
402 WILLIAM A. HASWELN.
which it has undergone a reduction in thickness. A similar
reduction is observable in the internal longitudinal layer of
muscular fibres (7.1. m.), which are well developed in all parts
of the strobila, and very conspicuous in transverse sections
owing to their highly refracting character, whereas in the
free proglottides they are barely discernible in transverse
sections, and in longitudinal appear as a few inconspicuous,
often degenerate, fibres.
The nervous system (fig. 3, fig. 4, n.c.) 1s In no way
remarkable, consisting of the usual head-ganglion in the
scolex, and the pair of longitudinal nerve-cords with their
branches and commissures. In the separate proglottides,
owing to the reduction in the thickness of the subcuticular
cellular and internal longitudinal muscular layers, the nerve-
cords come to be situated more superficially than in the
strobila. They meet anteriorly in the “head,” where there is
a slight thickening of the nature of a rudimentary ganglion.
As in many other forms, two of the four longitudinal
excretory vessels of the anterior region—the dorsal pair—
become reduced greatly in diameter in the posterior pro-
elottides. In the last proglottis these open on the exterior
at the posterior end. In the free proglottides (fig. 3) only
the ventral pair remain. These are very narrow towards the
anterior end, while posteriorly they are very wide and very
sinuous; their external openings are situated near together
at the posterior extremity. The excretory vessels in general
have a wall consisting of a thin layer of fibrillated proto-
plasmic material; but in the scolex and neck region the four
main vessels have a fairly thick layer of iongitudinal muscular
fibres.
Reproductive Organs.
The reproductive system will be best described first as it
appears in its fully developed condition in the free pro-
elottides.
The testis (fig. 3, te.) consists of numerous rounded lobes
extending from the neck to behind the genital aperture.
ON A CESTODE FROM CESTRACION. 403
They lie in the central or medullary region, and are thus
situated on a deeper plane than the vitelline glands. They
average about ‘(06 mm. in diameter. Hach lobe has a fine,
thin-walled efferent duct; the ducts of neighbouring lobes
anastomose to form a network. From this network are
derived larger trunks, which towards the anterior end, and
near the ventral surface of the proglottis, combine together
to form a single median vas deferens (s.d.). The latter is
a closely coiled, widish, thin-walled tube, situated in the
middle of the region in front of the genital aperture. Its
wall consists of a reticulated material with superficially placed
nuclei. No muscular layer was definitely made out, but
muscular fibres must be present, as in the living condition the
tube is observed to undergo peristaltic contractions. The
“prostate” cells described by various authors (see Braun, 5)
as occurring in certain Cestodes, are not present. This main
testicular duct is always packed full of sperms, and it plays
the part of a vesicula seminalis as well as a vas deferens.
It terminates by passing through the wall of the cirrus sac
and becoming the ejaculatory duct. The cirrus sac has a
wall composed of two layers of muscle. Within it, when the
cirrus is not protruded, lies coiled up a long tube, contmuous
internally with the vas deferens. This tube (fig. 5) has a
muscular wall, consisting of an outer thicker layer of longi-
tudinal fibres and an inner of circular fibres, Internal to this
is a homogeneous cuticular layer, beset on its inner surface in
the outer part of the tube with numerous excessively minute
spinules. Outside the muscular layer is a layer of cells
similar to the myoblasts of the oviduct and vagina. In the
space between the wall of the cirrus sac and the enclosed tube
are to be observed numerous muscular fibres which appear to
run about in every direction.
The outer end of the tube is continuous with the outer
extremity of the cirrus sac, and might be described as
invaginated within it were it not for the circumstance that its
inner end is not free, but passes through the wall of the sac
to become continuous with the vas deferens.
4.04. WILLIAM A. HASWELL.
The mode of protrusion of the cirrus is rendered evident on
an examination of hving animals and of sections of speci-
mens with the organs in various states. The strong muscular
wall of the cirrus sac contracts, and the narrow outer end
with which the invaginated tube is continuous becomes thrust
out through the genital opening. Further pressure causes
the tube to become evaginated as a narrow cylindrical process,
the cirrus, with a double wall, the space between the two
walls being continuous with the cavity of the cirrus sac. The
retraction takes places through the agency of the muscular
fibres that have been above referred to as situated in the
cavity of the cirrus sac; when the cirrus is protruded these
are put upon the stretch, and each of them is found to be
connected internally with one of the myoblasts in the wall of
the tube, and to run inwards towards the inner part of the
wall of the cirrus sac.
The ovary (figs. 3 and 6, ov.), as in many other Cestodes,
consists of two large lateral portions and a small median
isthmus connecting them together, the whole, on a dorsal or
ventral view, resembling a letter H, with the limbs thick and
near together and the transverse part very short. A trans-
verse section shows that each lateral portion is itself double,
consisting of a dorsal and a ventral lamina which coalesce
internally towards the isthmus. The margins of the lamine
are divided irregularly into a number of rounded lobes, but
these divisions are quite superficial, the substance of the
lamina consisting of a mass of ova with no trace of a tubular
structure, except that irregular fenestrae occur here and
there. The ova are somewhat smaller peripherally, largest in
the neighbourhood of the isthmus. The mature ova are
‘Ol mm. in diameter; their nuclei, ‘004 mm.; and their
nucleoli, "(002 mm. Their cytoplasm appears homogeneous
under the highest powers, binding them together in a small
quantity of retiform connective tissue. Knclosing the whole
ovary is a membrane having the appearance of a condensation
of the parenchyma, but perhaps of muscular character.
The isthmus, or connecting part, differs widely from the
ON A CESTODE FROM CESTRACION. 405
rest, and is to be looked upon rather as the beginning of the
efferent duct than as part of the ovary proper. It is enclosed
in a membrane continuous with that which encloses the
lateral portions. The contained ova, instead of being closely
ageregated together, are loosely distributed singly or in
groups (figs. 11 and 12).
The oviduct begins in a_ well-developed “ swallowing
apparatus” (figs. 6, 7, 8, 9, 11, 12, 18, 14, 15, 16, sw.), such
as has been described in various other Cestodes. This lies
on the ventral side of the isthmus of the ovary and opens
into its cavity. It is a bell-shaped structure, the wide mouth
ot which, directed towards the dorsal surface, opens into the
cavity of the isthmus of the ovary, while at the opposite
extremity a very much smaller aperture leads into the ovi-
duct proper. During life this swallowing apparatus was
observed to perform rhythmical pulsating movements, the
effect of which must manifestly be to seize the loose ova of
the isthmus, one by one, and to pass them backwards along
the oviduct. In sections it is found that the wall of the
swallowing apparatus 1s continuous with the investment of
the ovary and with the muscular layer of the wall of the
oviduct. It has the character of a dense layer of fibres
(figs. 15 to 16, sw. m.), which, though of extreme fineness,
must be muscle-fibres. These are for the most part arranged
circularly around the wall of the organ, but some are radial.
Surrounding this fibrous layer is a single layer of cells
(figs. 13 and 14, sw. my.) of irregular shape. Processes pass
from these into the fibrous layer, and there can be little
doubt that the majority of these cells are the myoblasts of
the fibres of the swallowing apparatus. A small number
(fig. 13) which give off processes both externally and in-
ternally are probably nerve-cells.
Through the oviducal opening of the swallowing apparatus
projects for a short distance a sort of plug perforated by a
circular aperture. The substance of this plug is continuous
with the epithelium of the oviduct; but, though it contains
several nuclei (fig. 16), it does not consist, so far as I have
406 WILLIAM A. HASWELL.
been able to ascertain, of definite cells. On its inner surface,
i.e. the surface turned towards the ovary, it is fimbriated
(fig. 14), and it is doubtless through the agency of these
fimbriz that the ova are seized during the movements of the
apparatus.
The oviduct (figs. 6 and 7, od.’) runs, at first, nearly
straight back from the swallowing apparatus, on the ventral
side of the shell-gland and receptaculum seminis, and is
joined by the narrow fertilise duct (f. d.) from the latter.
In this part of its course (fig. 8) it has an epithelium com-
posed of short prismatic cells. Internal to this is a thin
cuticle beset on its inner face with numerous slender hairs,
resembling cilia in appearance, but non-vibratile, which le
with their apices directed backwards, i.e. away from the
ovary, their arrangement thus being such as to prevent the
ova received from the swallowing apparatus from passing
forwards again towards the ovary. Hxternal to the epithe-
lium is a muscular layer composed of external longitudinal
and internal circular fibres. Surrounding this is a layer of
cells of the same general character as those that surround
the muscular layer of the swallowing apparatus. These
appear to correspond to the cells which Zschokke (20) looks
upon as glandular, and to those which Pintner! regards as
the formative cells of the swallowing apparatus. In view of
Blochmann’s* results on the subcuticular muscle, however,
and Sabussow’s (17) extension of the same view to the re-
productive ducts, | am more disposed to look upon these also
as myoblasts.
A little behind its point of junction with the fertilising
duct the oviduct bends sharply round towards the dorsal
side, and is joined by the main vitelline duct at the posterior
limit of the shell-gland. From this point it runs forwards
for some distance with a sinuous course on the dorsal side of
the isthmus of the ovary and of the vagina, and then runs
1 Sce Braun, 5.
2 F. Blochmann, “ Ueber freie Nervenendungen und Sinneszellen bei Band-
wiirmern,” ‘ Biol. Centralbl.,’ xv, 1895.
ON A CESTODE FROM CESTRACION. 407
straight forwards as a cylindrical tube with irregular dilata-
tions. As this part of the oviduct contains fully formed
eggs, and is something more than a mere passage, it will
be convenient to designate it ootype, or primary uterus.
Anteriorly it opens into the secondary uterus by a longi-
tudinal slit, the extent and position of which vary in different
specimens, situated on one side of the vagina.
After it becomes joined by the maim vitelline duct, the
oviduct changes its structure, the cuticular hairs are lost,
and there is no epithelium, the wall of the duct now con-
sisting of cuticle, muscular layer, and layer of myoblasts.
The uterus (figs. 3, 6, and 18, s. w.) is a cylindrical un-
divided chamber, extending from the level of the repro-
ductive aperture to the interspace between the anterior
portions of the lateral wings of the ovary. It has a lning
membrane composed of a single layer of cells. It has no
natural external aperture, but dehisces by the formation of a
longitudinal slit along nearly the whole, or only a limited
part of the length of its ventral surface. This dehiscence
readily takes place when the specimen is manipulated, more
especially when it is placed in sea-water, when the eggs are
observed to be suddenly discharged with the appearance of
a white cloud.’
The shell-gland is a compact oval body, ‘18 mm. in length,
which surrounds the oviduct where the vitelline duct joims it.
1 Shipley, in his description of the worms collected by Dr. Willey (19), in
referring to a species of Phylobothrium, states that in the oldest pro-
glottides the uterus had ruptured “about the centre of the dorsal surface.”
But there cau be no doubt that the surface on which the dehiscence takes
place is the ventral, and not the dorsal. This is made perfectly clear in the
case of the Cestracion species by the relative positions of the various parts of
the reproductive apparatus—as, for example, the vagina and vas deferens—
and by the disposition of the longitudinal vessels of the excretory system. It
may be remarked, however, that in the Australian land Planarian (Geoplana
Mortoni) Steel has confirmed by observation on the living animal Dendy’s
description of the rending of the dorsal body-wall on the discharge of the egg-
apsules (‘ Proc. Linn. Soc. N.S.W.,’ 1900, p. 573, pl. 34, fig. 10, and pl. 41,
fig. 6).
408 WILLIAM A. HASWELL.
Its cells, several hundred in number, are arranged in a radiat-
ing manner round the oviduct, their narrow inner extremities
evidently acting as ducts by which the secretion is dis-
charged. Their nuclei are large, a little less than ‘005 mm.
in diameter. Between the cells are a number of smaller
nuclei indicating the presence of a certain amount “of inter-
cellular tissue.
The vitelline glands (fig. 3, v.) extend throughout a narrow
belt of the lateral regions of the body from the neck to the
posterior end. The lobes are spherical or subspherical in
shape, and average about ‘03 mm. in diameter. Hach lobe
has its slender duct, which joins those of neighbouring lobes
to form larger ducts, and these again combine to form the
main lateral ducts (fig. 7, v. d.). These converge from both
sides towards the middle line, running on the ventral side of
the ovary, and finally unite to give rise to an impaired main
duct, situated slightly to the right of the middle lne. This
runs backwards and joins the oviduct as already described.
Near its termination it is usually distended with yolk, and this
dilated part (figs. 7, 8, and 9, v. 7.) (‘05 mm. in diameter)
might be looked upon as a yolk-receptacle. It is followed by
a constricted part with thickened walls (fig. 8, v. 7. ©.)
through which the yolk cells can only pass singly to enter
the oviduct. The yolk matter leaves the lobes of the glands
in the form of very regular spherical masses ‘012 mm. in
diameter, each of which contains one, or sometimes two,
rounded bodies which, as they are capable of being stained,
though only slightly, are very lable to be mistaken for
nuclei. These bodies will be further referred to in the
description of the egg. Meanwhile it is of importance to
emphasise the fact that they are not nuclei, and that the
vitelline masses in which they are lodged are not cells.!
The wall of the vitelline ducts consists of fibrillated proto-
plasmic material with nuclei at intervals. In the main duct
1 This is contrary to what is usually stated of Cestodes in general. Braun,
for example, states: ‘* Die Ansicht Moniez’s dass die Dotterzellen keine
echten sondern nur Scheinzellen seien entbelirt jeder Begriindung (5, p. 1468).
ON A CESTODE FROM CESTRACION. 4.09
the wall is thicker, and contains a large number of super-
ficially situated nuclei.
The vagina (figs. 6, 9, and 10, ra.) opens into the shallow
genital cloaca by a narrow aperture immediately in front of
the male aperture. The terminal part is somewhat dilated.
From this point it bends round the sac of the penis as a narrow
tube, which dilates again to a diameter of about ‘05 mm.,
as it runs straight backwards immediately above (1. e. on
the dorsal side of) the secondary uterus. When it reaches
the region of the ovary it again becomes narrower and more
sinuous. Hventually passing backwards on the dorsal side
of the isthmus, it becomes somewhat dilated again to form a
vesicle, the receptaculum seminis (figs. 6, 7, 9, 10, 11, and
12, r. s.). From the rounded posterior end of this a
narrow duct, the fertilising duct (f. d.), runs to join the
oviduct.
In the posterior part of its extent the vagina has a thickish
muscular wall consisting of external longitudinal and in-
ternal circular layers. Internal to this is a cuticle beset
with exceedingly minute spinules. External to the muscle is
a layer of cells resembling those cells of the oviduct which I
have supposed to be myoblasts. Anteriorly the muscular
layers become reduced, and longitudinal fibres alone are
present. The fertilising duct resembles the oviduct in
structure, but the cuticular hairs are absent. In the
posterior proglottides of the strobila (fig. 2) all parts of the
reproductive apparatus are represented, though neither the
male nor the female organs are mature, and there are no
egos in the uterus. The latter has a comparatively narrow
lumen surrounded by a thick layer of small cells; its aperture
of communication with the primary uterus is already
developed. In more anteriorly situated proglottides the
uterus is represented by a solid cord of small cells running
along on the ventral side of the vagina.
410 WILLIAM A. HASWELL.
Development.
In the case of P. Dohrnii, Zschokke (20) states that the
formation of eggs begins in the posterior proglottides of the
strobila. In the form now under consideration this is not the
case, eggs only occurring in well-developed free proglottides.
The only recorded observations on the development of any
member of the genus appear to be a few notes on P. thridax
by Moniez (14, p. 28). I can trace no correspondence what-
ever between the statements there made and what I have been
able to observe in the species from Cestracion.
The primary uterus contains only eggs with unsegmented
ova. ‘The entire egg is in the form of a thick spindle about
"045 mm. in length and ‘021 mm. in greatest breadth. The
shell is at this stage not yet fully solidified, so that the
shape is readily modified by pressure, and the eggs tend to
adhere together in masses. he shell consists of two distinct
layers
fibrillee —which run in the direction of the long axis of the egg.
The completed egg in the primary uterus contains (1) the un-
segmented ovum ; (2) a large number of small, bright globules
(3) one, or, more commonly, two, larger rounded masses.
The last two are the substance of the vitelline spherule.
When the eggs are acted upon by any weak acid the small
globules tend to run together into larger (2 [14], p. 28) masses,
and eventually these pass out through the shell at the ends of
the egg, so that in preparations fixed and stained by any of
the ordinary methods this constituent of the egg becomes
completely lost, there being left behind merely some irregular
granular matter, in which, presumably, the globules were
enveloped. ‘These globules, from their appearance and
behaviour, are most probably composed of oily matter.
The larger bodies derived from the yolk (see fig. 20) are of
an entirely different character. They are solid masses having
an outer homogeneous and an inner made up of fine
the central hilum and concentric lamination characteristic of
the calcareous corpuscles. ‘They become coloured, though
not strongly, by staining agents, the central mass colouring
ON A CESTODE FROM CESTRACION. 411
first. In fixed and stained preparations they become much
altered, having apparently become partly dissolved, and the
concentric lamination being no longer discernible, might very
easily be taken for nuclei. Like the oil globules, these bodies
consist, doubtless, of food materials ; but both these ingredients
of the yolk persist, not greatly diminished in bulk, to the
most advanced stage observed. Nothing was made out with
certainty as to the processes of maturation and impregnation.
The oosperm does not differ to any appreciable extent from
the ovarian ovum.
Very few, if any, unsegemented ova were found in the
secondary uterus. No definite history of the process of seg-
mentation could be traced, as there seemed to be great varia-
tion in the details. The first two segments (figs. 20 and 21)
are equal. One of these, or both, become divided into two
equal parts (figs. 22 and 23), and from the three or four
equal, or nearly equal cells thus formed, a number of smaller
cells become segmented off (figs. 24, 25, and 26). Eventually
the larger cells become reduced by division until a blastoderm
is formed consisting of a disc of small cells (figs. 27, 28, and
29), which are very irregular in size and shape, and present
no definite arrangement. This disc becomes thickened to
form a rounded mass, on the surface of which appears here
and there a flattened cell. In this stage there appears to be
no further cell-differentiation, except that there are present,
in the most advanced embryos, one or two pairs of very small
cells that become more intensely stained than the rest. It is
conjectured, from their arrangement in pairs, that these are
the cells destined to develop the hooks.
No hooked embryos were found in the uterus of any of the
numerous specimens examined. But of a number of eges
which had been kept in pure sea-water for five days, a large
proportion (figs. 30—52) were found to contain fully formed
active hexacanth embryos. It would thus appear that passage
to the exterior with the feces is, under normal circumstances,
the necessary condition for the development of the hooked
embryo.
412
ono uo
12.
13.
14.
15.
16.
ay:
18.
WILLIAM A. HASWELL.
LIvERATURE.
. BenepEN, P. J. Van.—‘‘ Recherches sur la faune littorale de Belgique :
Les Vers cestoides,”? ‘ Nouv. Mém. de |’Acad. Roy. de Belg.,’ t. xxv,
1850.
3ENEDEN, P. J. Van.—“ Mémoire sur les Vers intestinaux,” Suppl. aux
‘Comptes Rendus de |’Acad. des Sciences,’ 1861.
. Benepen, KE. Van.—* Recherches sur le développement embryonnaire de
quelques Ténias,” ‘ Archiv. de Biol.,’ vol. ii, 1881.
. BLancuarp, —.— Recherches sur |’organisation des Vers,” ‘ Ann. Sci.
Nat.,’ 3 sér., t. vii and viii, 1847.
. Braun, M.—“ Vermes ”’ of Bronn’s ‘ Thierreich.’
. Dresine, C. M.—‘ Systema helminthum,’ 1850.
. Leucxart, R.—‘ Die Parasiten des Menschen.’
. Lryton, E.—‘ Notes on Entozoa of Marine Fishes of New England,”
‘U.S. Fisheries Reports,’ 1886 (publ. 1889).
. Linton, E.—* Notes on Entozoa of Marine Fishes of New Wngland,”
part 2, ‘U.S. Fisheries Reports,’ 1887.
. Linton, E.—‘* Notes on Cestode Parasites of Fishes,” ‘ Proce. U.S.
National Museum,’ vol. xx, 1897.
. Lonnzerc, 0.—“ Bidrag till Kannedomen om i Sverige forekommande
Cestoder,” ‘ Bih. till K. Svenska Vetensk.-Akad. Handlingar,’ Bd. xiv,
1889.
LonneBeErG, E.—‘*‘ Helminthologische Beobachtungen von der Westkiiste
Norwegens: 1 Thi., Cestoden,” ‘ Bil. till K. Svenska Vetensk.-Akad.
Handlingar,’ Bd. xvi, 1890.
Linz, M.—* Ueber einen eigenthtimlichen Cestoden aus Acanthias,”
‘Zool. Anz.,’ xxiv, 1901.
Montez, R.— Mémoires sur les Cestodes,” ‘ Travaux de |’Institut Zool.
Lille,’ t. iii, 1881.
MonrIce.ui, F.—‘‘ Nota intorna a due forme de Cestodi,” ‘ Bollettino
dei Musei di Zoologia ed Anatomia comparata della R. Universita di
Torino,’ vol. vii, 1892.
Otsson, P.— Bidrag till Scandinaviens Helminthfauna II,” ‘ Kgl.
Svenska Vetensk.-Akad. Handl.,’ Bd. xxv, 1893.
Sanussow, H.—‘ Zur Histologie der Geschlechtsorgane von Trisenophorus
nodulosus, Rud.,” ‘ Biol. Centralbl.,’ Ba. xviii.
ScHavinsitanv, H.—“ Die embryonale Entwickelung der Bothrio-
cephalen,” ‘Jen. Zeitsehr. f. Naturw.,’ Bd. xix, Neue Folge, xii, 1885.
ON A CESTODE FROM CESTRACION. 413
19. Surrey, A. K.— Description of the Entozoa,”’ A. Willey’s ‘ Zool.
Results,’ part 5.
20. Zscnoxke, F'.—‘ Récherches sur la structure anatomique et. histologique
des Cestodes ” ‘Mem. de l'Institut nation. Génévois,’ t. xvii, 1S86—
1889.
21. Zscuoxxke, F.— Studien itiber den anatomischen und _histologischen
Bau der Cestoden,” ‘ Centralbl. f. Bakteriologie u. Parasitenkunde,’ i,
1887.
EXPLANATION OF PLATES 22—24,
Illustrating Prof. William A. Haswell’s paper ‘‘ On a Cestode
from Cestracion.”’
List oF Rererence Letters.
ec. Cirrus. ¢.s. Cirrus sheath. ez. Cuticle. d. Depression at posterior
end of free proglottis. d.v.m. Dorso-ventral muscular fibres. e.¢c. m. Exter-
nal layer of circular muscular fibres. e. 2. m. External layer of longitudina
muscular fibres. ea. Main excretory vessel. fd. Fertilising duct.
h. “Head” of separate proglottis. 7./.m. Internal longitudinal layer of
muscle. z.c. Nerve cord. o. d.' First part of oviduct. o.d.? Second part of
oviduct. ov. Ovary. ov. m. Median part or istlimus of ovary. p.u. Ootype
or primary uterus. 7.s. Receptaculumseminis. s.¢@. Sperm duct. s.g. Shell-
gland. s.w. Uterus. sw. “Swallowing apparatus.” sw. m. Muscular layer
of swallowing apparatus. sw. my. Myoblasts of swallowing apparatus. e.
Lobes of testis. v. Lobes of vitelline glands. va. Vagina. v.d. Vitelline
ducts. v.7. Vitelline reservoir. v.7.c. Constriction at posterior end of
vitelline reservoir. @. Male reproductive aperture. 9. Female reproduc-
tive aperture.
PLATE 22.
Fie. 1.—Scolex of Phyllobothrium vagans magnified.
Fie. 2.—Last proglottis of strobila magnified.
Fic. 8.—Free proglottis, dorsal aspect. Nervous system, blue; excretory
vessels, green; testicular ducts, red.
Fie. 4.—Portion of transverse section of strobila, showing integument and
muscular layers. x 600.
414 WILLIAM A. HASWELI..
Fic. 5.—Transverse section of cirrus. cz. Cuticle, with spinules. ¢.m.
Layer of circularly arranged muscular fibres. 7. m. Layer of longitudinal
muscular fibres. my. Layer of myoblasts.
Fic. 6.—General view of the female reproductive apparatus as seen from
the ventral side.
Fie. 7.—Dorsal view of the median part of the ovary and of the neigh-
bouring ducts.
PLATE 23.
Fic. 8.—From a series of longitudinal (horizontal) sections. Section
passing through swallowing apparatus, first part of oviduct and main vitelline
duct. x 450.
Fic. 9.—From the same series. Section dorsal to that represented in
Fig. 8, showing vagina, receptaculum seminis, and shell-gland. x 450.
Fre. 10.—From the same series. Section dorsal to that represented in
Fig. 9, showing receptaculum seminis and fertilising duct. x 450.
Fic. 11.—From a series of transverse sections. Section passing through
swallowing apparatus and median part of ovary.
Fie. 12.—Section immediately behind that represented in Fig. 11.
Fic. 13.—From an oblique series. Mouth of swallowing apparatus. x 600.
Fic. 14.—From a transverse series. Showing swallowing apparatus and
its relations to ovary. x 600.
Fic. 15.—From a transverse series. Showing relations of swallowing
apparatus to oviduct. x 600.
Fic. 16.—From a transverse series. Swallowing apparatus and oviduct.
x 600.
Fic. 17.—From a horizontal series. Section of oviduct at the point where
the ducts of the shell-gland open into it; an ovum in the act of union with a
yolk-zell,
Fic. 18.—Transverse section to show the relations of the primary uterus,
the vagina, and the ruptured secondary uterus.
PLATE 24.
All the figures drawn under Zeiss’s apochromatic 2-0 mm, objective and
compensation ocular 12, magnifying 1100 diameters.
Fic. 19.—Kgg with unsegmented ovum. T'rom preserved specimen.
Vic. 20.—Two-celled stage. Fresh specimen, showing the globules and
coneentrically laminated bodies of the vitelline mass.
Fic. 21.—Two-celled stage. Preserved specimen,
ON A CKSTODE FROM CESTRACION. 415
Fic. 22.—Three-celled stage.
Fic. 23.—Four-celled stage.
Fie. 24.—Stage of about eight, cells.
Fie. 25.—Surface view of blastoderm of a somewhat later stage than that
represented in Fig. 24.
Fic, 26.—Stage of about fourteen cells.
Fie. 27.— Surface view of dise-like blastoderm.
Fies. 28 anp 29.-—Disc-like stages seen edgewise.
Fig. 830.—Hexacanth embryo with the hooks retracted.
Fie. 31.— Hexacantl embryo with the looks everted,
Fic. 82.—EKgg containing hexacanth embryo. Fresh specimen.
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THE DEVELOPMENT OF LEPIDOSIREN PARADOXA.
417
The Development of Lepidosiren paradoxa.
By
J. Graham Kerr,
Regius Professor of Zoology in the University of Glasgow.
Part III.—Development of the Skin and its Derivatives.
With Plates 25—28.
CONTENTS.
The general epidermis
The buceal cavity
The teeth
The hypophysis
The central nervous system
The brain of the adult :
Development of main features of brain topography
Thalamencephalon
The hemispheres .
Mesencephalon
Rhombencephalon
The sense organs:
Olfactory organ .
The eye
Auditory organ
General remarks
Summary :
Explanation of Plate
INTRODUCTION.
PAGE
418
423
424
427
428
428
429
434
437
458
438
438
439
445
447
452
454.
Iy the followmg pages I give an account of the chief
results obtained from my investigation of the development
418 J. GRAHAM ‘KERR.
of Lepidosiren, in so far as they relate to the skin and
certain organs associated with it. Some structures which
ought logically to be described now have been deliberately
omitted: such are the external gills, which I propose to
consider at the same time as the branchial clefts. I also
make no attempt to describe the various organs with an
equal degree of fulness. In regard to some, which I have
found specially interesting, I give a fairly detailed account ;
in regard to others I am content. to outline the main
features.
I have endeavoured to avoid obscuring my description by
going into masses of minute detail, feeling that by so domg I
should often be trespassing in regions where individual varia-
tion, and the “probable error” of observation, make results
useless if not actually misleading.
Tur GENERAL EPIDERMIS.
It has already been shown that the first part of the
epidermis to assume a fairly definite shape is that covering
the ventral surface of the embryo, which is simply the
persistent roof of the segmentation cavity. The epidermis
covering the dorsal surface of the body, on the other hand,
develops later. It has also been shown that during gastru-
lation, in Lepidosiren, the dorsal hp of the blastopore is
composed of a mass of undifferentiated cells, showing no
distinction into layers. In this Lepidosiren differs from
Ceratodus, where, as Semon pomts out, the epiblast is
marked off by a distinct split right back to the lip itself.
Elsewhere than at the blastopore lp epiblast is formed by
delamination: from the large yolk-cells underneath. By
Stage 14, when the process of gastrulation is finished, the
embryo is already covered uniformly by a definite stratum of
epiblast composed of two layers of closely apposed flattened
cells, except just in the lp of the blastopore where the
germinal layers are still, and will remain for some time
THE DEVELOPMENT OF LEPIDOSIREN PARADOXA. 419
undifferentiated. From the first the epiblast is thicker in
the region overlying the archenteron, the cells of which it is
composed being here somewhat columnar. The fate of this
thickened region, which is destined to give rise to by far the
most complicated product of the ectoderm, the central nervous
system, may conveniently be left out of consideration until a
little later.
In regard to the general ectoderm there is little change
to chronicle for a considerable period. In an embryo of
Stage 25 it is still two-layered, the bounding surfaces of the
two layers being smooth and parallel. Prominent yolk
granules are still present in the cells, and the outer layer has
formed on its surface a fine but distinct cuticle. When the
tail begims to form the ectoderm at the tip of this thickens,
its cells assuming a cuboidal form, but remains two-layered.
The general ectoderm retains its two-layered condition
for some time. Increase in thickness by division of the
lower layer cells begins at a period varying from about
Stage 52 to about Stage 55.
In various young Lepidosirens, which have been fixed in
strong Flemming’s solution, I have been able to make out that
certain of the ectodermal cells are provided with tail-like
processes of cell substance, closely resembling the tails so
characteristic of the ccelenterate epithehal cells (PI. 25, fie. 1).
The tails of the ectodermal cells in Lepidosiren are very
difficult to observe, showing up distinctly only in well-fixed
material in which the cells are shehtly separated from one
another. They run along the mner surface of the epidermis,
forming a kind of plexus-hke layer. Into this layer pass
also processes from the underlying mesenchyme cells, so that
it forms an organic connection between ectoderm and mesen-
chyme.
The glandular structures of the fully formed skin of
Lepidosiren (PI. 25, fig. 2) are one of its most charac-
teristic features. The tall unicellular mucus glands, which,
in the adult, form a palisade-hke arrangement through the
whole thickness of the skin, begin to appear about Stage 35
420 J. GRAHAM KERR.
as ordinary cells of the epidermis, whose cytoplasm assumes
a clear vacuolated appearance, the whole cell remaining in
form and size like its neighbours. By Stage 38 the gland-
cells have become predominant by their size, and they are
also elongating in shape.
The multicellular glands appear about the same time as
downgrowths of the deep layer of the epidermis, and here
again we find that the rudiment is solid, and the cavity
appears secondarily. This we can naturally not put down,
as we do in the case of certain other organs arising similarly,
to any such simple cause as the presence of yolk. By Stage 38
a large cavity has appeared, but it is not yet open to the
exterior.
Cement Organ.
A remarkable local development of epidermal gland-cells
is afforded by the cement organ, which, as indicated before,
retains through life the crescentic shape shown by Thiele to
be characteristic of the organ in its early stages in Batra-
chians. It is a curious point, however, to which my attention
was first drawn by my friend Mr. Bles, that in the Amphibia .
the organ is derived from the superficial layer of the epi-
dermis, not the deep layer as in Lepidosiren.
The first indication of the cement organ appears about
Stage 23 (Pl. 25, fig. 3 4) as a sheht thickening of the deep
layer of the epidermis, the superficial layer passing over it
hardly affected. By Stage 25 the thickening has considerably
increased, and the superficial layer now shows signs of break-
ing down over the middle of the gland, so that here the deep
cells are exposed to the external medium (fig. 5 3B). By
Stage 31 the gland is fully functional. Its cells are tall and
columnar with nucleus at the base, and protoplasm showing
peripherally a clear transparent appearance.
During the later stages of development the glandular
surface becomes involuted slightly, and at the same time its
lower edge becomes tilted up somewhat, so that the organ
projects conspicuously above the adjoining skin surface.
THE DEVELOPMENT OF LEPIDOSIREN PARADOXA. 421
Degeneration of the Cement Organ.—The process of
atrophy of the cement organ is a comparatively rapid
process, taking place about Stage 35. It is illustrated by
fig. 5D.
In the early stages of degeneration the glandular epi-
thelium becomes penetrated by vascular loops, and leucocytes
begin to concentrate in its neighbourhood. At a later stage
(e.g. Stage 35, fig. 3p) there are crowds of leucocytes
collected about the gland, and it is now seen that they are
laden with fatty and other granules, the product of their
active metabohism. The glandular part of the ectoderm
becomes gradually consumed, and the adjoining epidermis
becomes shrivelled and has its surface thrown into wrinkles
as the gland cushion diminishes in size.
Pigment Cells.
About Stage 35 branched pigment cells begin to appear in
the ectoderm. I believe that these are all mesodermic in
origin. In sections from embryos of about this stage many
examples of pigment-laden chromatophores may be seen in
process of migration into the ectoderm.
The only case, in fact, that I have found of pigment
granules being formed to any conspicuous extent in epidermal
cells is that of the pigment layer of the retina.
Changes in Chromatophores caused by Alteration
in the Amount of Incident Light.
I have already referred (Part I, p. 320) to the remarkable
difference in the appearance of a youne Lepidosiren
during the day and mght. A Lepidosiren of Stage 38,
which by day is of a deep rich brownish black, becomes at
night-time quite colourless, the change being associated with
the withdrawal of the dendritic pseudopodia of the chro-
matophores. An inspection of Pl. 25, figs. 4a and 4.8, will
4,22, J. GRAHAM KERR.
illustrate the appearance of the skin of a Lepidosiren of
the stage mentioned during the day and durimg the night.
The night specimen had been exposed to faint lamp-lght for
several minutes, and consequently the retraction of the
pseudopodia is not quite complete.
From fig. 4 it will be seen that the black chromatophores
tend towards two distinct types, differing in the appearance
of the contained pigment and in the degree of ramification of
the pseudopodia.
In type A, which is the less numerous, the pigment is very
black, the cell body is compact, and the pseudopodia are
long and comparatively slightly branched, and often present
a varicose appearance. In type B the contained pigment is
less opaque, of a brownish colour, and the cell body is often
very irregular in shape, projecting in great trunks from
which arise numerous short and very irregular pseudopodia.
Of these type B appears to be the more highly sensitive to
light, a much fainter amount of light sufficing to cause
extrusion of its pseudopodia.
When in a state of maximum expansion the pseudopodia
frequently anastomose both with their neighbours and with
those of other chromatophores. Anastomosis often takes
place between pseudopodia belonging to chromatophores of
the two different types. This, together with the presence of
intermediate forms, indicates that the two types are not
really distinct, but are merely the extremes of variation of a
single type.
It is instructive to compare vertical sections through the
skin in different light conditions. Fig. 5 illustrates such
sections from (1) a young Lepidosiren of Stage 38, killed
at 9 p.amn., by faint lamp-light (fig. 5c), (2) one of the same
brood taken from deep shade at 2 p.m. (fig. 5 B), and
(3) a rather younger Lepidosiren taken from an open
white enamelled dish with clear water and exposed to bright
diffused dayheht (fig. 5 A).
In (1) the chromatophores are in their state of maximum
contraction, and | may mention that the scattered chromato-
THE DEVELOPMENT OF LEPIDOSIREN PARADOXA, 4238
phores deep down in the substance of the body are also
contracted.
In (2) the chromatophores have their pseudopodia fully
extruded. In the case of pigment cells within the epidermis
the pseudopodia pass between the cells up towards the
surface. ‘The chromatophore tends to push its pseudopodia
towards the hght; their movements are positively helio-
tropic. In the case of chromatophores lying in the super-
ficial layer of the dermis the cells flatten themselves out
against the lower surface of the epidermis, forming with their
pseudopodia a practically continuous heht-proof coat.
In (5) the chromatophores are seen to have their pseudo-
podia at the maximum of extension.
THe “ Stomopzum.”
In the young Lepidosiren up to Stage 30 there is no
stomodzum present; the enteric rudiment, solid in this
region and sharply marked off from surrounding tissues by
its cells being packed with large yolk granules, extends right
up to the external epiblast.
About the stage mentioned a change is seen to be setting
in in the anterior part of the enteric rudiment, corresponding
to what will become the buccal cavity. The superficial layer
of the still solid rudiment is seen to be approximating in
character to the ectoderm. Its yolk granules become finely
broken up, showing that active metabolism is taking place ;
protoplasm and nuclei are becoming more abundant. In this
way there arises a layer of definite epithelium continuous
anteriorly with the external epiblast, sharply marked off
from the embryonic connective tissue outside it, but im-
ternally passing without any sharp boundary into the yolk-
laden mass inside (cf. Pl. 25, fig. 64). It is, as it were, as if
an influence were spreading inwards from the external epi-
blast, gradually transforming the original “ endoderm” yolk-
laden cells into ectoderm like itself. I find no evidence of
424, J. GRAHAM KERR.
an actual bodily involution of ectoderm such as is ordinarily
associated with the term stomodzeum. On the contrary, the
“ stomodeeal ”’ cells
perfectly gradual transition between the
and the typical yolk-laden endoderm cells shows quite con-
clusively that the former are beg derived from the latter.
The buccal rudiment retains its solid character till about
Stage 31. About this time the cells in its interior begin to
degenerate and break down, and so give rise to the cavity of
the mouth.
The tooth germs begin to appear long before there are any
traces of lumen in the buccal cavity.’ Already in Stage 32
they may be detected.
Development of the Teeth.
One of the most striking points brought out by Professor
Semon’s researches on the development of Ceratodus has
been the way in which the so characteristic tooth plates are
formed by the joming together, by dermal bony trabecule, of
originally separate denticles. On coming to consider the
tooth development of Lepidosiren I not unnaturally ex-
pected to find a similar state of affairs, and I was accordingly
much astonished on using appropriate macerating media to
fail completely to discover separate denticles. I then turned
to young specimens of Ceratodus, and had no difficulty in
completely confirming Semon’s description. In Lepido-
siren the only possible reminiscence of such a stage in tooth
1 In Urodele Amphibians the teeth similarly develop before a lumen is
formed, and the lining of the buecal cavity appears to arise in them just as
in Lepidosiren and Protopterus. Rése (Schwalbe’s ‘ Morphologische
Arbeiten,’ Bd. iv, S. 182), in describing the development of the teeth in
Urodeles, talks of the buceal cavity being “ mit Dotterplattchen und abgestos-
senen schollenformigen Epithelzellen ausgefiillt.”” On the contrary, I should
say, from a study of my own sections of Urodele embryos (Amblystoma
and Triton). that the buecal cavity has not yet arisen at the stage of which
Rose is speaking. In fig.6B I figure a section of the mouth region of an
Amblystoma of the stage in question for purposes of comparison with the
corresponding section from Lepidosiren.
THE DEVELOPMENT OF LEPIDOSIREN PARADOXA. 425
arrangement is to be found in the fact that in the young
individual the teeth are furnished with definite prominent
pointed cusps—each probably representing the tip of an
originally simple denticle,—although in ontogeny they de-
velop as a perfectly continuous ridge from the beginning.
Text-ric. 1.—Sagittal section through head region of a Protopterus
larva (Stage 33). Cam. Zeiss a*, oc. 4.
b.c. Buccal cavity. d.p. Dental papilla. ep, Pineal body.
p. Paraphysis. ¢. ‘Thyroid rudiment.
The first obvious rudiments of teeth occur about Stage 30
in the form of a thickening of the oral epithelium,! under
which the mesoblast becomes concentrated as it were, the
nuclei being crowded much more closely together than
elsewhere.
By Stage 31 the thickening is growing downwards into
the mesoblast so as to border on each side a ridge-like
“ yapilla ” of mesoblast (Pl. 26, fig. 7 a).
The first traces of hard structure in the tooth appear about
Stage 52, when a conical calcareous cap appears beneath the
1 For general topographical relations of this see Text-fig. 1.
4.26 J. GRAHAM KERR.
enamel organ. It is difficult to arrive at a certaim opimon on
the morphological nature of this first formed cap. It adheres
stronely to the enamel organ, as shown by the torn surfaces
when the two structures have been pulled apart in process of
preparation, and in many cases it 1s for a time sharply marked
off from the underlying dentine. On the other hand, it
differs from ordinary enamel in the much larger proportion
of organic matter, which causes it to remain quite distinct
even in decalcified specimens.
On the whole, [ am inclined to look upon this structure as
beg enamel, though of a somewhat modified kind.
The structure of the palatopterygoid teeth about Stage 35
may be gathered from the sections represented in figs. 7 B, 7 ¢,
and 7 p. The enamel forms a distinct cap tapering off
towards its edges, and sharply marked off from the under-
lying dentine. It shows a faint striation perpendicular to its
surface. In undistorted sections the flat mner ends of the
enamel cells abut close against it (fig. 7D).
At this stage there is a quite definite though still thin
layer of dentine lying within the enamelcap. The broadened
outer ends of the odontoblasts come into close contact with
one another, and form, to the eye, a quite continuous mass
(fig. 7p). As they pass into this their protoplasm shows a
development of fine fibrillee crossmg one another in all
directions. Traced still further out the fibrillar mass gradu-
ally takes on more and more deeply the stain which, in
Heidenhain’s hematoxylin preparations, indicates the pre-
sence of calcareous matter. The thin outer layer is, im fact,
fully calcified dentine, on its inner side passing by impercep-
tible gradations into the ordinary protoplasm of the odonto-
blasts, on its outer side marked off from the enamel by a
sharp boundary.
In Stage 36 (fig. 7 ») the formation both of dentine and of
the bony trabecule which form the spongy basal support for
the tooth is seen to have made considerable progress. ‘The
ridges of the tooth now approach the surface of the oral
epithelium, which is becoming thin over their apices prepara-
THE DEVELOPMENT OF LEPIDOSIREN PARADOXA. 427
tory to breaking through. 'The enamel layer is closely fused
with the underlying dentine; the sharp line separating the
two has completely disappeared, and it is only possible, by
the use of very high powers, to distinguish the enamel by its
clear appearance without any obvious structure from the
dentine, which still shows a faint reticular or fibrillar struc-
ture—the remnants of the more obvious structure of the
same kind in the uncalcified odontoblast.
Finally, in Stage 38 (fig. 7 F), when the young Lepidosiren
has already begun to feed, the teeth have broken freely
through the oral epithelium, the enamel organ having disap-
peared entirely except for a vestigial flap (fig. 7 F, e.0.) stick-
ing up all round the base of the tooth. The enamel is now
no longer to be detected in my sections: it has probably been
worn off, being doubtless, from its larger proportion of
organic matter, much less hard than ordinary enamel. The
mass of dentine has much increased in size. A little later its
central portion assumes the hard glassy character of the
vitrodentine of the adult (“Enamel,” ‘Tomes’ ‘ Dental
Anatomy,’ fifth edition, p. 263).
Hyporpnwysis.
The hypophysis is somewhat obscure in Lepidosiren.
In Stage 254 it is visible as a somewhat wedge-shaped in-
erowth of the deep layer of the epiblast.
In Stage 294+ the deep end of the structure has become
slightly swollen, with indications of a longitudinal split in its
middle; the portion connecting this with the ectoderm is
thinned down to a narrow thread occupying the space between
the closely approximated front end of the gut and the floor of
the fore-brain.
At a stage very slightly later (30) the connecting isthmus
is nipped through, while the expanded extremity, whose
split is now widening out into a definite cavity, les as a
closed sac beneath the infundibulum.
d
428 J. GRAHAM KERR.
Eventually the hypophysis becomes here as elsewhere
closely united with the infundibulum, its dorsal portion
becoming partly penetrated by tubular outgrowths of the
latter (saccus vasculosus, cf. p. 432, Text-fig. 2, H).
CrentrRAL Nervous System.
As the brain of the adult Lepidosiren! has never before
been investigated in the fresh condition, I give on Plates
26 and 27 figures illustrating its conformation, and showing
the roots of the cranial nerves, including those of the fourth
and sixth, whose existence in Lepidosiren has hitherto been
doubted. By a comparison with Burckhardt’s figures of
Protopterus it will be seen that the two brains are very
similar. In dorsal view the only difference is in the relative
size of the different parts. In Lepidosiren the mid-brain
region is relatively longer, the thalamencephalon relatively
shorter than in Protopterus. In my figure I have not
shown the extensive system of outgrowths from the saccus
endolymphaticus which here, as in Protopterus, overlies and
to a great extent hides the region of the hind brain.
In the ventral view of the brain the cerebral hemispheres
are not sharply marked off from the thalamencephalon. The
swelling at the base of the olfactory nerves is much more
conspicuous, owing to the smaller size of the post-olfactory
lobe which underlies them. The lobi inferiores are much
more strongly developed, the hypophysis is more rounded in
form, and the hind brain is like the cerebral region less broad
from side to side as compared with the thalamencephalon.
In the side view of the brain the most striking difference
1 In dissecting the brain of Lepidosiren one is struck by the extra-
ordinary development of richly ramifying blood-vessels within the cranial
cavity, forming a packing all round the brain. ‘This may possibly be an
adaptation to the times at which it is impossible to make the blood rich in
oxygen, during the final stages in drying up of the swamps, or during casual
rainfalls in the dry season.
THE DEVELOPMENT OF LEPIDOSIREN PARADOXA. 429
from what is found in Protopterus occurs in the cerebral
hemisphere in the much smaller development of the post-
olfactory lobe. The “lobus hippocampi” described by
Burckhardt for Protopterus is less distinctly marked off in
Lepidosiren, but is still distinctly visible.
The Development of the Main Topographical
Features of the Central Nervous System.
I now proceed to describe in outline the main features in
the brain and spinal cord of Lepidosiren. The minute
structure and details of histogenesis I propose in this general
account of the development to leave completely on one
side.
On Pl. 27, fig. 10, are given a series of drawings of the
brain in side view, and on Pl. 26, fig. 8, are given selected
stages as seen from the dorsal aspect, and an inspection of
these figures will suffice to give a clear idea of the evolution
of the external features of the brain without any elaborate
verbal description.
In my description of the early stages in development the
brain and spinal cord were left (this Journal, vol. 45, p.
23) when they were still in the condition of a partly solid
rudiment. From the beginning the anterior or brain region
is distinguished by its greater width.
At about Stage 20 or 21 a slight constriction appears
marking off the region of the hind brain from the region in
front of it.
At about Stage 25 a transverse wrinkle in the floor of the
brain begins to appear to mark the commencement of cranial
flexure.. By Stage 26 (fig. 10 4) this has become well marked.
A slight bulging on each side of the thalamencephalon at this
stage marks the rudiment of the cerebral hemisphere.
By Stage 29 (cf. fig. 10 8) the cranial flexure has become
more pronounced, and a depression of the brain-roof has
begun to show itself in the region of the anterior limit of the
hind brain. The anterior corner of the hind brain has grown
430 J. GRAHAM KERR.
out to form a prominent knob on each side, and the cerebral
hemisphere has become more distinct.
From Stage 30 to 32 (figs. 10 c, p, and ©) the chief changes
consist in the close approximation of the infundibular region
to the floor of the hind brain, in the appearance of the pineal
outgrowth, and in the commencing forward growth of the
two hemispheres. It is to be noted that up to nearly Stage
32 there is no obvious separation of mid-brain from thalamen-
cephalon. In a brain of about Stage 35 (fig. 10 F) the chiet
advance consists in the considerable growth forwards of the
hind brain on each side, so as in side view to completely hide
the floor of the mid-brain. The roof of the thalamencephalon,
forming for the most part the pmeal cushion,’ is now quite
sharply marked off from the roof of the mid-brai. From
now onwards to Stage 38 changes in external conformation
are but slight, as will be seen from fig. L0G, the chief one
being in the upward growth of the hemisphere-roof, so that
it, with the pineal cushion, rises to about the same horizontal
line as the summit of the mesencephalon.
In the last stage (Stage 39), which I figure (fig. LOH), a
marked advance towards the adult condition is seen, the
cerebral hemispheres having undergone a™large increase
in antero-posterior length. The olfactory lobe is already
marked off.
The chief change subsequent to this consists in the further
ereat elongation of the brain axis.
Dorsal Aspect of the Brain.
The earliest stage which I have figured (fig. 8 a, Stage 31)
illustrates (1) the relatively enormous size of the hind brain ;
(2) the fact that mesencephalon and thalamencephalon form
a single perfectly definite brain region, on the roof of which
the pineal body has appeared; and (3) the paired independent
rudiments of the hemispheres.
In the view of Stage 35 (Pl. 26, fig..88) the forward
' Zirbelpolster.
THE DEVELOPMENT OF LNPIDOSIREN PARADOXA. 451
growth of the hind brain on each side is seen, the elongation
of the mesothalamencephalic region and its distinct division
into a mesencephalic part behind, and a thalamencephalic
portion in front, the latter thin-roofed, and forming a kind of
pillow or cushion (pineal cushion), upon which the pineal
body rests. Lastly, the elongation of the cerebral hemispheres
has now begun.
In the brain of Stage 38 (fig. 8c) this elongation of the
cerebral hemispheres, of the mid-brain, and of the lateral
angles of the hind brain is seen to have gone on still further.
Finally, in the adult brain (fig. 8 p) the great elongation of
the antero-posterior axis is very obvious, affecting all regions
of the brain except the thalamencephalon. It will be noticed
that the lateral angles of the hind brain have lagged behind
in this lengthening, so that now they do not project forwards
at all. QUule — — - —
| 7+38 Lp
472 ROBERT GURNEY.
The pleopods are now well developed, each exceeding the
length of the next succeeding segment. The first four pairs
consist of a broad basal part bearing a long exopodite and a
short stump representing the endopodite, but there are no
setee and no trace of segmentation. The fifth pair, on the
sixth segment, are simple unbranched appendages. The
telson is exactly the same as in the preceding stage.
The Megalopa (fig. 13) —Measurements (average of ten
specimens) :
Length of carapace. ‘ . 36 mm.
Breadth across third lateral spine eas 2 iy Mee
Length of antenne . : ee 5 ae
The last larval stage passes by a single moult to the
Megalopa, which is distinctly recognisable as Corystes,
though retaining certain features characteristic of the zowa.
The rostrum and dorsal spine are still present, though very
greatly reduced.
The rostrum has now the form of a broad plate extending
forwards between the eyes, its lateral margin arched upwards
and crenulated. Its extremity is trifid, the median process
representing the last trace of the original long rostral spine
and retaining the orange chromatophores of the previous
stage, the lateral processes by which it is flanked beg new
formations. A few hairs are borne upon the upper anterior
surface of the rostrum.
The dorsal spine is now an inconspicuous orange-red
process, situated not immediately over, but somewhat behind
the heart. From it a ridge runs forwards for some distance
along the middle line of the carapace.
On either side of the middle line, in the region of the
stomach, there is a single short spine on the dorsal surface.
These spines appear first at this stage, and are lost again
with the next moult.
Laterally the carapace bears three strong teeth on either
side, the first immediately behind the eye, and the third
above the first ambulatory leg. The postero-lateral margin
METAMORPHOSIS OF CORYSTES CASSIVELAUNUS. 473
of the carapace is fringed with a number of setz. The
appendages have now taken on essentially the form of those
of the adult. The second antenne are considerably longer
than the carapace, many-jointed, and provided with the
characteristic dorsal and ventral row of sete. The antenne
have, as already described, the same function of serving as a
respiratory tube as they have in the adult. The mandible
palp is now three-jointed, the distal jomt bearing a number
of setee and overhanging the mouth opening in front.
The first maxilla differs from that of the preceding stage
in the form of the endopodite, which is now not jointed, and
bears but a single well-developed seta. ‘This reduction in the
number of sete is remarkable from the fact that in the adult
there is a rich clothing of sete.
There is but little change in the form of the inner lobes,
and the only change from this stage to the condition in the
adult consists in a relative reduction of the superior lobe and
an increase in number of spines.
In the second maxilla there is a great increase in size of
the scaphognathite and simplification of the structure of the
endopodite (fig. 12).
The first and second maxillipedes show an intermediate
condition between the swimming limb of the zoza and the
masticatory limb of the adult. The two-jointed exopodite is
practically unchanged, except that in the first pair it bears
but five terminal sete, and in the second pair eight. ‘The
endopodite of the first pair (fig. 15) is no longer jointed, but
has not acquired the lamellate form characteristic of Corystes.
The two basal joints are richly setiferous at their imner
margin, and the epipodite is greatly developed. The endo-
podite of the second maxillipede (fig. 17) has practically the
adult form, while the podobranch and small arthrobranch
are both developed.
The third maxillipede develops directly to the adult form,
the second joint of the endopodite having the characteristic
anterior prolongation. ‘he remaining thoracic legs have in
all essential respects the form of those of the adult. The
474, ROBERT GURNEY.
abdomen still retains some larval characters. The lateral
spines of segments 2—5 are still retained, and the telson still
shows traces of bifurcation, being deeply indented posteriorly.
The five pairs of pleopods have the shape characteristic of
the typical Brachyurous Megalopa. Those of the first
four pairs each consist of a stem bearing a long exopodite
armed with numerous long ciliated setae. The endopodite is
very small, and interlocks with that of the opposite append-
age as a retinaculum. ‘The last (fifth) pair of pleopods have
no endopodites, and are shorter than the telson itself.
First Post-larval Stage (fig. 14).—Measurement :
Length of carapace . : . 40 mm.
Breadth (across third lateral spines) POEs
Length of antenne . : Pet) i fh
The cast skin of the specimen from which these measure-
ments were taken had the following dimensions :
Length of carapace . ; . 34mm,
Breadth . : : Sy gd eas
Length of antenne . : «pared:
The Megalopa stage lasts, according to my observations,
from eighteen to twenty days, but possibly a more abundant
food supply in natural conditions would somewhat shorten
the period.
The young Corystes has now attained the structure of the
adult in almost all respects. ‘The rostral spine is reduced to
an insignificant tubercle lying at the base of the indentation
between the two anterior spines. The dorsal spine is com-
pletely lost, though a small orange chromatophore still marks
its position on the carapace. The dorsal surface of the cara-
pace is smooth, the median ridge of the previous stage and
the two anterior dorsal spines having disappeared.
Besides the three lateral teeth of the Megalopa a fourth
tooth is developed behind on each side close to the posterior
edge of the carapace, so that the number characteristic of
the adult is attained. The cephalo-thoracic appendages show
no changes worth noting, except that the endopodite of the
METAMORPHOSIS OF CORYSTES CASSIVELAUNUS. A475
first maxillipede has attained its final lamellar form (fig. 16).
The abdomen, however, has changed considerably. It is now
kept normally bent up under the body, the young crab having
taken definitely to a burrowing habit. The first two seements
are broad and flattened at the sides, while the remaining
segments narrow out posteriorly and bear no lateral spines.
All the segments bear sete on their lateral margins. The
telson has now an evenly rounded posterior margin.
The pieopods are no longer swimming organs, haying lost
all their sete. The first four pairs remain biramous, and of
about the same size as before, but the fifth pair is reduced to
a simple stump. There is still no appendage upon the first
abdominal segment, so that apparently in the female this
appendage never develops, while in the male it is retarded
till at least the second post-larval stage. I have hitherto
obtained no later stage than that now under consideration,
so that I cannot say at what period the distinctive sexual
characters appear. The specimens of the first post-larval
stage in my possession show also no difference in the relative
size of the chelipeds.
CoNCLUSION.
The Corystide, though placed by Milne Edwards (1834)
and by Heller (1863) among the Oxystomata, have by more
recent authors, such as Claus and Miers (1886), been assigned
to the Cyclometopa. The resemblance between the Corystidee
and the true Oxystomata has been shown by Mr. Garstang
(1897, etc.) to be largely superficial, and due to adaptive
modifications of an essentially different character, though
directed to the same ends. He has, in fact, brought forward
clear evidence that the Corystide and the Oxystomata have
been independently derived from Cyclometopous ancestors.
This view is to some extent supported by my observations
on the development of Corystes, though the great uniformity
in the structure of the zowa throughout the Brachyura pre-
vents any conclusion being drawn from the earlier stages.
4.76 ROBERT GURNEY.
In fact, it must be confessed that the most striking feature
of the zowa of Corystes, namely, the great length of the spine,
recalls the zozas of such Oxystomata as Dorippe and Ethusa
—forms from which it differs essentially in other respects—
more than those of the Portunidee.
Still the final stages of the metamorphosis show that the
peculiar emarginate rostrum of the adult (which recalls that
of the Oxystomata) is preceded by a three-toothed rostral
prominence which exactly resembles that found in most
Portunids. That the central tooth represents more than a
mere entogenetic stage in the reduction of the long rostral
spine of the larva is also confirmed by the retention of a
trifid rostrum in the adult of Pseudocorystes and Trachycar-
cinus (Faxon).
The existence of this Portunid stage in the development of
Corystes was, I understand, the subject of a verbal communi-
cation made by Mr. Garstang to the Toronto meeting of the
British Association in 1897 under the title ‘On Recapitula-
tion in Development, as illustrated in the Life-history of the
Masked Crab (Corystes).” As Mr. Garstang has been unable
hitherto to write up his observations for publication, and as
he informs me that the material at my disposal is more
complete than in his own case, I am glad to be able to give a
full account of the metamorphosis, and to confirm his obser-
vations. I may here express my indebtedness to him for his
kind advice and many suggestions during the carrying out of
my work.
PiymoutnH ; May, 1902.
BIBLIOGRAPHY.
1. Cano, G.— Sviluppo postembryonale dei Dorippidei, Leucosiadi, Corys-
toidei, e Grapsoidi,’’ Napoli, ‘Atti della R. Accad. d. Sci. Fisiche e
Matem.,’ 1891.
2. Ciaus, C.—‘ Untersuchungen zur Erforschung der genealogischen Grund-
lage des Crustaceensystems,’ Wien, 1876.
METAMORPHOSIS OF CORYSTES CASSIVELAUNUS. 477
3. Faxon, W.—‘ Reports on an Exploration off the West Coasts of Mexico,
Central and South America, and off the Galapagos Islands: XV, The
Stalk-eyed Crustacea,” ‘Mem. Mus. Comp. Zool. Harvard Coll.,’
xvill, 1895.
4. Garstanc, W.—‘‘ The Habits and Respiratory Mechanism of Corystes
Cassivelaunus,” ‘Journ. Mar. Biol. Ass.,’ iv, pp. 223—232, 1896.
5. Garstane, W.—‘ The Functions of Antero-lateral Denticulation of the
Carapace in Sand-burrowing Crabs,” ‘Journ. Mar, Biol. Ass.,’ iv,
pp. 896—401, 1897.
6. Garstanc, W.—‘On some Modifications of Structure Subservient to
Respiration in Decapod Crustacea which burrow in Sand,” ‘ Quart.
Journ. Mic. Sci.,’ xl, 1898, p. 211.
7. Heiter, C.—‘ Die Crustaceen des Siidlichen Europa,’ Wien, 1863.
8. MityE Epwarps, H.—‘ Histoire naturelle des Crustacés,’ 1834.
9. Prentiss, C. W.—‘‘ The Otocyst of Decapod Crustacea: its Structure,
Development, aud Functions,” ‘ Bull. Mus. Comp. Zool. Harvard Coll.,’
xxxvi, No: 7, 1901.
10. Wexpon, W. I’. R.—‘ Note on the Functions of the Spines of the Crus-
tacean Zowa,” ‘Journ. Mar, Biol. Ass.,’ i, n. s., 1889-90, p. 169.
EXPLANATION OF PLATES 29—31,
Illustrating Mr. Robert Gurney’s paper on ‘The Metamor-
phosis of Corystes Cassivelaunus (Pennant).”
All figures drawn with the aid of the camera jucida.
Vie. 1.—( x 32.) Zowa of the first stage, showing distribution of chro-
matophores.
Fig. 2.—(x 100.) Telson of the first zoa.
Fic. 3.—( X 47°5.) Zowa of the second stage.
Fic. 4.—( x 100.) Telson of the second zowxa.
Fie. 5.—( x 47°5.) Telson of the third zowa.
Fig. 6.—(X 35.) Third maxillipede of the first post-larval stage.
Vie. 7.—(x 26.) Zowa of the fourth stage.
Fig. 8.—(x 65.) Second maxilla and thoracic appendages of the third
zowa.
Hp. 1—Kp. 3. Epipodites of maxillipedes 1—3.
a l—a3. Arthrobranchs 1—8.
p', p?. First and second pleurobranclis.
mxp*, Third maxillipede.
478
Fic.
Fic.
Vie.
Fic.
Fie.
Iie.
Fie.
Ge
Fie.
9.—(x 260.)
100561703)
11.—(x 105.)
12.—(x 65.)
13.—(X 20.)
14.—( x 20.)
15.—(X 45.)
16.—( xX 40.)
17.—(x 45.)
ROBERT GURNEY.
Second maxilla of the first zowa.
Second maxilla of the second zowa.
Second maxilla of the third zova.
Second maxilla of the Megalopa.
The Megalopa.
The first post-larval stage.
First maxillipede of the Megalopa.
First maxillipede of the first post-larval stage.
Second maxillipede of the Megalopa.
ARTIFICIAL PARTHENOGENESIS AND FERTILISATION. 479
Artificial Parthenogenesis and Fertilisation :
A Review.
By
Thomas HH. Bryce.
. Tats article is an effort to gather together, in so far as they
relate. to the phenomena of fertilisation in the sea-urchin
ego, the results obtained by experiments. It does not pre-
tend to consider the problem of fertilisation as a whole, nor
the phenomena save in Hchinoderms, and no attempt will be
made to establish comparisons with other forms in which the
details may to some extent differ. ‘The limitation to one form
is in so far appropriate, that practically all the experiments
have been made on Hchinoderm eggs.
I have personally studied fertilisation in the egg of Echinus
esculentus—specially in sections,—and though I have
nothing fresh to add to the description of the facts, this
article may in a measure be considered a sequel to a paper
on maturation in the same form. In that paper my attention
was chiefly directed to the chromosomes, and I did not follow
out the results of observers in the experimental field, but
as some of the phenomena described are of interest in con-
nection with these results, I shall take the opportunity of
returning to them.
The two mitotic divisions characteristic of the maturation
phases, differ markedly from those which take place in the
segmentation phases. In many respects there is a close
resemblance to phenomena observed in the eges of ‘Toxo-
pneustes, which develop parthenogenetically under the
influence of magnesium chloride solution (Wilson, 1901),
VOL, 46, PART 3.—NEW SERIES. GG
480 THOMAS H. BRYCE.
On the dissolution of the nuclear membrane the site of the
germinal vesicle is occupied by a “kinoplasmic” mass,
derived either entirely from the nuclear network, or also
partly from protoplasm differentiated on the distribution of
the nuclear substance into it. In fixed material this area has
a fibrillar appearance. This may be the result of the fixing
reagents used, but in any case it indicates the accumulation
at this part of protoplasm which has undergone some change
in constitution physical or chemical. In the nuclear area,
out of this material, asters are formed, and ultimately the
first polar amphiaster. Besides the asters concerned in
the formation of the bipolar figure, there are secondary
asters, which seem to have only a temporary existence.
In some few cases multipolar figures were observed. No
structure recognisable as a centrosome or centiole was found
before the germinal vesicle broke down, and therefore the
centrosome was either derived from the nucleus, or formed
de novo in the nuclear area. ‘The astral radiations are
confined to a small and superficial part of the egg, and a very
unequal division results in the formation of the polar bodies.
When the two divisions are over all the radiations and the
remains of the kinoplasmic area disappear, the cytoplasm
assumes its alveolar structure throughout, the nucleus retires
from the surface, and no centrosome can be recognised
in relation to it.
On the breaking down of the germinal vesicle the greater
part of the nuclear material disappears as such, and not only
is a change in the constitution and distribution of the proto-
plasm to be recognised, but experiment proves that the egg
has undergone a physiological change of state. Whereas a
spermatozoon can neither fertilise an egg with the germinal
vesicle intact, nor a fragment without the nucleus (Delage,
1901), after the polar bodies are formed the egg becomes
capable of fertilisation.’ This cytoplasmic maturation, de-
pendent, probably (Delage, 1901), on the influence of the
! For evidence and theories regarding the influence of the stage of matura-
tion in Amphibian eggs see Bataillon (1901).
ARTIFICIAL PARTHENOGENESIS AND FERTILISATION. 481
nuclear sap set free from the germinal vesicle, ig accompanied
by the conversion of the large vesicular nucleus related to the
metabolic changes underlying the growth of the ovum, into a
small morphologically equivalent nucleus, possessing the same
number of chromosomes as the sperm nucleus. This has been
proved by their enumeration when the nuclei undergo in-
dependent transformation, and the number is one half that
found in the segmentation divisions.
The eggs thus matured remain, in the case of the sea-urchin,
for a considerable time quiescent within the ovary before they
are discharged—for the process of ripening in the ovary
is a gradual one. When discharged into sea water it seems
that, like the eggs of some other forms (O. Hertwig, 1893,
p- 239), after lying for many hours unchanged, the sea-
urchin eggs show spontaneously, karyokinetic transforma-
tion; for instance, R. Hertwig (1896) observed in eggs which
had been deposited prematurely during transport, analogous
changes to those produced by treatment with strychnine.
This phenomenon is one apparently of wide range. In an
interesting review entitled “ Giebt es bei Wirbeltieren Par-
thenogenesis’”’ (1900), Bounet, after examination of all the
literature up to that date, comes to the conclusion that,
according to our present knowledge, the phenomena in verte-
brates are due to degenerative divisions, and in meroblastic
eggs to fragmentations, and the alleged parthenogenetically
divided tubal, uterine or laid eggs, are either over-ripe, and
therefore badly fertilised, or are eges normally fertilised
with defective spermatozoa. In the light of the facts of
artificial parthenogenesis, it may be that this segmentation
in unfertilised eggs, at least in certain invertebrates, is an
effort in the direction of true parthenogenesis which is abor-
tive, the egg dying before the tardy process is accomplished.
In 1876 Greeff described parthenogenetic development in
Asterocanthion. The eggs were obtained from animals
early in the season, before the spermatozoa were mobile, and
the blastulee formed differed from those preduced in normal
fertilisation. O. Hertwig (1890) recorded some observations
482 THOMAS H. BRYCE.
on spontaneous parthenogenesis. In confirmation of Fol, he
found that eggs from fully-matured animals did not seement
spontaneously, but only after a considerable time underwent
changes considered pathological. The nucleus enlarged
more and more, and after ten to fifteen hours the eggs
died and fragmented. Only among hundreds of eggs here
and there one had divided into two. At Trieste, however,
in a season when the animals were late in maturing, and at a
time when males were rarely got, he observed in a limited
number of cases (in Asterias glacialis, and Astero
pecten) that after the polar mitosis had occurred, the nucleus
did not come to rest, but continued to divide. There resulted
an irregular division, but here and there a blastula was found
which had no vitelline membrane. Into the interesting
observations and suggestions regarding the failure of the
second polar body extrusion, and the union of two vesicular
nuclei in the egg, we cannot here enter. The main point
established was, that fully-matured eggs did not develop
parthenogenetically, but that in some few cases immature
eges did divide irregularly, and in a small number of cases
blastulee were formed. A number of observers have
described the occurrence of natural parthenogenesis in
Kehinoderms, and it is an open question; but apart from its
possible relation to immaturity of the ovum, the sources of
error in the matter of infection by spermatozoa are so many,
and the causes which artificially start parthenogenetic
development in certain cases are so slight, that all cases of
so-called “ natural parthenogenesis” are open to suspicion,
but even granting that it may occur, it is a matter of no great
moment in the question of “artificial parthenogenesis.” It
would be only additional evidence of the fact, that there is in
these forms a tendency to parthenogenetic development,
which, however, does not normally occur.
Mitotic division may be excited in unfertilised eggs in a
variety of ways.
Kirst, by increasing the degree of concentration of the
sea water (Morgan, Hunter), or by increasing the osmotic
ARTIFICIAL PARTHENOGENESIS AND FERTILISATION. 483
pressure in various other ways (Loeb). This may be
done by adding various inorganic salts to the sea water,
especially the salts of magnesium, potassium, sodium, and
calcium, in definite proportions (Loeb, Morgan, and others),
or by adding sugar or urea (Loeb). Other salts have also
given results, e.g. chloride of manganese (Delage). The
effect is produced when the eggs, after being left from
half an hour to two hours in the solution, are transferred to
pure sea water, and is due to the disturbance of the osmotic
pressure leading to loss of water by the egg, followed by
rehydration (Bataillon, Loeb, Giard, etc.), not to specific
chemical stimulation.
The nuclear activity may also be roused by other chemical
bodies, as strychnine (Hertwigs, Morgan), chloroform, ether,
alcohol, by lack of oxygen (Mathews), by very dilute hydro-
chloric acid (Loeb, Delage).
Further, purely physical agents may have the same effect—
heat (Mathews, Bataillon, Delage, Viguier), cold (Morgan,
Greeley), and, most important, agitation (Mathews). Mathews
had previously proved for Asterias, and Morgan for Arbacia
also, that shaking of unripe eggs caused them to form the
polar bodies—the shaking presumably causing dissolution of
the nuclear membrane. Ripe eggs of Asterias, but not of sea-
urchin, act in the same way, but only after they have lain
some time in water; after two hours larve begin to appear
on shaking; after four hours, hard shaking produces a large
proportion of larvae, and the mere transference of the eggs
by a pipette from one vessel to another is sufficient to form a
few larve. A few hours later the slight amount of shock
experienced in the transference of the eggs, causes a large
number to begin to develop, though they do not go beyond
the late segmentation stages. At this time shaking causes
all to develop, but none reach the blastula stage. Loeb and
Fischer have extended this observation to the Annelids,
Cheetopterus and Amphitrite.
The mitotic phenomena produced artificially are apt to be
irregular, and the division of the cell body is often unequal
4.84 THOMAS H. BRYCE.
when it occurs. Thus the nucleus may divide repeatedly
without division of the cytoplasm, and then the egg
may break into as many segments as there are nuclei
(Wilson and others). It is only in a relatively small pro-
portion of eggs that division is regular enough to permit of
development to the larval stage. Further, the eggs of the
same species behave capriciously to the same agents under
different conditions (temperature, etc.), and the eggs of closely
allied species seem to react differently to the same agent.
There is no reasonable doubt, however, that true artificial
parthenogenetic development has been demonstrated for the
Kchinoderms—sea-urchins and star-fish—and for at least two
Annelids, though the same amount of independent testimony
is not available for the latter.
Actual development to a larval stage has been obtained
only by certain of the agents enumerated above.
1. Increase of Osmotic Pressure-—The most successful
results (Loeb, 1902) are obtained at a temperature about
20° C., by the addition of the chlorides of potassium or sodium
to sea water, the optimum degree of concentration being
determined by experiment for each set of observations.!
After the eggs have remained in this for half an hour to two
hours, the optimum being again tested by experiment, they
are restored to normal sea water.
Sea-urchins (Loeb, Wilson, Giard, Prowazek, Delage,
Viguier, Hunter). Annelids: Chatopterus, Amphitrite,
Nereis (Loeb, Fischer).
2, Agitation. — Asterias (Mathews), Cheetopterus and
Amphitrite (Loeb, Fischer), but not sea-urchins, (Mathews,
Viguier).
3. Hlevation of Temperature.—Asterias during maturation
(Delage) ; not for ripe eggs (Greeley, Viguier).
4. Depression of Temperature.—Asterias (Greeley) ; not
sea-urchins (Viguier).
' Loeb (1902) uses a stock solution of 23 n. HCl, and adds this in different
proportions, 8, 10, 12, 14, 16, 18 e.em., to 100 ¢.c. of sea water in six
vessels to determine the best grade of concentration,
ARTIFICIAL PARTHENOGENESIS AND FERTILISATION. 485
5, Exposure to weak HCl in sea water, and subsequent
restoration to pure sea water. Asterias (Loeb, Delage).
6, Continuous exposure to a solution of a specific chemical
substance at the same osmotic pressure as normal sea water.
Potassium Chloride. Cheetopterus (Loeb).
Calcium Chloride. Amphitrite (Fischer).
The Ions of potassium and calcium are said to be specific
for these forms respectively.!
With regard to the influence of the state of maturation,
Delage gives results to show that in Asterias glacialis,
when the eggs are placed in sea water to which is added an
equal quantity of a solution of HCl, raising the molecular
concentrations of the mixture to 0°660, different results are
got according to the stage of maturation. Among the eggs
placed in the liquid before maturation, 20 per cent. of
blastule were got, at the appearance of the first polar
body 95 per cent., and after the appearance of the second
polar body 5 per cent., while none of the controls showed
any normal segmentation.
From all this it seems that changes in the osmotic
pressure between the egg and its surrounding medium, and
mechanical agitation, are the chief agents so far as yet
1 Delage (‘ Compt. Rendus de l’Acad. des Sciences,’ October 13th and 20th,
1902) announces that he has found an agent which is as certain and effective
as the spermatozoon, in producing development to advanced larval stages, in
Asterias. It is sea water aérated by carbonic acid gas, and at the same osmotic
pressure as ordinary sea water (or lower?). When the eggs, at what he
calls the “ critical stage ’—i.e. when the nuclear membrane of the germinal
vesicle is dissolved, up to the expulsion of the first polar body—are placed in
this, and after one hour transferred to pure sea water, practically all the eggs
develop. His view is, that the maturation is arrested temporarily, and on
restoration to pure sea water, the carbonic acid gas is quickly eliminated and
division proceeds ; but it is not partial, as in the polar mitoses, but complete,
and goes on to the formation of the normal larval forms. The result is not
obtained at a stage after the polar bodies are extruded and the ovum has again
come to rest, nor is it applicable in sea-urchin, in which the maturation is
over before the ova are shed. His theory as to the action of the gas is, that
it is a temporary poison which arrests maturation completely, and is quickly
removed afterwards without altering the characters of the protoplasm.
486 THOMAS H. BRYCE.
known, which tend to the production of artificial partheno-
genesis, but that in the case of the Annelids there is evidence
to show that certain Ions may have a specific effect.
According to Loeb (1902) the solutions must act, first, by
favouring the solution or dissolution of the nuclear mem-
brane; and second, by changing, in some sense, the physical
properties of the protoplasm (viscidity, etc.).
Mathews (1900), as a conclusion from his experiments on
Arbacia eggs, pointed out that the known methods of causing
liquefaction in protoplasm will induce karyokinesis in these
eggs, and also shows that loss of water has a liquefying
action.
Before considering further the bearing of the physiological
and physico-chemical conceptions regarding fertilisation, I
shall proceed to the morphological changes which have been
described in unfertilised eggs which undergo parthenogenetic
development.
R, Hertwig (1896) studied the changes in the egg after
treatment with strychnine. On the breaking down of the
nucleus, half spindles, and in a few cases whole spindles,
supposed to arise from the fan spindles, were formed. The
fan spindle fibres he regarded as derived from the achro-
matic network of the nucleus. The chromosomes derived
from the nucleoli became attached to the primary rays.
Later, protoplasmic rays also appeared, centering on the
focal point of the half spindle. At this central poimt, and
derived from the central parts of the rays, there appeared a
rounded body resembling in every way a centrosome, though
none such was to be found before the nucleus broke down.
The body was an ovocentrum, formed from the achromatic
portion of the nucleus, and, according to Hertwig, the
‘individualised centrosome is ultimately a derivative of the
nucleus—is, in fact, an achromatic nucleus.
Doflein (1897), contrariwise, examined the phenomena
of karyokinesis of the sperm nucleus in eggs which, after
fertilisation, had been treated by chloral solution after the
manner of the experiment of O. and R. Hertwig. The nuclei
ARTIFICIAL PARTHENOGENESIS AND FERTILISATION. 487
did not unite, but underwent independent transformation.
Doflein, like R. Hertwig, considered: the middle piece of the
spermatozoon as equivalent to the centrosome, and from the
experiments concluded, that from the centrosome a complete
spindle could form, and out of this, again, the achromatic
nuclear network. Thus, compared with Hertwig’s results, the
ripe sperm nucleus contains all the parts, even as the ripe
ege nucleus, which are necessary for a further development.
In Hertwig’s results we have evidence of a centrosome
arising from the nucleus de novo. Morgan, in 1896,
described the formation of artificial astrospheres in the
cytoplasm of the eggs of Arbacia treated by salt solutions,
and from his further observations published in 1899 and
1900 he decided, that in spite of certain differences these
artificial astrospheres corresponded to the normal spheres
which occur at the apices of the spindles in the sezmentation
stages; further (1900), that both artificialand normal spheres
are due to accumulation of a specific substance, and that the yolk
spheres are excluded from the substance of the astrospheres.
His view of the astral radiations is that they serve to
transport the chromosomes, but are not concerned in the
division of the cytoplasm.
Evidence of free formation of the centrosomes is found
also in the appearance of asters in the cytoplasm in various
forms, Hchinus among them, on the breaking down of the
germinal vesicle in maturation.
Boveri (1901) in essence accepted Hertwig’s definition of
the structure described by him as an ovocentrum, and _ its
origin apparently de novo. He argued that phylogenetically
the centrosome is an individualised cytocentrum, derived
from a centro-nucleus in which the centrosome or its
equivalent is not differentiated from the chromatin nucleus.
To the nucleus of the sea-urchin egg must necessarily
be attributed the properties of a centro-nucleus, with the
capacity of producing out of itself, under the action of
certain stimuli, individualised centrosomes, when such fail to
be supplied in the normal way in fertilisation. If even under
4.88 THOMAS H. BRYCE.
similar conditions, the sperm centrosome be present, the cyto-
centrum remains latent. The centrosome looked at in this
way, is not a specific cell organ in the sense that it must
consist of a specific chemical substance, but that parts of a
substance contained in the nucleus, undergoing certain
changes, and grouping themselves together, are organised
into a centrosome,
Thus the ovocentrum of the sea-urchin egg is not to be
considered an individualised centrosome, but an intra-
nuclear latent cytocentrum, and the nucleus is a centro-
nucleus. Thus the centrosome in such a case is not some-
thing strictly new, but arises by the transformation in a
definite manner of a cytocentrum already present. It isa
case not of new formation, but of “ reparation.” ‘ Gervisse
Centronuclei sind im stande unter bestimmten Bedingungen
Centrosomen zu reparieren,”
Morgan’s artificial astrospheres he did not admit to have
true centrosomes—the essential character of capacity for
division was not proved for them.
This brings me to Wilson’s very interesting and important
paper on the morphological phenomena in parthenogenetic
eggs.
The main results are that under the influence of the
magnesium chloride solution, not only are asters produced
de novo in connection with the nucleus, but also in the
cytoplasm. “Not only the asters connected with chromo-
somes (nuclear asters), but also the supernumerary asters
unconnected with nuclear matter (cytasters), may multiply
by division; the cytasters contain deeply staining central
granules indistinguishable from centrosomes, that divide
to form the centres of the daughter asters. ‘These asters
operate with greater or less energy as centres of cyto-
plasmic division. ‘Typical cytasters, often containing deeply
staining central granules resembling centrosomes, are formed
in the magnesium solution in enucleated egg fragments
produced by shaking the unfertilised eggs to pieces, and
these asters likewise may multiply by division, though
ARTIFICIAL PARTHENOGENESIS AND FERTILISATION. 459
no cytoplasmic cleavage takes place. The cleavage centro-
somes first make their appearance outside the nucleus, but
directly on the nuclear membrane, and the evidence renders
it nearly certain that they arise by the division of a single
primary egg centrosome that is formed de novo. All the
evidence goes to show that the cleavage centrosomes are of
the same general nature as the central bodies of the cytasters.”
Among many interesting details I will refer here only to
the changes described for eggs which underwent segmenta-
tion, and were capable of developing into swimming embryos,
because in certain particulars they are reminiscent of what
takes place in the formation of the first polar amphiaster.
I may summarise as follows :—(1) The first change that
occurs is a coarsening in the appearance of the protoplasm,
better marked in eggs treated by stronger solutions. (2) A
primary radiation appears centering on the nucleus, better
marked in eggs treated with weaker solutions. (3) A varying
number of secondary radiations appear in eggs especially
treated with stronger solutions. The extent of the primary
radiations is inversely in proportion to the number of the
secondary radiations. These latter appear as vague clear
spots in the cytoplasm, which gradually become surrounded
with radiations, and finally assume the form of asters. They
always appear in situ, and do not change their position till a
later period. (4) Coincident with the appearance of the
radiations there is a gradual growth of the nucleus. (5)
Round the nucleus appears a clear perinuclear zone of hyalo-
plasm. (6) The nuclear membrane fades out, and a vague
irregular clear space is left, to which the hyaline zone con-
tributes. (7) The rays then diminish, and, indeed, almost
disappear.
The eggs at this point were restored to pure sea water, and
after a pause the radiations reappear and advance centri-
fugally towards the periphery. In eggs capable of develop-
ment the principal rays are now focussed on two centres at
opposite poles of the nuclear area, which now forms a spindle
connecting the two asters. If the amphiaster is typical,
4.90 THOMAS H. BRYCE.
division proceeds as in normal fertilisation. If more than
two asters are formed from the nuclear area, multipolar figures
form, and irregular cleavage results. If there is only a single
radiation which does not resolve itself into a bipolar figure,
the egg never properly segments, but there are regularly
alternating phases of nuclear transformation.
Analysing the meaning of the phenomena, Wilson says, ““ We
may therefore state that the first general effect of the stimulus,
whether the magnesium solution or the spermatozoon, is to
arouse an activity of the cytoplasm, one result of which is the
establishment of a centripetal movement of the hyaloplasm
towards one or more points at which the hyaloplasm accumu-
lates.’ The rays in this view are the expression, in part at
any rate, of centripetal currents, and the substance flowing
in, is the hyaloplasm or interalveolar substance. The hyalo-
plasm spheres at the centres of the asters are local accumula-
tions of this hyaloplasm. In fixed material, studied in
sections, the radiations are fibrillar in appearance, and as
they stain much more deeply than the general network
the hyaloplasm in the rays must probabiy have undergone
some physical or chemical change. ‘he centrosome is a
well-defined body of considerable size and of spongy con-
sistence, composed of intensely staining granules, which often
give the centrosome the appearance of a minute nucleus
containing a chromatin reticulum. ‘The hyaloplasm spheres
in the living egg correspond to the centrosome, the clear area
round it, and the innermost darkly staining radiated zone of
the aster taken together.
hus Wilson has proved that structures which cannot be
distinguished morphologically from “true centrosomes”
appear in the cytoplasm de novo; and further, that they
divide to form the apices of bipolar figures, even in enucleated
fragments.
In a recent paper Meves (1902), using Boveri’s nomen-
clature, expresses the view that the centrosome is only the
mantle of the centriole, and is only present in rapidly-dividing
cells like the blastomeres. The ‘ Doppelkérchen” of the
ARTIFICIAL PARTHENOGENESIS AND FERTILISATION. 491
tissue-cells are to be considered as centrioles, and ‘‘ nur von
den Centriolen nicht aber von den Centrosomen, kann daher
gelten, dass sie allgemeine und dauernde Zellorgane sind.”
The results of Morgan and Wilson can only then be held to
prove that centrioles under certain conditions may, by the
action of salt solutions, be excited to form centrosomes and
radiations round them, for their results might be explained by
a multiplication of the two centrioles which the egg has
derived from the last division of the division period, and the
distribution of these centrioles through the cell. Even in
enucleated fragments there is no proof that the fragment did
not contain the centriole of the cell.
Such a supposition admits of neither proof nor disproof,
and the presence of a free “centriole” in the unfertilised
sea-urchin ege@ has not been demonstrated. I have seen in
young oocytes minute bodies, stained black with iron
hematoxylin—sometimes double bodies,—but I have not been
able to convince myself that they are more than accidents of
staining and fixing.
Turning now to the phenomena of fertilisation in the sea-
urchin, there is to be recognised (1) a local stimulation at
the place of contact of the chosen spermatozoon,! manifested
by the streaming out of the protoplasm to form the entrance
cone. (2) A general stimulation, manifested by the throw-
ing off of the vitelline membrane, and by a change in the
constitution of the protoplasm. It becomes more viscid for a
time (Morgan) ; a funnel-shaped area of darkly staining sub-
stance follows the path of the sperm head (Wilson). (3)
A protoplasmic movement focussed on the situation of the
middle piece giving rise to the sperm aster. This appears
soon after the entrance of the spermatozoon, when the head
has begun a movement of rotation. The rotation goes on
Buller (1902) has studied the question of the bearing of chemotaxis on
fertilisation in Echinoderms. His conclusion is that chemotaxis plays no
réle in bringing the sexual elements together. The meeting is a matter of
chance. The passage through the gelatinous coat is radial in direction, and
probably purely mechanical, though possibly due to stereotaxis.
4,92 THOMAS H. BRYCE.
through 180° till the base of the conical sperm head is
directed inwards. The rays of the aster now extend widely,
and at their centre is a clear area. Meantime the sperm head
becomes converted into a small round nucleus. The move-
ment of the sperm head is, at first, radial; then there is a
change, and it assumes a new direction towards a point not
quite in the centre of the egg; when this change of path is.
taken up the egg nucleus begins to move towards the point
where the nuclei ultimately meet (Wilson and Giardina).
The aster now comes in contact with the egg nucleus, and
as the nuclei approach, the clear area at its centre spreads
out over its side. ‘The aster then divides and the nuclei
conjugate. The radiations now die down during a pause in
which the nucleus grows in size (Wilson), to redevelop again
focussed at the poles of the nucleus.
According to Hertwig, Doflein, Erlanger, and Wilson’s
earlier account, the centrosome corresponds to the whole
middle piece, but later Wilson described the middle piece as
cast aside, and in the centre of the aster is a small darkly-
staining granule. Boveri (1901) represents the sperm
centrosome as a spherical body smaller than the middle piece,
and containing within it two centrioles shortly after its
entrance into the ege.
Various other observers have represented a dark-staining
eranule at the centre of the aster. My own observations
are inconclusive, and do not warrant me in expressing an
opinion.!
1 The character of the fully-formed centrosome in the sea-urechin egg is
still subject to difference of opinion. The form in which I see it in osmic
acid material is that of a largish sphere of very finely alveolar structure. In
Wilson’s papers on magnesium and etherised eggs, “it appears as a well-
defined body of considerable size, consisting of intensely stained granules,
which often give the centrosome exactly the appearance of a minute nucleus
containing a chromatin network.” This becomes in the anaphases more homo-
geneous, and flattens down into a plate-form, which in the telophases often
lies directly on the membrane of the newly-formed nucleus precisely as Boveri
(1901) has described for Echinus. Boveri (1901) represents it in several
forms. In one set of preparations it is a largish sphere of very finely alveolar
ARTIFICIAL PARTHENOGENESIS AND FERTILISATION. 4985
The essential difference between the processes seen in
magnesium eggs and normal fertilisation is that whereas in
fertilisation there is only one, and that a definitely localised
point of astral activity, in the magnesium eggs there are a
number of foci, and development in large measure depends
on the accident of their number in the nuclear area.
There is the same want of unity of purpose that is seen in
polyspermic eggs, in which the number of points of astral
activity depends on the number of spermatozoa which gain
an entrance.
It has lone been recognised that the union of the nuclei
and the initiation of division are co-ordinated, but in a
measure independent factors in fertilisation. Partheno-
genetic development under artificial agents is the iatest
proof of this. The possibility of the development of
enucleated ego fragments when entered by a spermatozoon,
as described by Boveri, and afterwards named merogony by
Delage, is another. Hither nucleus is sufficient in itself.
With the problems underlying the nuclear conjugation
this article is not concerned. It starts from the assumption
that the union of equivalent nuclei is the end of fertilisation,
but not the means (Boveri).
The cause of the nuclear conjugation is not as yet under-
stood. The first possibility is that the aster is concerned in
bringing them together. Giardina (October, 1902) brings
the latest suggestion on this line. Starting from the basis
of the alveolar structure of protoplasm, he suggests that the
aster is the expression of both centripetal and centrifugal
currents. The centrosome is concerned in the diffusion of
chemotrophic substances into the egg, while at the same time
structure. In another set, in which the centrosome had reacted differently,
there is a centriole within the centrosome, which divides before the centrosome,
so that it is double in the metaphase. In Wilson’s earlier account there was
no central body, but in later descriptions there was a mass of granules in a
well-defined sphere, which succeeded a single granule of earlier stages. In
my previous paper, I regret that I misrepresented Professor Wilson’s nomen-
clature by referring to this as his centrosome. The sphere, as a whole, is
named the centrosome. See note to page 314, “ The cell, ete.,” 1900,
4.94, THOMAS H. BRYCE.
the hyaloplasm flows in towards the centre. He points out
that the germ nucleus does not move till the rays of the aster
have reached it, and the aster has assumed a position of
equilibrium towards the centre of the egg. The union is
thus the result of the chemotactic forces of which the aster
is the expression.
Wilson (1901 8) shows, however, that the nuclei may
unite in the entire absence of an aster. When eggs, im-
mediately after fertilisation, are placed in a weak solution of
chloral (O. and R. Hertwig), or ether (Wilson), no aster is
developed, but when replaced in sea water the rays reappear
and the nuclei unite. In a certain proportion of cases,
which will be referred to later, the nuclei remain apart and
undergo independent transformation ; but in some instances,
also while the eggs are still in ether, the nuclei enlarge, and
later conjugate in the entire absence of an aster, This
happens, however, only when the spermatozoon has entered
at a point not too far from the egg nucleus. Giardia holds
that this fact, and the other—that the nuclei quickly unite
whenever the eggs are put in pure sea water, and the aster
develops,—makes Wilson’s observation insufficient to exclude
his hypothesis. Other explanations, such as mass attraction
and direct chemical attraction, both observers reject.
Wilson thinks the latter improbable. Again, the idea of
protoplasmic currents such as suggested by Butschli,
Erlanger, and Conklin, is not proved by actual evidence in
normal conditions in the sea-urchin egg (Wilson). The
changes of shape of the germ nucleus might suggest
amceboid movement on its part; but, again, this does not
apply to the sperm nucleus, which travels through a longer
path (Wilson). The changes in form might be due to the
exercise of chemotactic forces on the nucleus (Giardina).
The phenomenon described by Boveri (1888) under the
name of “ Partial Fertilisation,” has recently been worked
out in detail in Boveri’s fixed preparations by Teichmann
(1902). The method by which the results were obtained
was that eggs which had lain fourteen hours in unrenewed
ARTIFICIAL PARTHENOGENESIS AND FERTILISATION, 495
sea water were fertilised with spermatozoa, which were
treated with a ‘05 per cent. solution of potassium hydrate
until only a few were mobile. While polyspermy occurred in
more than half the eggs, the remainder were fertilised by a
single spermatozoon. In these cases, however, the sperm
nucleus did not unite with the germ nucleus, but the aster
became detached from it, and advanced alone to the germ
nucleus, a bipolar figure was formed and division proceeded.
The sperm nucleus took no share in the process, but passed un-
altered into one of the blastomeres. later, however, either in
the two- or the four-cell space, it broke up into its chromosomes,
which entered into the equatorial plate of the cell in which it
was included, and which now divided like its neighbours.
Such eggs were capable of developing to the blastula stage.
The question presented itself: Was this aster and the
amphiaster the result of the activity of an ovocentrum, or
were they the product of the sperm aster?
_ In monospermic eggs Teichmann found the early stages
very scarce, and, though very suggestive, too few for absolute
proof, and the phenomena seen in dyspermic eggs are
described to fill the gap. It may be admitted that in these
eges, in spite of the apparent inactivity of the sperm nucleus,
the sperm aster with its centrum is the operative factor in
starting the developmental process. The appearances are
very similar to those in the etherised eggs described by
Wilson (1901). In that form, as in Hchinus, the nuclei con-
jugate when they are very unequal in size, and before the
division of the aster. In Asterias and other forms an amphi-
aster is developed before conjugation, and the nuclei are
nearly equal in size. In the experiments the union was
delayed, as in “ partial fertilisation,” and the amphiaster was
formed before the conjugation.
Among 'l'eichmann’s observations I shall refer only to those
of monospermic eggs. The main feature is the detachment
of the sperm aster from its nucleus, its application to the egg
nucleus, and its normal division, followed by normal segmen-
tation. The fate of the sperm nucleus depends on the
vou. 46, PART 3.—NEW SERIES. HH
4.96 THOMAS H. BRYCE.
position it assumes in the egg, relative to the cleavage
plane. If it lies outside the equatorial plate of the spindle
it passes unchanged into one of the blastomeres; if it
lies within the field of the first spindle, it does not
actually unite with the chromatin of the female nucleus, but
its chromatin undergoes a marked relaxation. Though it
shows a marked resistance to the tractive forces, it is drawn
out and torn into several shreds. It thus passes undivided
into one of the blastomeres, and no chromatin elements
derived from it are found at the poles of the spindle. In
several cases where the nucleus lay exactly at the equator,
and the traction of the poles was nearly equal, it was
observed that the chromatin mass was much broken up, and
was torn into two parts. The cleavage of the cell body may
have helped to complete the division. The loosening of the
sperm chromatin mass in the first spindle seems to have
broken its power of resistance, for when the next division is
initiated, the two nuclei lying side by side in one of the
blastomeres unite in the equatorial plate stage, and the
chromatin of both is equally distributed in the next division.
The number of chromosomes is now different in the blasto-
meres, sometimes double, perhaps quadruple in some cases
(though an accurate count was not possible), as if the chromo-
somes of the sperm nucleus emerged in double number,
though the first division was suppressed.
The sperm nucleus in the case where it has not lain within
the power of the spindle in the first division, may now, in
analogous fashion, be caught in the second division, to unite
later with the chromatin of one of the blastomeres of the
four-cell stage, or it may even pass over into the eight-cell
stage, as seen in living eggs by Boveri.
Teichmann concludes that the radiations are derived from
the sperm centrosome as their starting point, and on the
supposition that the centrosome is introduced by the sperma-
tozoon into the egg, that it has suffered less from the chemi-
cal reagent than the nucleus. The centrosome behaves as
in ordinary fertilisation, the sperm nucleus is passive, and
ARTIFICIAL PARTHENOGENESIS AND FERTILISATION. 497
seems to form no hindrance to the normal processes in the
ego, and it seems to be of no significance, whether it enters
earlier or later into union with one of the descendants of the
ego nucleus. The difference between the phenomena of
“partial fertilisation’ and the normal process is the non-
union of the nuclei. In certain cases Boveri (1890)
described an independent transformation of the nuclei
under normal conditions, but the elements from both entered
into the equatorial plate of the first cleavage spindle, and
normal division took place. The mere want of union is not
of moment, if the sperm nucleus hes near enough the germ
nucleus to be influenced by the nuclear fluid of the egg
nucleus. ‘lhe absence of this hastening factor may explain
the fact that the karyokinesis of the sperm nucleus in
enucleated fragments is much slower than in the cleavage
spindle. But such an explanation alone will not hold for
cases of dyspermy in these experiments, where the sperm
nuclear descendants remain far behind the derivates of the
ego nucleus, and it must be assumed that a change has taken
place in the sperm nucleus itself, a kind of paresis, produced
by the potassium hydrate. This holds for the monospermic
egos also, and explains why, even in spite of its position, the
nucleus does not enter into union.
Another factor is a change in the egg, defined as an over-
ripeness. In many cases the germ nucleus is a stage ahead,
compared with the normal process, of the centrosome. When
the centrosome met the egg nucleus, the latter must already
have been in a way prepared for division, and this great
readiness to enter into division may be part explanation of
the lagging behind of the sperm nucleus. ‘here has not
been time for the sperm nucleus to undergo transformation
before the egg nucleus has submitted to division. Teichmann
does not explain in what the over-ripeness consists. It may
perhaps be that, since the eggs had lain fourteen hours in
unrenewed sea water, the early preparatory stages of the
natural transformation had supervened, which takes place in
eggs after lying long in sea water.
2)
498 THOMAS H. BRYCE.
Regarding the main point, it may be admitted that the
aster and its centrosome here concerned is that belonging to
the sperm nucleus, and that in its behaviour we have a
beautiful demonstration of the independence of the two
factors in fertilisation, or, in other words, of the two fune-
tions of the spermatozoon, and that the two functions have
been disturbed in unequal degree. At the same time the
egg protoplasm, after the fourteen hours’ sojourn in
unrenewed sea water, was approaching to that stage in
which it acquires spontaneously the tendency to develop
astral activities, and it might be held that the conditions are
the same as In magnesium eggs in which, as a result of a
general stimulation, asters and centresomes which are
unconnected with the nucleus appear de novo in the
cytoplasm. While the results are of interest in connection
with the apparent independence of the factors in fertilisation,
they also show how they are co-ordinated together. The
sperm nucleus becomes dissociated from the aster, and fails of
union, because it has not undergone the transformation which
properly corresponds to the phase reached in the cycle of the
centrosomal changes. Further, while the nuclei may be re-
solved into chromosomes before union, and yet unite in the
equatorial plate stage, a certain stage in the transformation
of the dense mass of chromatin of the sperm head into
a nucleus with distinct chromatin network, must be reached
before union can take place. ‘This seems to show that several
co-ordinated factors are at work in the nuclear conjugation.
Further insight into the behaviour of the factors in
fertilisation is given by an experiment described by Ziegler
(1898). This consisted in carrying newly-fertilised eggs by
a gentle current of water in his compressorium against
threads of cotton wool. The egg was caught on a thread
and nearly cut through, leaving only a slender bridge of
protoplasm between the two portions of the egg. ‘The one
contained the sperm nucleus, the other the germ nucleus.
While the sperm nucleus regularly divided, followed by
division of the cytoplasm, the egg nucleus merely underwent
ARTIFICIAL PARTHENOGENESIS AND FERTILISATION. 499
alternate changes of disintegration and reconstruction with-
out division of the cytoplasm. Radiations appeared and dis-
appeared, and after three cycles, owing to the segmentation
of the portions containing the sperm nucleus, the ege-nuclear
portion became detached and disintegrated. In another
experiment the egg-nuclear portion underwent changes of
form suggesting abortive attempts at cleavage. The mitotic
transformation of the egg nucleus was not synchronous with
that of the sperm nucleus, but always a little behind.
These observations show that under the conditions of the
experiments the egg nucleus is excited to division without
direct contact with the sperm nucleus cr aster, but that the
mitotic phenomena are ineffective to produce cytoplasmic
cleavage. Ziegler refers this to the general stimulation of
the egg by the spermatozoon, manifested also by the throw-
ing off of the vitelline membrane. Boveri has shown that
the same phenomena occur in egg fragments produced by
shaking some minutes after fertilisation, and he (1902) refers
to cases of this kind in which he has observed divisions of
the nucleus followed by cell cleavage. The division was
repeated a second time, and thus the four-cell stage was
reached, but development then ceased. Another example of
the effect of this general stimulation is to be seen (Boveri,
1902) in the cases in which the egg is incited to throw off the
polar bodies by the entrance of the spermatozoon.
This brings me to a further reference to Wilson’s observa-
tions on etherised eggs (1901 B). As has already been said,
under this treatment the sperm and germ nuclei remain
apart and undergo independently karyokinetic transforma-
tion. ‘The most striking fact is that, while the sperm aster
often gives rise to a perfect and symmetrical bipolar figure, the
ego nucleus in a great number of cases produces a monaster,
which seems at first incapable of resolving itself into a
bipolar figure.” In typical cases the ege nucleus gives rise
to a monaster such as described by Hertwig (’96), and such
as occurs In magnesium eggs. While the egg monaster does
not at first give rise to a dicentric figure, it does so later, as
500 THOMAS H. BRYCE.
may be gathered from the description of an egg continuously
observed in the living state. At the height of its develop-
ment the egg monaster lay at one side, the sperm amphiaster
at the other, and no spindle was formed between them. The
ege divided into three cells, two larger and somewhat
irregular containing two daughter sperm nuclei, and a small
one in which the single egg nucleus re-formed. At the second
division each of the sperm nuclei gave rise to a perfect amphi-
aster, and divided into two, the accompanying cytoplasmic
division resulting in the formation of two complete cells and
one binucleate cell. The single egg nucleus gave rise to a
tetraster, and divided into three cells, one binucleate, the
nuclei of the latter quickly fusing together. The embryo
now consisted of six cells—three containing maternal, three
paternal nuclei. At the ensuing division fifteen cells were
formed, of which eight larger ones contained paternal nuclei,
while seven much smaller ones containing maternal nuclei lay
in a definite group at one side. The egg observed afterwards
died. Wilson has not seen an egg monaster become dicen-
tric at the first division, but the above observations prove
that it may operate as an effective division centre, without
establishing a spindle connection with either of the sperm
asters, and that it may divide later. A centrosome was
demonstrated in the monaster, in the same form as in the
sperm aster, and as in magnesium eggs. The possible action
of the chemical as the exciting agent of the karyokinetic
transformation was excluded by control experiments, and it
was therefore concluded, that it was due to a stimulus effected
by the spermatozoon, as in Ziegler’s experiment. These
observations, added to the results obtained in the magnesium
eggs, “demonstrate that under appropriate stimulus the egg
inay give rise to a centrosome capable of progressive division,
but the etherised eggs show in the clearest manner that this
centrosome is less effective than the sperm centro-
some.”
I shall not venture on the general problem of the asters and
centrosomes. It will suffice for the present purpose if it be
ARTIFICIAL PARTHENOGENESIS AND FERTILISATION. 501
admitted that, without prejudice to the question either of the
individuality, or the persistence of the centrosome, the body
and its aster represent a kinetic phase of protoplasm, which
reveals itself in cycles of activity, and that the centrosomes
and asters constitute together, in some sense, a divisional
apparatus, though that term is not used in any definite
mechanical sense.
The egg both before and after maturation lacks the power,
for and by itself, to produce in normal circumstances such a
divisional apparatus as will regularly and equally divide the
cell.
In Ziegler’s and Boveri’s experiments on separated portions
of the egg containing only the ege nucleus, a divisional
apparatus is called up under the general stimulation of the
spermatozoon ; but it is ineffective, or only very partially
effective. In the etherised eggs it is slow in appearing, and
less effective than that associated with the sperm nucleus.
In magnesium eggs the effect of the disturbance of equilibrium
is to cause a change of state in the protoplasm which results
in the differentiation at many foci of kinetic centres, and it is
only in the cases where a single such centre, which divides
into two, appears in the nuclear area, or at most two centres,
that normal division proceeds.
In fertilisation there is only one kinetic centre, and this is
localised on the middle piece of the spermatozoon. Its
activities are rapidly unfolded, and dominate all the other
latent astral activities of the egg. ‘The latent capacity of
both nucleus and cytoplasm to give rise to centrosomes is in
this case wholly inhibited.”’! By union of the nuclei its
activity is transferred to the cleavage nucleus, and “ becomes
a part of an activity on the part of the ege nucleus that
would have ensued even had the germ nuclei not united.”!
Thus it may be said that the spermatozoon supplies the
lack in the egg, by providing a powerful and effective
“divisional apparatus.” How is this effected? Does the
spermatozoon act by giving a general or diffused stimulus
1 Wilson, 1901 a, pp. 581, 582.
502 THOMAS H. BRYCKH.
to the ege, or by disturbing the general equilibriam in some
such way as the loss of water does, when the normal osmotic
relations are disturbed ? Or does the spermatozoon carry into
the egg some specific chemical substance which produces a
local differentiation, of which the centrosome and the aster
are the expression? Or does it import “ a highly active centro-
some or centroplasm about which the cytoplasmic energy is
brought to a focus?”
Boveri (1902) holds that a general stimulation of the egg,
with the sperm head as the point of predilection for the
formation of the aster, as in magnesium eges the ege nucleus
is the point of predilection, is insufficient as an explanation.
There is much rather something special present in the sperma-
tozoon, which determines that the aster shall appear at that
point, and that poimt only ; and thus he thinks that still the
appearances may best be described as being due to the intro-
duction of a centrosome. Hven admitting—which, as has just
been indicated, he does not—that the spermatozoon acts like
Loeb’s agents, and in view of the demonstration by Morgan
and Wilson that their effect is to cause the egg to produce
centrosomes de novo, only a modification of secondary
importance would be required in his theory of fertilisation,
viz., that instead of saying that the spermatozoon brings a
centrosome into the egg, it would be necessary to say that it
causes the formation of a centrosome in the egg, from the
division of which the rest follows.
Taking the sperm aster as the manifestation of activities
produced by the spermatozoon, and looking to its sharp
localisation on the site of the middle piece, it seems reason-
able to suppose that the localised excitement is the effect of an
agent operative in fertilisation, and that it is probably related
to the middle piece; but the actual continuity between the
centrosome of the spermatozoon and that in the aster has not
been absolutely demonstrated, and the new facts in regard to
the centrosome put the matter in another light. Thus it
remains for the future to decide which of the two latter
alternatives stated above shall be adopted, and perhaps after
ARTIFICIAL PARTHENOGENESIS AND FERTILISATION. 505
all there is only a formal difference, for the fundamental
problem is the same when the question is raised how the
centrosome exercises its activities.
Loeb (1901), on the physico-chemical side, suggests that a
catalytic substance is carried by the spermatozoon into the
egg, that is one which accelerates physical or chemical
processes which would occur without it. The K ions act as
catalysers, and the loss of water acts also, though less
directly, in the same way, and it may be that it gives rise to
substances which act catalytically. Inasmuch as in Cheetop-
terus the normal development does not show the character-
istics of a treatment of the eggs by K, it is probable that
normal fertilisation is not brought about by K ions.
Delage (1901) considers that the egg is in an unstable
state of equilibrium, which is readily upset by various
agencies—loss of water, heat, etc., and he lays some weight
on the specific action of the salts. He finds that the
chloride of manganese has, for Asterias, a specific action
superior to that of the alkaline salts. ‘Together with his son,
he showed that in the case of the sea-urchin there was less
magnesium chloride in the sperm than in the eggs, by about
1 per cent., so that this salt could not have a specific action.
Among other possible factors in the action of the sperma-
tozoon, he gives prominence to its abstraction of water from
the cytoplasm, During maturation the nuclear sap is shed
into the cytoplasm; until this is effected by the solution of
the nuclear membrane, fertilisation is not possible ; it is just
at this “critical stage” in Asterias that he finds artificial
parthenogenesis most hable to occur. In the specialisation
of the sexual elements, the egg thus becomes rich, while the
spermatozoon has become poor, in water. After the sperm
head has entered the ovum it increases in size by abstraction
of fluid from the egg protoplasm, and this abstraction of
water by the sperm nucleus has to be reckoned with as a
possible factor in fertilisation.
Apart from the large assumptions involved in such an
hypothesis, the facts of ‘‘ partial fertilisation, and the local-
VOL. 46, PART 3.—NEW SERIES, I]
504. THOMAS H. BRYCE.
isation of the aster on the middle piece, are in opposition
to it.”
It has been suggested that the centrosome is the seat of
formation of a ferment. Mathews (1901), from the results of
his experiments on the eggs of Arbacia, believes that “ what-
ever the details of the process may prove to be, the essential
basis of karyokinetic cell division is the production of localised
areas of liquefaction in the protoplasm.” “The centrosome
might be a liquefying enzyme.”
Experiments on this line have been tried, but without
definite result. Pieri’s results (1899), from which he sup-
posed he had obtained a ferment ‘ ovulase,” have not been
confirmed. Dubois (1900) showed that there was no
question of a ferment being obtained by Pieri’s methods.
He made various experiments on sperm and eggs, from
which he concluded that there was evidence of the existence
of a “ zymase,” which he provisionally named ‘ Spermase,”
in the spermatozoa, and in the egg a substance, at least
modifiable by ‘‘spermase,” provisionally named “ Ovulase.”
Spermase cannot enter the egg by diffusion or osmosis, but
only by a mechanical means, which is the raison d’étre of
the spermatozoon. Winkler’s experiments (1900) are also
inconclusive. He used sperm shaken for half an hour in
distilled water and filtered five or six times through three-
fold filter-paper. ‘The filtrate was added to sea water, the
precaution being taken of keeping the mixture at the same
degree of concentration as the sea water. While the sperm
in heated sea water produced no results, the liquid caused in
the case of Spherechinus and Arbacia eggs, though in a rela-
tively small number, the beginnings of segmentation. ‘These
results may have been due to osmotic influences,
Loeb (1900) states that up to that date he had found no
enzyme save papain which had an effect in causing the egg
to segment, and it was uncertain whether this was not due
to some accidental constituent of the enzyme preparation used.
Gies (1901) made a complete study of the effects of extracts
of sperm made by the ordinary methods for the preparation of
ARTIFICIAL PARTHENOGENESIS AND FERTILISATION. 505
enzyme solutions. His results were wholly negative, and he
concluded that, used in certain proportions and under certain
conditions at any rate, such extracts did not possess any
power of causing proliferation of the ripe ovum. No
evidence could be furnished of the existence of a zymogen
in spermatozoa. Extracts of fertilised eggs in the earlier
stages of development seemed likewise devoid of any seg-
mental activity. ‘he results, Gies adds, do not, however,
certainly show that enzyme action is impossible, after, or at
the time of union of the spermatozoon with the ovum, within
the latter.
The same negative result was got this spring by R. T.
Lieper at Millport Marine Biological Station, using an
extract of sperm prepared by spreading fresh sperm on
sheets of glass, then drying in air and sun, and afterwards
triturating the dried extract in sterilised sea water. The
filtrate from this fluid produced no segmentations, though
control experiments with eggs from the same ovaries
normally fertilised, nearly all developed.
LITERATURE.
Artota.— Atti Soc. Ligrest. Sc. Nat. e Geogn.,’ ann. xii, fase. 3.
Bataruton, 1900.—‘ Compt. Rendus de |’Acad. des Sciences,’ t. exxxi, p. 115
BararLton, 1901.—‘ Compt. Rendus de l’Acad. des Sciences,’ t. exxxii, pp
852, 1134.
BatarLion, 1902.—‘ Compt. Rendus de l’Acad. des Sciences,’ t. exxxiv, p.918
Barattton.— Archiv fiir Entwickelungsmechanik,’ t. xi, Heft 1.
Batattton.— Archiv fiir Entwickelungsmechanik,’ t. xii, Heft. 4.
Bonnet, 1900.—Merkel und Bonnet- Ergebnisse u. s. w.,’ 1900,
Bovert, 1888.—‘ Sitzungsber. des Ges. f. Morph. und Physiol. in Miinchen,
Bd. iv, Heft 2. ;
Bovert, 1901.—‘ Zellen Studien,’ Heft 4, Jena, 1901.
Bovert, 1902.—* Das Problem der Befruchitung,’ Jena, 1902.
Bryce, 1902.—‘ Quart. Journ. Mier. Sci.,’ vol. xlvi.
Buuier, A. R., 1902.—‘ Quart. Journ. Mier. Sci.,’ vol. xlvi.
506 THOMAS H. BRYCE.
DetaGe, 1899.—* Archiv. de Zool. Expér.,’ t. vii.
Decace, 1901.—‘ Rev. Gen. des Sciences,’ 12 année, No. 19.
Dexace, 1901.—‘ Archiv. de Zool. Expér.,’ t. ix, Nos. 2, 3.
DorteEin, 1897.—‘ Arch. f. mikroskopische Anat.,’ Bd. 1.
Dusors, 1900.—‘ C. R. Soe. Biol.,’ Paris, vol. lii, p. 197.
Fiscner, 1902.—-‘ Amer. Journ. of Physiology,’ June 2nd, 1902.
Granrp, 1900.—‘C. R. Soe. Biol.,’ Paris, vols. lii, liii.
Giarpina, 1902, a.—‘ Anat. Anzeiger,’ Bd. xxi, No. 20.
Grarpina, 1902, d.—‘ Anat. Anzeiger,’ Bd. xxii, Nos. 22, 33.
Giss, W. J., 1902.—* Amer. Journ. of Physiol.,’ vol. vi.
Greerr, It., 1876.—* Sitzungsber. der Ges. zur Beford. der gesammten Natur-
wiss. zu Marburg,’ No. 5, quoted from O. Hertwig, 1890.
Greevey, A. W., 1901.—‘ Amer. Journ. of Physiology,’ vol. vi, p. 296.
Hertwic, O. and R., 1887.—*‘ Jenaische Zeitschrift fiir Naturwissenschalt,’
BileexXe
Hertwie, O., 1890.—‘ Jenaische Zeitschrift fiir Naturwissenschaft,’ Bd. xxiv
Hentwie, O., 1893 —*‘ Die Zelle und die Gewebe.’
Hertrwie, R., 1896.—* Uber die Entwick. des unbefurchteten Seeigeleies
Leipzig, 1896.
Hunter, 8. J., 1901.—‘ Amer. Journ. of Physiology,’ vol. vi, p. 177.
Lorp, J., 1900.—‘ Amer. Journ. of Physiology,’ vol. ili, No. 9.
Lorn, J., 1900.—‘ Amer. Journ. of Physiology,’ vol. iv, p. 178.
Lors, J., 1901.—‘ Amer. Journ. of Physiology,’ vol. iv, p. 423.
Logs, J., 1902.—* Archiv f. Entwickelungsmechanik,’ vol. xiii, Heft 4.
Lorx, J., 1902.—‘ Archiv f. Entwickelungsmechanik,’ vol. xiv, Heft 4.
Maas, O., 1901.—‘ Sitzungsber. d. Ges. f. Morph. und Physiol. in Miinehen,’
Bd. xvii.
Matuews, A. P., 1900.—‘ Amer. Journ. of Physiology,’ vol. iv, p. 3438.
Matuews, A. P., 1901.—‘ Amer. Journ. of Physiology,’ vol. vi, p. 142.
Marturws, A. P., 1901.—‘ Amer. Journ. of Physiology,’ vol. vi, p. 216.
Meves, I’., 1902.—‘ Verhand. des Anat. Ges. Halle,’ 1902, p. 132.
Morean, I’. H., 1896.—*‘ Arch. f. Mntwickelungsmechanik,’ Bd. iii Heft 3.
Moreay, I’. H., 1899.—* Arch. f. Entwickelungsmechanik,’ Bd. viii, Heft 3.
Moreay, I’. H., 1900.—‘ Arch. f. Eutwickelungsmechanik,’ Bd. xii, Heft 2.
Norman, W. W., 1896.—‘ Arch. f. Entwickelungsmechanik,’ Bd. iii, Heft 1.
Prent, J. B., 1899.—* Archiv. de Zool. Expér., Notes et Revue,’ vii, 3.
Prowazek, S., 1900.—*‘ Zool. Anzeiger,’ No. 618, p. 358.
ARTIFICIAL PARTHENOGENESIS AND FERTILISATION. 507
TrIcuManN, Ernst, 1902.—‘ Jen. Zeitschrift fiir Naturwiss.,’ n. F., Bd. xxx,
Heft 1.
VicurER, 1900.—‘ Compt. Rendus Acad. Sci.,’ Paris, t. exxxi, p. 118.
Vicuier, 1901.—‘ Compt. Rendus Acad. Sci.,’ Paris, t. exxxii, p. 1436.
VicuIER, 1902.—‘ Compt. Rendus Acad. Sci.,’ Paris, t. exxxv, p. 60.
VieurEr, 1902.—‘ Compt. Rendus Acad. Sci.,’ Paris, t. exxxv, p. 197.
Wepexinp, 1901.—‘ Ber. tib. der Verhand. d. 5 internat. Zool. Congress,’
Berlin, 1901.
Wutson, Ep. B., 1901.—‘ Arch. f. Entwickelungsmechanik,’ Bd. xii, Heft 4.
Witson, Ep. B., 1901.—-‘ Arch. f. Entwickelungsmechanik,’ Bd. xiii, Heft 1.
Wink Ler, Hans, 1900.—‘ Nachr. K. Ges. Wiss. Gottingen, Math. Phys. K1.,’
1900, Heft 2, p. 187.
ZiEGLER, H. E., 1898.—‘ Arch. f. Entwickelungsmechanik,’ Bd. vi, Heft 2.
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CONTENTS OF No. 184.—New Series.
MEMOIRS:
The Movements and Reactions of Fresh-water Planarians: a Study
in Animal Behaviour. By Raymonp Peart, Ph.D., Instructor in
Spee in the University of eae Ann Avnae Michigan,
U.S.A. : : ; : , : ‘
On the Dividers IV. On the Central Complex of Cephalo-
discus dodecalophus, Mcl. By A. T. Mastrerman, M.A,,
DSc; Lectur er on Zoology, School of Medicine, Edinburgh. (With
Plates’ 32, 33) : : : _ : : :
On Hypurgon Skeati, a New Genus and Species of Compound
_Ascidians. By Icerna B. J. Sorzas, B.Se.Lond. (With Plates
34,35) Sy bgt Bees ee
The Anatomy of Arenicola assimilis, Ehlers, and of a New Variety
of the Species, with some Observatious on the Post-larval Stages.
By J. H. Asuwortn, D.Sc. (With Plates 36,°37)
TitLe, Contents, aNnD INDEX.
wt ———KLK
PAGE
509
715
729
737
MAR 12 1903
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 509
The Movements and Reactions of Fresh-water
Planarians: a Study in Animal Behaviour.'
By
Raymond Pearl, Ph.D.
(Instructor in Zoology in the University of Michigan, Ann Arbor,
Michigan, U.S.A.)
ConrTENTS.
PAGI
A. INTRODUCTION . : : i s Bill
B. RESUME OF LITERATURE . : : : 5 le
I. Morphological and Systematic — . ‘ +, BLS
II. Physiological ; : : ; . 520
c. MarTEeriaL : , 523
p. Hapits anp Natura. History : t ; 25
I. Occurrence and Distribution E . 526
II. Activities 3 ‘ E 3 = Hey
a. Sensitivity : ; : : » b27
4. Secretion of Mucus : 2 : . 529
c. Periods of Activity and Rest
d. Formation of Collections
5
5
e. Movement on Surface Film : m5
III. Food ; ' : : 4 i
5
5
5
“ot Ee Cs
LV. Defecation ‘ :
V. Summary of Factors in Behaviour
rE. Normat Motor Acrtivitirs
Mm CO Oo to OO Ww OH WH OH CS DW WO WD DW ®W 2
S wo
I. Locomotor Movements 539
a. Gliding : F 539
1. Rate of Gliding Movement 545
2. Direction 548
' Contributions from the Zoological Laboratory, University of Michigan,
Ann Arbor, Michigan, No. 58.
von. 46, PART 4.—NEW SERIES. LL
510 RAYMOND PEARL.
PAGE
b. Crawling Movement : ; ; . 548
1. Direction . ; : ~ boo
Y, Stimuli which induce Graminge 5 Syl!
c. Movement on the Surface Film : 552
d. Relation of the Movements of Triclads to tigse of ones
Forms : - = 553
Il. Non-locomotor Mawaments ' : DDD
. Contraction of the Body. : ; . 555
i Extension of the Body —. ; : . 556
@ Rest, ; ; . sbby
1. Formation of Walleetions ; ; . 566
d. The Effect, of Operations on Movement : . 570
F. REACTIONS TO STIMULI. F ¢ - 576
T. Reactions to Mechanical Stimnuly : : /. 696
a. Methods : 3 : ; 5 56
). Description of Reactions 577
1. Reactions to Stimuli applied to tie Head Region 577
a. Reactions to Strong Stimuli 578
B. Reactions to Weak Stimuli 582
2. Reactions to Stimuli applied to the Middle Region 5
the Body 4 . 588
a. Reactions to Strong Stimuli ‘ . 588
B. Reactions to Weak Stimuli ‘ 589
8. Reactions to Stimuli applied to the Posterior Bevin
of the Body. A 1og2
4. Reactions to Stimulation of the Ventral Statice . 594
5. Reactions of Resting Specimens to Mechanical
Stimuli : . 595
6. Reactions to Stimuli given by Open Proce-
dure . 5 DUo
7. The Effect of Meehaniedt Emaeaue to Maveineah 5 BS)
ce. The General Features of the Reactions to Mechanical
Stimuli. ; , . 600
d. The Mechanism of the mason : . 602
1. The Relation of the Brain to the Baactions . 602
2. The Neuro-muscular Mechanism 606
e. Features in the General Behaviour of the rgatiem mined
the Reactions to Mechanical Stimuli explain . 619
J. Summary : ‘ . 693
Il. Reactions to Food and Ghemtcate Sismalt ; . 623
. Food Reactions . . 694
1. Food Reactions of Spaiment aiden Saratinid . 637
2. Summary of Food Reactions. 5 . 640
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 511
PAGE
6. Reactions to Chemical Stimuli—Chemotaxis . > 643
1. Reactions to Localised Chemical Stimuli . . 643
a. Methods : : . . 643
B. Results A : : . 646
2. General Summary : Se Oni!
3. Unlocalised Action of @henen's : . 669
IT. Thigmotaxis and the Righting Reaction . . 670
a. ‘Thigmotaxis ‘ ‘ : 2 600
b. The Righting Reaction —. d : 2) 83
The Mechanism of the Reaction — . : . OG
ec. Summary : : 3 : . 684
IV. Electrotaxis : ; : : . 685
. Methods 4 : 2 ; . 685
; Results ; : f = 685
ec. Mechanism of the Reachions F : . 690
d, Summary : ‘ : : mE OOD
V. Reaction to Desiccation . : : ; (695
VI. Rheotaxis : 697
G. GENERAL SUMMARY AND Drecoetee OF esters : . 698
1. List oF LITERATURE : . ? : YG
A. INTRODUCTION.
THe present study has for its purpose the analysis of the
behaviour of the common fresh-water planarian into its com-
ponent factors. It is well known that, aside from the
researches of a few investigators on a small number of forms,
we have little detailed knowledge of the behaviour of lower
organisms. It is coming to be realised, too, that knowledge
of what an animal does is just as important in the general
study of life phenomena as a knowledge of how it is con-
structed, or how it develops. But it must be admitted that
until quite recent times the study of the activities of living
things was a much neglected field in biology. ‘lhe
publication of the ‘Origin of Species’ gave the biological
pendulum a swing towards the study of phylogeny, from
which it is only just beginning to return.
As a consequence of this concentration of interest on other
subjects, we possess an accurate and full knowledge of the
512 RAYMOND PEARL.
activities of very few lower organisms. ‘The behaviour of the
Protozoa has been quite fully described and analysed by the
work of Verworn (’89) and Jennings (’97, 799, 99a, ’99b,
’99e, : 00, : 00a, :00b, :00c,: 01, Jennings and Moore : 02).
In the earlier work of Verworn the general features of most
of the reactions of the Protozoa are described, special atten-
tion being paid to the rhizopods. The reactions of the
Infusoria have been very thoroughly worked out by Jennings.
In the case of the Infusoria we now know exactly the
mechanism of the reaction to a large number of stimul. The
reactions and general behaviour in the case of two groups of
echinoderms are quite thoroughly known from the early work
of Preyer (86, ’87) on the starfish and the recent brilliant
work of von Uexkiill (96, ’96a, 799, :00, : 00a) on the sea-
urchin. These few instances are the only ones in the
literature where the movements and reactions of an organism,
or group of organisms, have been investigated in any com-
prehensive ‘‘ monographic ” way, There is a great body of
literature dealing with isolated reactions in a variety of forms,
but the thorough investigation of the activities of animals in
a way comparable to that in which their morphology has
been investigated remains in large degree yet to be done.
It, appeared highly desirable that this sort of knowledge be
extended, and it was with this idea in mind that this work
was undertaken. The form used, Planaria, was chosen for
several reasons. In the first place, it has come to be a sort of
paradigm for work on regeneration, and its biology from that
standpoint is already well known. Furthermore, in some one
or more of its species it is an almost universally distributed
form and can always be ebtained in quantities. Finally, and
particularly, it is a representative of an animal type about
whose activities we know only the most general facts. It is
a symmetrical aquatic organism of low organisation, and its
behaviour is rather complicated. The importance of possess-
ing a detailed knowledge of the activities of a bilaterally
symmetrical, free-moving, low organism will be apparent
when it is considered that such an organism has never been
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 015
made the subject of such a study. The behaviour of typically
unsymmetrical organisms, the Infusoria, has been analysed,
as has also that of some radially symmetrical animals, and in
both cases there is found to be a very close interrelation-
ship between the general form of the body and the reactions.
To investigate, then, in a comprehensive way the activities
of a bilaterally symmetrical organism standing low in the
animal series was the purpose of this work. ‘The most
general problem which presents itself is the establishment
of the animal’s position in the objective psychogenetic series.
Are its activities relatively simple or are they complex ? Do
they fall under the same general type as those of the
Infusoria or those of the higher organisms, or do they occupy
an intermediate position? Another general problem of im-
portance is whether there is any marked correlation between
the behaviour and the form of the body, such as has been
found to obtain in so marked a degree in the case of the
Infusoria and the rotifers (vide Jennings, loc. cit.). We
have in the flat-worm a symmetrical animal; are its reactions
of a symmetrical type? Besides these broad fundamental
problems there are, of course, a large number of subsidiary
questions which readily suggest themselves in connection
with a work of this sort. These need not be specifically
mentioned here, but will be brought out in the course of
the paper.
As to the scope of the work as actually done, the following
” of the animal
may be said:—The general “ natural history
was studied as completely as possible. All the normal move-
ments were studied in detail. ‘The reactions to mechanical
stimuli; the food reactions and reactions to chemicals in
general ; electrotaxis ; thigmotaxis; rheotaxis; the righting
reaction; the reaction of cut and regenerating pieces ; and
hydrotaxis and the reactions during desiccation were investi-
gated. No work was done on the phototaxis or thermotaxis,
A study of the phototaxis was omitted for two reasons ; first
on account of the fact that during the progress of this in-
vestigation Parker and Burnett (: 00) reported their work on
o14 RAYMOND PEARL.
the same subject, and furthermore on account of lack of
opportunity. As a result of some incidental observations
made during the course of this work, it has appeared that it
would be profitable to extend the work of Parker and
Burnett, and this, together with a study of the thermotaxis, I
hope to be able to do in the future. Another field for further
work is afforded in the study of the reactions of regenerating
individuals. As this subject did not fall immediately into
the general plan of this work, but comparatively little atten-
tion has been given to it, yet the work done gives much
promise of important results to be gained by more extended
investigations.
So far as possible the details of the movements and
reactions will be described fully. It is not easy to see why
there is not as much need for a complete knowledge of
details in physiological work as in morphological, yet in
much of the recent work in comparative physiology only the
most general results are reported. To gain a knowledge of
the details one must do the work over again. While such
more or less general papers are easy to read, and put the main
results in such a form as to be easily accessible, yet it is
believed by the writer that the solid foundations of com-
parative physiology and psychology must consist of detailed
“fine” work, just as has been the case in morphology. It
seems to the writer that the tendency to abandon the detailed
descriptive method in favour of the extreme experimental
method in biological work is unfortunate. Both ways of
working are methods of getting at the truth, and, as proven
by their results, both are good methods. The current notion
of the sufficiency of the experimental method to the exclusion
of others is not only an evident exaggeration of the facts in
the case, but, in the opinion of the writer, the exclusive use
of the “ crucial-experiment ”? method in work upon the move-
ments and reactions of organisms has in some cases hindered
rather than helped us to gain a clear understanding of the
phenomena. ‘The importance of close observational work in
the study of animal behaviour has been strongly emphasised
MOVEMENTS, BTC., OF FRESH-WATER PLANARIANS. 515
recently by Whitman (99). The aim in the present work
has been to get as extensive and detailed a knowledge as
possible of the behaviour of the organism by direct observa-
tion before resorting to experiments.
At this point I wish to acknowledge my indebtedness to
the officials of the laboratory in which this work has been
done. To Professor H. 8S. Jennings, under whose general
oversight this investigation has been prosecuted, I wish to
extend my heartfelt thanks for his uniform kindness in
freely giving advice, suggestion, and kindly criticism of im-
measurable value. Any adequate expression of my indebted-
ness to him is impossible. I further wish to express my
thanks to Professor Jacob Reighard for the numerous
facilities which I have enjoyed during my stay in his
laboratory, and for his kindly interest, which has made work
there a pleasure. Finally, I desire to acknowledge my in-
debtedness to Professor F. C. Newcombe, of the Botanical
Department of the University of Michigan, for many
valuable suggestions and advice.
B. Resume or LITERATURE.
But little has been done on the physiology of the move-
ments or on the psychology of the ''urbellaria, and, as in
the case of most of the literature dealing with these subjects,
what has been done has been in comparatively recent years.
Investigators of the old ‘ natural-history ” school which
flourished before the time when Darwin’s work changed the
course of zoology seem not to have given much attention to
planarians, while the later systematists and morphologists
for the most part carefully avoided any reference to the
activities of the forms which they studied.
I. Morphological and Systematic.
Among the papers devoted primarily to the systematic or
morphological treatment of the group, there are occasional
references to points in the behaviour of the organisms which
516 RAYMOND PEARL.
are of importance from the present standpoint. Among
such references the following may be noted :
Moseley (’74), in a paper concerned principally with the
anatomy and histology of the land planarians, devotes a sec-
tion to a discussion of the habits of these forms. He com-
ments on the “avoidance of light” (negative phototaxis) of
land and aquatic planarians, and discusses the habitat and
food of the animals. He reaches the conclusion that all
planarians are carnivorous, but gives no account of the
method of feeding. He quotes Rolleston as having found
that Planaria torva and Dendrocelum lacteum in a
dish in which had been placed a freshly killed earthworm
“crowded on to the worm’s body and soon sucked all the
hemoglobin out of it, leaving it white and pulpy.” Brief
mention is made of the habit of the land planarians of
secreting a mucous thread and hanging from it as a mollusc
does. Finally, the method of movement of Bipalium with
the head raised and waved from side to side as the animal
proceeds is described. ; eee
E (tri: excitation.
and 11) .
Negative responses (0) ;
The positive, responses in all these experiments were very
definite and characteristic. I have obtained the same results
in many other series of experiments, which need not be
recorded in detail. ‘The experiments show very clearly that
in order for the animal to give positive responses to weak
stimuli it is necessary that it be in an unexcited condition,
These results have also an important bearing on the question
of the mechanism of the positive response, in that they show
conclusively that the reaction does not depend on the stimu-
lation of special sense organs located in the head regions
alone.
Weak mechanical stimulation of the dorsal surface in the
middle region of the body is usually without any effect other
than the causing of a slight local contraction at the point
stimulated. If any specific effect on the whole animal is
produced, it is merely a change from the gliding to the
crawling movement, such as results from strong stimulation
in the same region.
3. Reactions to Stimuli applied to the Posterior
Region of the Body.—By “ posterior region of the body ”
I mean that part of the body from the pharyngeal region to
the posterior end. This region is not sharply marked off
physiologically from the middle region, and it is impossible
to say in any given individual at just what level the demar-
cation will be found. The physiological distinction between
the two regions is founded on the fact that it is possible by
unilateral stimulation of the middle region of the body to
produce a change in the direction of the movement of
the animal as a whole, while in case of the posterior region,
MOVEMENTS, BTC., OF FRESH-WATER PLANARIANS. 093
as will be shown, this cannot be done. On this account it
will not be necessary in the description of the reactions to
sharply distinguish between the effects of stimulation of the
margins and of the dorsal surface, as has been done in the
previous cases.
Strong mechanical stimulation of the posterior region of
the flat-worm produces as a specific reaction an immediate
change from the gliding to the crawlmg movement. ‘The
direction of the crawling is the same as that of the gliding ;
that is to say, the worm keeps on in a straight line, taking
itself directly and in the quickest possible way away from
the stimulus. The duration of the crawling movement
following stimulation of the posterior region varies with the
relative intensity of the stimulus and the physiological con-
dition of the specimen. ‘The most usual number of the
strong, crawling contraction waves following strong stimula-
tion is three or four. We may get a smaller number than
this, and very frequently do, but in the species studied I
have very rarely seen more than four of the general con-
tractions following a single stimulus. This is evidently all
that would be necessary under normal circumstances, since
four of these strong contractions will carry the animal a
considerable distance ahead, and probably out of reach of
the stimulating agent. ‘he weaker the stimulus is, the fewer
are the contractions and the shorter the distance crawled.
In some individuals it is at times almost impossible to induce
the crawling movement except by repeated stimulation.
Such specimens will merely draw up the posterior end in a
single crawling contraction after stimulation, and then im-
mediately relapse into the glide. If a strong stimulus is
repeatedly given at the posterior end the crawling is con-
tinued, becoming more and more rapid. This is the only
effect of continued stimulation in this region, there being no
summation effect corresponding to that produced by stimu-
lating the anterior end. No different effect is produced by
stimulating the margins of the posterior region of the body
from what takes place when the point stimulated hes near
594. RAYMOND PEARL.
the middle line. There is no turning towards or away of
any part of the body. The lack of any special effect of
unilateral stimulation is not surprising, for the reason that
rapid movement in a forward direction will get the animal
away from harmful stimuli affecting this region, in the long
run, more quickly than any other. Further, there would be
no advantage in the production of a positive reaction by
stimuli at the posterior end. If we think of these reactions
as having been developed by natural selection there would
be no possibility of such a reaction having arisen, for the
reason that practically any favourable stimulus would be
encountered by the anterior end before it possibly could be
by the posterior. Very weak mechanical stimulation of the
posterior end of the body causes only a local contraction at
the point stimulated.
4, Reactions to Stimulation of the Ventral Sur-
face.—In the descriptions of the reactions to mechanical
stimuli up to this point we have been considering stimuli
applied to the dorsal surface and to the margins of the body.
It may be well to describe briefly what the reactions in
response to localised stimulation of the ventral surface are.
This matter can best be tested when the animal is moving on
the under side of the surface film, with its ventral side
uppermost. It might be supposed before the trial was made
that this habit of the animal would afford ideal conditions for
testing its reactions to ventral stimulation, but, as a matter
of fact, the conditions are anything but ideal. ‘The flexibility
and elasticity of the surface film makes it almost impossible
to touch it with a stimulating point anywhere within a radius
of a centimetre about a planarian without causing the animal
to be jerked bodily to one side or the other, quite sharply
and for some little distance. This is, of course, a mere
mechanical effect, which takes place with lifeless bodies also.
Furthermore, as has been mentioned in an earlier section, it
appears to be very difficult for planarians to quickly change
the direction of their movement when on the surface film (as
is necessary in reacting to stimuli). On account of these
MOVEMEN'IS, ETC., OF FRESH-WATER PLANARIANS. 595
conditions it is very difficult to get any certain and trust-
worthy results from the stimulation of the ventral surface.
My results have been as follows :—strong stimulation of the
anterior end on one side of the middle line causes the
negative reaction just as when the stimulus is applied at a
corresponding point on the dorsal surface. For mechanical
reasons the response is not as extensive as when the animal
is on a solid, but there seems no doubt of its character. The
positive reaction to weak stimuli I have not been able to
produce in any certainly recognisable form in response to
stimulation of the ventral surface, but I think this negative
result is due probably to the external conditions, and not to a
real failure of the organism to react. Strong stimulation of
the posterior end of the body causes the gliding to change
to the crawling just as under other conditions. Very
strong mechanical stimulation of the ventral surface of the
body causes the animal to let go its hold and pass down
to the bottom.
5. Reactions of Resting Specimens to Mechanical
Stimuli—A resting specimen gives no response whatever
to weak stimuli which are still strong enough to produce a
definite reaction when the worm is in the active condition.
The stimulus is simply below the threshold of the resting
animal’s sensitiveness. To stronger stimuli the reactions
correspond in form with those given by the active animal,
but are less pronounced. For example, rather strong stimu-
lation at the anterior end induces a weak negative reaction ;
similar stimulation of the posterior end sets the animal off
into the crawling motion. Strong stimulation of any part of
the body besides producing the characteristic reaction for
that region (that is the negative reaction) will also in most
cases start the animal into movement. ‘This will always be
the case if the stimulus is of sufficient strength, or is several
times repeated. As would be expected from the low sensi-
tiveness of the resting flat-worm, it is impossible to call
forth from it any positive reaction.
.6. Reactions to Stimuli given by Operative Pro-
596 RAYMOND PEARL.
cedure.—Hvidently when a planarian is cut the cutting
induces a strong stimulation, which is of the same kind as
that induced by ordinary mechanical stimuli, only much
more intense. ‘The immediate effects of operations may then
be taken up in this section.
If we take first the typical case given by cutting the
animal transversely in two in the region between the pos-
terior border of the head and the origin of the pharynx, and
make the cut by a single stroke of a sharp scalpel, we find
that the effect on the anterior piece is precisely the same as
that of an ordinary strong mechanical stimulation of the
same place. That is, this piece merely changes from the
oliding to the crawling movement, and after giving three or
four crawling contractions settles down again into the
elide. This is the same result essentially as that obtained
by Norman (: 00) and earlier by Loeb (94 and :00). In the
behaviour of the posterior piece in this experiment under
discussion there is a great deal of variation. In about 70
per cent. of all cases in which I have observed the results of
such an operation, the posterior piece crawled backwards
as a result of the cut. In the remainder of the cases the
piece either stayed in the same place and contracted
violently, or else glided ahead. The amount of the back-
ward crawling when this occurs varies greatly, from a
short distance involving only one longitudinal crawling con-
traction to several times the length of the worm, the move-
ment lasting in this latter case for over a minute. In order
that this backward crawling may appear in a well-marked
and distinct form it is necessary that the posterior piece be
above a certain size. Very small posterior pieces after
operation usually remain quiet.
A cut so made as to split the anterior end of the body in
the middle line in most cases causes the worm to crawl back-
wards just as does a transverse cut. In some cases this, as
well as other operations, merely causes the animal to contract
violently and squirm about at the same place. Splitting the
posterior end of the body in the middle line causes the parts
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 597
on either side of the cut to give violent longitudinal con-
tractions, while the worm as a whole starts crawling ahead ;
that is, it changes from the gliding to the crawling
movement.
Oblique cuts produce essentially the same effects as would
transverse cuts in the same part of the body, i. e. forward
crawling of the anterior piece, and usually backward crawling
of the posterior piece. This is true unless the cuts are very
oblique, so as to form very acute angles with the sagittal
plane of the body. In such cases the effects produced more
nearly resemble those obtained in complete longitudinal
splitting of the body. If the body is split completely into
two parts longitudinally, there is usually very little pro-
gressive movement of either piece afterwards. ‘The pieces
contract strongly on the cut sides very soon after the opera-
tion is performed, so that they take on the form of a bow,
which in many instances becomes a nearly complete circle.
This being the case, any progressive movement, either by
gliding or crawling, is nearly or quite impossible. Cuts
involving only a small portion of one side of the body
produce, if in the anterior region, the characteristic negative
reaction given to other strong mechanical stimuli, while if in
the posterior region they cause the crawling ahead.
Cuts made on the resting animal produce essentially the
same effects as on the gliding specimen. Unilateral cuts
have the same effect in producing the negative reaction.
7. The Effect of Mechanical Hindrance to Move-
ment.—A series of experiments was performed on Dendro-
coelum, sp., with reference to the behaviour of the animal
when progressive movement was made impossible, and yet
the animal was stimulated strongly at the same time. ‘These
conditions can be realised by thrusting a needle through the
centre of the body from above, and then holding it fixed in
position. The results of this procedure varied somewhat,
according to the portion of the body through which the
needle was thrust. In case the hindrance is in the posterior
region of the body, e. g. at a point just behind the posterior
598 RAYMOND PEARL.
end of the pharynx, the effect immediately following the
thrusting in of the needle is a strong longitudinal contraction
of the whole body. After this first strong contraction the
animal remains perfectly quiet in the contracted form for a
varying length of time (in some cases as long as five minutes,
but usually less). After this period of quiet a series of
rhythmical waves of contraction pass longitudinally over the
still contracted body. The purpose of these waves is
evidently to loosen the restraining object by making the
hole in the body through which it passes larger. This is the
same behaviour that I have observed in the deposition of the
large egg. ‘This process of rhythmical longitudinal contrac-
tion 1s continued for a time; then the animal stretches to its
extreme length, attaches the anterior end to the substrate,
and attempts to crawl away. The movement of the anterior
end is precisely the same as in crawling. The animal turns
and twists and struggles violently in this attempt to crawl
away, and the cilia beat strongly. If the needle occupies a
position near the edge of the body this first struggle will
usually be sufficient to tear the body loose from the needle, so
that the animal may then move ahead freely. Such specimens
will, of course, have a large jagged wound in one side of the
body, which, however, closes in and heals in a short time. In
case the first struggle of the extended animal to crawl ahead
is not effective, that is if the needle is too far in towards the
centre of the body to make the tearing out possible, the
animal, after continuing the struggle for a time, contracts
strongly longitudinally and goes through the whole series of
stages of quiet, rhythmical, longitudinal contraction and
attempted crawling again. The only difference between the
first and succeeding series of trials is that the stages in which
the animal is strongly contracted longitudinally tend to
become shorter with each repetition.
In case the needle is thrust through the body in front of
the pharynx, the strong longitudinal contraction appears as
before, and is followed after some time by an extension of the
part in front of the needle, while the rest of the body re-
MOVEMENTS, ETO., OF FRESH-WATER PLANARIANS. 599
mains quiet and contracted. This short anterior region,
including hardly more than the head, goes through the
crawling movements, but on account of its small size is very
ineffective so far as pulling the body away from the needle
is concerned. In my experiments I have never seen any
worm succeed in getting free from a needle put through the
body in this position.
This general behaviour of the animal in response to restraint
of movement is very interesting, especially in the cases where
the restraint is at the posterior end, as showing the relation
between the behaviour and the capability of regenerating.
The organism tears itself loose from a restraining body with
entire nonchalance, as it were, and its confidence is well
founded because no permanent harm comes from the action.
The lost and wounded parts are regenerated and healed in a
short time. The behaviour takes advantage of the ability to
regenerate. Whether the form of behaviour (pulling away
from restraining objects) or the power of regeneration and
reparation appear in the organism first we cannot say, for
either might very well follow, in a more or less remote causal
connection, the other. What we do know is that at present
there is a very nice condition of mutual adaptation between
the two things.
The effect of the hindrance of a rather light weight at the
posterior end of a worm is to induce the crawling movement.
This can be seen in case the animal is feeding on a small
piece of food material, and, as frequently happens, starts into
movement before the pharynx is withdrawn. The piece of
food attached to the end of the pharynx is dragged along
behind, and the movement is the crawling. Frequently, also,
in feeding experiments pieces of food will get stuck to the
posterior end of the worm by means of the mucous secretion
of the body, and these have the same effect in inducing the
crawling movement.
Having now obtained a descriptive basis we may pass to a
discussion of some general features of these reactions. We
may first take up—
600 RAYMOND PEARL.
c. The General Features of the Reactions to
Mechanical Stimulii—From the above description it
appears that the nature of the reactions to mechanical stimuli
depends upon several factors. These are
1. The intensity of the stimulus.
2. The localisation of the stimulus.
3. The physiological condition of the organism.
The reactions given may be of several different kinds, de-
pending on the factors mentioned above. These are chiefly
as follows:
1. The resting individual may begin locomotion.
2. The gliding movement may be changed to the crawling
movement.
3. The forward movement may be transformed to move-
ment backward.
4, The animal may turn away from the source of the
stimulus (the “ negative ”’ reaction).
5. The animal may turn towards the source of the
stimulus (the ‘ positive ” reaction).
It is evident that the reactions last named—the negative
and positive reactions—are the most important and most
interesting from the theoretical standpoint. It is of the
greatest interest to note that these two qualitatively opposite
reactions’ are induced merely by differing intensities of
stimuli, the stimuli being otherwise identical throughout.
lt is to be noted further that the positive and negative
reactions have the characteristics of purely reflex acts. Hach
reaction has a perfectly definite and characteristic form.
While, in some cases, which of the two reactions will be
given in response to a particular stimulus depends on the
physiological condition of the organism, yet it 1s practically
always either one or the other of the typical reactions. Only
very rarely do we get any deviation from the type forms,
and in such cases the reaction is evidently a combination of
easily recognisable components of the two typical complexes
of reflexes.
These two reactions are evidently not single simple
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 601
reflexes, but are complexes of several simple reflex acts. It
may be well to present in tabular form the different com-
ponents in each of these reactions, indicating by the position
in the table the relations of the parts.
Component Phases of the Reactions to Mechanical
Stimuli, with special reference to the Head
Region.
PosITIve. NEGATIVE.
A. Momentary stopping of pre- A. Same as in positive.
vious movement. Referred
to as ‘‘ pause” or ‘ hesita-
tion’ in description.
B. Longitudinal extension of the 6. Longitudinal contraction of ante-
anterior end to greater or rior end of greater or less
less extent. Amount de- intensity. ‘Tends to make A.
pends on previous extension. appear more pronounced and
Usually distinctly noticeable. longer in duration.
/C. Turning towards one side, viz. C. Turning towards one side, viz.
that stimulated. This side that not stimulated. Defined
is defined by the position of as in positive. No sharp
the source of the stimu- “ orientation.”
lus, not structurally, Sharp
“orientation.”
C’. Raising of anterior end. This
takes place at the same time
as C.
D. Movement towards stimulus. D. Movement away from stimulus.
Direction determined by Direction determined as in
position taken by anterior positive.
end at termination of C.
Time relations are indicated by vertical position in the table. Components
occurring at the same time are included in braces.
Kach of the components before D may be considered as a
single reflex, and thus there are in one case four and in the
other case three simple reflexes which go to make up the
whole reaction. ‘That these reactions are composites of the
distinct parts is evidenced, first, by direct observation of the
reactions themselves; and second, by the fact that it is
602 RAYMOND PEARL.
possible by varying the strength of the stimulus to produce
only certain parts of the whole reaction without the
remainder, and, furthermore, that a part of one reaction may
in rare instance be combined with a part of the other
(v. sup., p. 587).
d. Mechanism of the Reactions.—A question which
is of the greatest importance in all work on the reactions of
organisms is, what is the mechanism of the reaction? In
the case of the flat-worm this becomes, what is the neuro-
muscular mechanism of the reactions? Very little direct
evidence bearing on this question can be obtained from the
reactions themselves. Taking the positive and negative
reactions as they occur, there are several different sets of
muscles and of nerve connections by means of which they
might conceivably be brought about. ‘The best evidence on
the question is the indirect evidence from operation experi-
ments, in which parts of the mechanism are injured or
removed.
1. Relation of the Brain to the Reactions.—The
first specific problem which may be taken up may be stated
thus: is the brain necessary for the performance of the
normal reactions to mechanical stimuli? Or, in other words,
will a planarian from which the brain has been removed
react normally to stimuli? This question can be answered
from the study of specimens which have been cut in two
transversely, and consequently we may proceed at once toa
description of the reactions of the pieces resulting from such
an operation. A typical specimen is cut in two transversely
at the level of a point about halfway between the head and
the origin of the pharynx, as shown in Fig. 16. As has been
mentioned above, the cut itself acts as a strong mechanical
stimulus, and the immediate effect of the operation is to set
both pieces crawling, the anterior one ahead and the
posterior one usually backward.
If now the pieces are allowed some hours to recover from
the immediate effect of the operation, and then stimulation is
tried, the following results are obtained :—With the anterior
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 6083
piece A, containing the brain, the results are entirely similar
to those obtained in case of the normal animal. Strong
unilateral stimulation of the head causes the negative
reaction, weak stimulation of the same sort the positive
reaction. Stimulation at the posterior end causes the crawl-
ing movement to appear, and altogether the appearances are
essentially the same as in the normal complete specimen.
The posterior piece B (lacking the brain) behaves in a
somewhat different manner. If the anterior end of this piece
is given a stimulus of moderate intensity anywhere on the
cut surface the piece will usually start crawling straight
backwards. This is almost always true for a short time
after the operation, and is especially well shown in such
specimens as started crawling backwards as a result of the
cut. When from twenty-four to forty-eight hours have
elapsed after the operation this tendency of posterior pieces
Fie. 16.—Operation diagram. Heavy line indicates cut.
to crawl backward on stimulation of the anterior end begins
to grow less marked, and, as regeneration proceeds, finally
disappears. In many such posterior pieces I have been able
to produce this backward crawling in a very pronounced
form, and of comparatively long duration (three or four
minutes at a time). The character of the movement has
been described above. If the stimulus is applied to one side
or the other of the anterior end of such a posterior piece,
instead of squarely against the cut surface, a well-marked
negative reaction is produced; that is, the anterior end
turns away from the stimulus just as a whole animal would.
The reaction is very definite, and of precisely the same
character as the normal negative reaction. The only
difference to be observed is that in proportion to the strength
of the stimulus the reaction is not so pronounced as in the
604 RAYMOND PEARL.
normal animal, this being due to the generally lowered tonus
in such a piece. I have not been able to obtain any positive
reaction (i.e. turning towards the stimulus) in such a
posterior piece after operation. Stimuli which are at all
effective produce the negative response. This experiment
has been tried many times, but always with the same result ;
the positive reaction never appears. If the posterior end of
such a posterior cut piece is stimulated the crawling move-
ment is produced just as in case of the normal complete
animal. As has been noted in connection with the move-
ment, there is a general reduction of tonus in the posterior
pieces resulting from transverse cuts. This low tonus in-
volves not only the motor functions, resulting in slower
movement, but also to a less extent the sensory functions.
Such a piece is somewhat less sensitive to mechanical stimuli
than normally. The cut surface is more sensitive to
mechanical stimuli than any other part.
Now it will be seen from the above description of the
reactions of a piece from which the brain has been removed,
that the most striking difference in the behaviour of such a
piece from that of a normal animal is to be found in the
absence of the positive reaction.
There are three conceivable possibilities as to the cause of
the absence of the positive reaction in pieces from which the
head has been removed. First, the positive reaction might
be due to the stimulation of certain sense organs which are
removed by the operation. But this is decisively negatived
by the fact that in an entire worm stimulation of points
posterior to the level of the cut removing the anterior end
will cause the positive reaction.
Second, it might be conceived that the reaction is brought
about by a special localised muscular mechanism, which is
removed or destroyed by the cut. But there is no evidence
of the existence of such a mechanism ; and further, it will be
shown later that the ordinary musculature of the body, which
is of course uninjured in the posterior part, is sufficient to
bring about the reaction.
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 605
Finally, the positive reaction might im some way be a
specific function of the brain, which is removed by the
operation. As the evidence seems to be decisive against the
first two possibilities this seems probably true. Is this
because the brain contains a special “ centre”? whose function
it is to produce the reaction ?
There is no reason to think of the reaction as a function of
the brain in the sense that that organ forms a centre which
originates the impulses which cause the reaction. On the
contrary, it seems much more probable that the loss of the
brain causes the loss of reaction for the following reason.
It has been shown that removal of the brain causes a general
lowering of the tonus of the organism, and further that the
appearance of the reaction in a normal animal is closely
dependent on the tonic condition of the organism. Probably,
then, the chief reason for the non-appearance of the positive
reaction in posterior pieces is that in these the conditions of
general tonus are so changed by the loss of the brain that the
reaction is no longer possible. Expressing it in another way,
the animal is too sluggish to give the positive response.
This being the case, it would be expected that it might be
possible to induce the positive reaction in a decapitated
specimen provided the tonus were raised in some way. Asa
matter of fact, as will be shown later, positive reactions to
certain chemical stimuli have been observed in a few cases
(cf. p. 649). In its form and mechanism the positive reaction
is not directly dependent upon the brain.
Summing up the evidence on the relation of the brain to
the reactions of the flat-worm, it may be said that all the
reactions to mechanical stimuli shown by the normal animal,
with the single exception of the positive reaction, are given
by specimens from which the brain has been removed.
The relation of the brain to the positive reaction is, in large
part, so far as evidence can be obtained, an indirect one, viz.
it is necessary for the maintenance of the proper tonic
conditions of the organism. ‘Thus far there is no evidence of
any special “ centre ” functions of the brain, similar to those
vol. 46, PART 4,—NEW SERIES, RR
606 RAYMOND PEARL.
supposed to exist in the cortical centres, for example, of a
mammal.
2. The Neuro-muscular Mechanism.—In the negative
reaction to mechanical stimuli the anterior end of the body
is turned sharply away from the source of the stimulation,
while in the positive reaction it is equally sharply turned
towards the source. These relations immediately suggest the
following questions :—Is the negative reaction the result of a
crossed impulse, which, originating at the point stimulated,
crosses over to the other side of the body and causes the
contraction of the longitudinal muscles on that side, thus
producing the turning away from the stimulus? What is the
course of the nerve impulse which produces the positive
reaction ? What sets of muscles are concerned in the pro-
duction of each reaction ?
The discussion of the negative reaction may be taken up
first. If the nervous impulse producing this reaction
crosses the body to produce a contraction on the side
opposite from the stimulus, the experiment cited in the
section above shows that this crossing cannot occur entirely
in the brain, but must also occur in some part of the body
posterior to the brain; or at any rate, be capable of so doing
in a quite normal fashion immediately after removal of the
brain. In this experiment where the body has been cut
in two behind the brain, the posterior piece performs the
negative reaction in a quite normal way immediately after
the operation. ‘This experiment may be carried farther,
and the animal cut in two transversely in places nearer
and nearer to the posterior end of the body. In all of these
cases, until the piece becomes too small to show definite
movements of any sort, the negative reaction may be obtained
by strong unilateral stimulation. ‘This shows conclusively,
then, that if the negative reaction is to be considered a
crossed reflex, there must be all along the body a series of
cross-commissures which are at all times ready to bring about
in co-ordinated perfection a result with which they have never
previously had anything to do. This conclusion seems in-
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 607
evitable because, as has been shown above, unilateral
stimulation of the posterior region of the body in a normal
individual does not cause the negative reaction, but instead
merely causes the animal to move ahead faster by crawling.
If these paths for the crossing of impulses which are so
immediately effective after the operation are present in the
uninjured specimen, one would expect the reaction to be of
quite a different character from what actually occurs. A
stimulus applied near the posterior end would naturally cross
over at once and produce a bending on the opposite side at
the same level. Or the stimulus might diffuse, so that the
entire opposite side would be affected and the worm would
become uniformly curved on that side. But as a matter of
fact we find that the turning affects only the anterior portion
of the body. If it is urged that after operation the crossing
of impulses takes place through the general protoplasm the
difficulties encountered are no less, for it must be shown how
passage of an impulse through the protoplasm to cause a
perfectly well co-ordinated reaction can appear so quickly and
produce such perfect results at once. If tested immediately
after the operation, before the general lowering of tonus is
felt, the reaction time for the negative response of a posterior
piece of the body will not differ appreciably from that of a
normal worm. Now, according to the views of the advocates
of the theory that after operations involving loss of nervous
tissue, impulses may be conducted through the general
protoplasm, it is held that such conduction is always at first
appreciably slower than in nervous tissue. It would also
seem on purely a priori grounds that this must be true.
Thus it is seen that there are serious objections to the view
that the negative reaction is the result of a contraction on the
side of the body opposite to that stimulated—that is, that it is
a crossed reflex. The question now arises, if the reaction is
not produced in this way, in what other way can it be
produced? Kvidently it is quite possible that the anterior
part of the body can be turned away from the stimulus by a
lengthening of the side stimulated, quite as well as by a
608 RAYMOND PEARL.
shortening or contraction of the opposite side. We may now
consider the evidence as to whether or not the turning away
is actually due to a lengthening of the side stimulated.
Very little evidence can be obtained regarding this from
observation of the normal moving animal, because the
general appearance in the turning would be the same
whether it were due to a shortening of one side or a
lengthening of the other. The results from certain sorts of
operation, however, give definite evidence on the question.
A specimen split longitudinally in the posterior end, as
shown in Fig. 17, a, and the cut was extended forward to the
posterior border of the head region. Several days were
A
B =
b x
Fie. 17.—a. Operation diagram. 0b. Showing side A supported on
B. For further explanation see text. (The pharynx is omitted
for the sake of clearness.)
allowed for recovery from the shock of the operation, care
being taken to prevent the two parts from growing together
again. By this time the cut edges had healed well, and the
specimen was in good condition for experimentation. The
results of mechanical stimulation were as follows: strong
stimulation of the head or anterior part of the body on either
side caused the negative reaction; the anterior end turned
away from the stimulus. But it was possible to tell in this
case which of the two pieces or halves of the body were
effective in producing the turning. It could be seen clearly
that the half stimulated, immediately on stimulation, flattened
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 609
out slightly ventrally, thus bringing the ventral cilia in close
contact with the bottom, as is necessary for their effective
working. At the same time it lengthened along its outer
side, thus forcing the anterior end around towards the side
opposite from the stimulus. That the ‘side opposite ” had
nothing to do with the turning could be observed in many
cases directly, for this side (B) would remain in an almost
entirely relaxed condition after the stimulus was given, and
not get any effective hold on the bottom so that it could
affect the movement. It was further possible by a little
manipulation to get the piece B laid over on A so as to be
practically entirely supported by it, as shownin Fig. 17,6. If
with such conditions the worm was stimulated rather strongly
on the A side of the head, it gave a strong negative reaction,
the point about which the turn was made being as far back
asa. Evidently with part B up on the dorsal surface of A,
and consequently having no hold on the bottom, it could
have no effect in the reaction. The reaction must have been
due to the side A alone. The same thing could be shown by
very gently lifting on a needle the side B so that it was not
in contact with the bottom, and then stimulating A, when
again the negative reaction occurred. This experiment I
have repeated with variations many times, but always with
the same result, showing that the side stimulated is the
effective one in producing the turning.
It may be mentioned here that the effect of strongly
stimulating the posterior end of either of the two pieces of a
specimen slit in this way was to cause a local contraction of
the piece stimulated, and a crawling movement of the short
portion of the body in front of the slit. ‘This crawling was
not very effective, since so small a portion took part in it,
but it is of interest to note that what crawling appeared
involved only the uncut part of the body.
It being established that the side stimulated produces the
turning, the question may be raised, how, supposing in these
longitudinally split individuals that this side does produce
the reaction, is it known that it does this by lengthening
610 RAYMOND PEARL.
along its outer margin rather than by actively contracting on
its inner cut margin? This question may be answered by
operative experiments of a different character. If the side
stimulated, acting independently, produces the reaction by
lengthening on its own outer side, then an isolated longitu-
dinal half of the body ought to be able to give only one
reaction wherever stimulated, or, in other words, it ought
always to turn towards the same side. Furthermore, such a
piece onght always to turn towards the cut edge, since only
on the side opposite to this has it a margin possessing
the necessary circular muscles for extension (vide sup.,
pp. 556, 557). On the other hand, if the contrary view is
correct, that the turning away is due to contraction of the
longitudinal muscles on the side opposite that stimulated,
Fie. 18.—Showing the appearance of a longitudinal half of a
planarian when at rest.
then such an isolated longitudinal half of the body ought to
be able to turn either way, according to the localisation of
the stimulus, since there are longitudinal muscle-fibres along
the cut edge as well as along the other. We may determine
from experiments which of these two views is correct.
Unfortunately, it is impossible to get any clear evidence
on this point from entirely separated longitudinal halves of
the worm. When a planarian is split in two lengthwise
each of the pieces immediately becomes strongly contracted
longitudinally on the cut side, the apparent purpose of this
reaction being to reduce the exposed surface at once to a
minimum, After this strong contraction has taken place,
giving the piece the form shown in Fig. 18, no further
progressive movement can take place, and the general tonus
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 611
becomes immediately very much lowered. In view of these
facts it is impossible to get any very trustworthy results
from the stimulation of such a piece.
There is another operation, however, which, while it does
not isolate completely two longitudinal halves of the body,
yet does separate into longitudinal halves the essential
reacting parts, namely, the head regions. This is the
splitting of the worm in the middle line for a short distance
back from the anterior end, as shown in Fig. 12. After this
operation the two anterior pieces move about violently and
independently for a time, taking all the various positions
shown in Fig. 19. The animal soon recoyers from the imme-
Sy ee -
ma
Pi) e
Fie. 19.—Diagram showing the different positions taken by the two
components resulting from longitudinal splitting of the head.
diate effects of the operation, glides about in a normal way,
only at a rather slow rate, and responds well tostimul. The
anterior piece keeps comparatively straight, there bemg much
less tendency to contraction on the cut side than when the
split extends the whole length of the body. The reactions of
such a specimen to mechanical stimuli are as follows. ‘lo
stimuli applied at the posterior end along the sides of the
body the reactions are precisely the same as those already
described for the normal individual. Stimulation in the
regions aa (Fig. 20) of moderate or strong intensity produces
the negative reaction, The organism turns away from the
612 RAYMOND PEARIL.
side stimulated quite as promptly and in the same way as
does a normal specimen. If now the cut edges A and B (Fig.
211) are stimulated in the same way (a needle may best be
used for this) the specimen will always turn towards the
stimulus. This can best be brought out by describing a
typical case in which a series of fifty stimulations in the
regions A and B were made ona favourable individual cut in
this way. In thirty-nine of the reactions the animal turned
towards the stimulated side. That is, if the stimulus was
applied at A the animal turned in the direction of the arrow
a; while if B was the stimulated edge the reaction was in the
direction of the arrow b. In eight of the remaining eleven
trials the reaction was indifferent. The animal stopped at
Fic. 20.—Operation diagram. See text.
stimulation and then started moving straight ahead again,
the stimulus evidently having been ineffective so far as
special reaction is concerned. In only three cases out of
fifty did the specimen turn away from the stimulus. Since
it required the greatest care in manipulation to give the
stimulus to one eut edge without touching the other side,
especially in view of the fact that the animal was moving all
the time, it seems very probable that in these three cases a
stimulus was accidentally given to the side which it was not
intended to stimulate. The same general result of turning
' After this operation the two parts of the head usually take the position
shown in this figure after the first’ spasmodic movements following the opera-
tion have ceased.
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 613
towards the stimulus when applied to the cut edge was
obtained in several other series with this same specimen, and
many times with other specimens similarly mutilated. — It
will be seen that this is the result which would be expected
if the turning away is due to lengthening of the side stimu-
lated. Stimulation of either side of the cut portions, inner
or outer, causes turning in the same direction, and that
Fic. 21.—Diagram to show the reactions to mechanical stimuli and
their mechanisms in the case of a specimen in which the head
has been split longitudinally. For further explanation see text.
direction is the one in which turning would be caused pro-
vided each piece did actively lengthen on its outer side.
There seems to be no reason whatever, if the turning away
were due to contraction of the side opposite that stimulated,
why the specimen should not turn away from stimuli applied
to the cut inner edges. This it does not do. ‘There seems
to be no escape, then, from the conclusion that the turning
614 RAYMOND PEARL.
away from the stimulus (negative reaction) is due to a
lengthening of the side stimulated.
It may possibly be objected to the last experiment that
the impulse from a stimulation at, for example, B (Fig. 21)
took the path indicated by the dotted line in that figure, and
caused a contraction on the left side of the body, so that
really the observed turning was the result of a contraction on
the side opposite that stimulated. To this objection it may
be answered that by stimulating different points alone the
edge B it is possible to cause the point about which the
turn occurs asa pivot to be located anywhere along the linea y.
It is very evident that contraction of muscles in the region N
can have nothing whatever to do with turning of the right
piece about the point x So this objection is without force.
As the process of regeneration of a cut longitudinal half of
the body goes on, the piece will straighten out from the
curved form it takes after the cut is made, and it is conse-
quently possible to obtain specimens in which the regenera-
tion of the missing half of the body has produced only a very
small amount of new tissue, and which are at the same time
nearly straight in outline and able to make progressive move-
ments. The reactions of such partially regenerated speci-
mens are of importance as throwing light on the normal
mechanism of the reactions. The reactions of a typical
specimen of this sort may be described in detail. On October
10th, 1901, a small piece of the anterior end of a specimen of
P. maculata was isolated. The piece was cut as nearly as
possible in the form shown in Fig, 22, a. On October 16th
the piece had the form shown in Fig. 22, >. A narrow strip
of new tissue had formed down the right side, and the forma-
tion of the outline of the head and of the right eye was just
beginning. At this time the reactions of the specimen were
as follows. Stimuli applied at y caused the head to turn
sharply away from the stimulus (typical negative reaction).
This reaction was quite like that given by a normal individual
stimulated in the same way. Stimulation at x, however,
produced no trace whatever of a negative reaction, On
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 615
stimulation at this point the specimen contracted longitudi-
nally, and then started moving ahead again in exactly the
same direction in which it was going before stimulation. It
was impossible to induce any turning away following stimula-
tion of the side 2, although this was tried many times.
Now it is evident that this specimen comes very near
to being an isolated longitudinal half-planarian. All the
structures of the original one half are present, and there
is only a very little of the other side of the body produced in
the line of new tissue, down the originally cut edge. In this
new tissue there is probably very little differentiation, and the
muscle layers are not well formed. It was brought out above
(p. 610) that an isolated half of the body ought to be able to
Fie. 22.—a. Operation diagram. b. Piece which regenerated from A
in Diagram a. The new tissue is indicated by stippling.
give only one reaction, or, in other words, ought to be able to
turn the body in only one direction in response to stimulation,
provided this turning is due to an extension of the stimulated
side. We find precisely this result in the regenerating speci-
men just discussed. It turns away from stimuli applied at y
because on that side are present all the muscles necessary for
extension just as in a normal animal. It does not turn away
from stimulation of the side v because it has not the necessary
muscles for extension on that side. On the view that the
turning away is due to contraction on the side opposite that
stimulated, there is no reason why stimulation at « should not
cause the animal to turn away from the stimulus, because the
opposite side (y) has all its muscular mechanisms intact.
616 RAYMOND PEARL.
The reason why the specimen in this last experiment does
not turn towards the stimulus when stimulated on the side a,
is apparently because the regeneration has proceeded only far
enough to produce just enough new tissue to form the
beginning of a new side to the body. 'l'his new side receives
the stimulus and is sufficiently potent to determine the re-
action of the whole (the straight longitudinal contraction),
but is lacking in the mechanism necessary to produce its own
proper reaction, the negative reaction. On the other hand,
in the case of the individual with the split anterior end, each
piece turns towards the stimulus after stimulation of the cut
edge because here only one half the organism is present
either to be stimulated or to react; there is not even the
beginning of the formation of a new side along the cut edge.
Putting all the evidence together, I think it must be re-
garded as demonstrated that the turning away from the
stimulus in the negative reaction to mechanical stimuli is due
to an extension of the side of the body stimulated. ‘This
extension is brought about by the contraction of the circular
and dorso-ventral muscle-fibres—probably also assisted by
the transverse and oblique systems of fibres—in the region
stimulated. This reaction is a simple reflex act involving
only the side stimulated. The normal organism, so far as this
response is concerned, is to be considered as composed of two
identical, but in a certain sense independent longitudinal
halves. Thus, representing these halves diagrammatically,
asin Fig. 23, a, the evidence presented indicates that stimula-
tion of one side of the worm, as A, causes a reaction in that
side, and, so far as essential features of the directive reactions
go, only in that side. The movements of half A after its
stimulation determine and, in fact, cause the reaction of the
wholeanimal. Furthermore, these longitudinal halves retain
their individuality as halves if they are isolated from each
other. A separated half-worm (longitudinal) reacts as a
half-worm, just as it did when in connection with the other
half in the body, and not, as might perhaps be expected on
a priori grounds, as a whole worm. It reacts as a whole
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 617
worm only after a new half has been regenerated along its
cut edge. The various stages in the change from the
reactions as a half-worm to those as a whole worm can be
followed step by step as regeneration proceeds. he new tissue
formed along the cut edge very quickly takes on some of the
functions of a side. When only a narrow strip has been
formed it serves for the reception of the stimulus, and hence
stops the reaction of the opposite side, as in the experiment
last discussed. ‘To make the meaning more clear, reference
may be made to diagrams b and ¢ of Fig. 23. In b is repre-
sented, in a straightened position, the half B of anormal worm
a aads sa
sae iye seen erere re 1
ee ae er ee
b.
c
A
Fig. 23,—Diagrams to show the relations of the halves of the body
of Planaria to the reception of stimuli, and the reactions
thereto. See account in text. (The pharynx is omitted for
the sake of clearness.)
immediately after being separated from the other half, while
c represents the same half after regeneration has begun and
a strip of new tissue has been formed down the cut edge.
Now stimulation of the cut edge of b causes the anterior end
of the piece to turn towards the stimulus, 1. e. to give its
own proper negative reaction (cf. experiment given above on
shtting anterior end). This is because in this case it is
side B that is stimulated, although along its inner edge.
Stimulation along the right-hand edge of ¢ does not cause
the turning towards the stimulus, because in order that this
618 RAYMOND PEARL.
may take place it would be necessary for the side B to give
its proper negative reaction. It cannot do this because it is
not directly stimulated, but the new very small side A is
stimulated. ‘This side may not have the necessary muscles to
give a negative reaction itself—as in the experiment
described above,—yet may receive the stimulus and so
indirectly prevent B from reacting. Another way of ex-
pressing this same fact is by saying that in regenerating
longitudinal halves of planarians the physiological middle
line remains at the line of the former cut edge for some time
after regeneration has begun.' In connection with this
discussion of the reactions of half-animals it is greatly to be
regretted that Willey (97) did not get any data on the
reactions of the remarkable form Heteroplana. In this
form we have a natural “ half-planarian,” or very nearly that.
One side is so greatly atrophied as to be practically absent.
It seems to me very probable that this organism would react
to stimuli in much the same way that a longitudinally split
specimen of Planaria, which had begun to regenerate,
does.
I do not wish it to be understood from the analysis of the
negative reaction which has been given that I intend to
maintain that in this reaction the side opposite that stimu-
lated never contracts longitudinally. It probably often does
this, especially in cases of very strong stimulation which cause
a general excitation and reaction of the whole body. I have
merely wished to show that the fundamental basis of the
negative reaction is the extension of the side stimulated. It
seems to me quite possible that it may be shown by close
analysis in other cases that supposedly crossed reflexes are
not fundamentally such at all.
We may now pass to a brief consideration of the mechanism
of the positive reaction of the planarian to mechanical
1 I have records in my notes of experiments which show that in the case of
oblique cuts the physiological middle line remains at the cut edge until after
the new head is well formed in the new tissue on the oblique edge. Lack of
space forbids detailed description of these experiments here.
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 619
stimuli. As has been shown above, removal of the anterior
end of the body containing the brain causes the disappearance
of this positive reaction, and this result is probably due rather
to the lowering of tonus than to the removal of any special
centre having the causation of this reaction as its function.
Additional evidence on this view that lowering of the tonus
is the chief cause of the disappearance of the reaction is found
in the fact that other injuries to the head, such as longitudinal
splitting, which produce a lowering of the general tonus, also
cause the disappearance of the positive reaction.
This very close dependence of the reaction on the general
tonic conditions of the organism makes its analysis difficult,
but it seems most probable that its mechanism is as follows :—
a light stimulus, when the organism is in a certain definite
tonic condition, sets off a reaction involving (1) an equal
bilateral contraction of the circular musculature, producing
the extension of the body; (2) a contraction of the longi-
tudinal musculature of the side stimulated, producing the
turning towards the stimulus (this the definitive part of the
reaction); and (3) contraction of the dorsal longitudinal
musculature, producing the raising of the anterior end. In
this reaction the sides do not act independently, but there is
a delicately balanced and finely co-ordinated reaction of the
organism as a whole, depending for its existence on an
entirely normal physiological condition. It is to be noted,
however, that the definitive part of the reaction, namely, the
turning
for
This point is one of fundamental importance for the general
is a response of the side of the body stimulated.
theory of the reactions.
The mechanism of the other reactions to mechanical
stimuli are evidently very simple. The crawling movement,
which must be considered as the specific reaction to mechanical
stimulation of the posterior region of the body, is due to
rhythmical contraction of the longitudinal musculature. The
only other reactions to mechanical stimulation are local con-
tractions, whose mechanism is evident.
e. Features in the General Behaviour of the
620 RAYMOND PEARL.
Organism which the Reactions to Mechanical
Stimuli explain.—That much of the behaviour of pla-
narians in their natural surroundings is the result of the re-
actions above described is very evident to any one watching
them. Among specific features of this sort in which these
reactions play a part may be mentioned the escape from
enemies or harmful surroundings, the getting of food (to be
discussed in detail later), the localities chosen for coming to
rest, the behaviour on meeting solid obstacles in the path of
movement, the passing on to the surface film, ete. All
of these need not be discussed specifically, as their relations
will be evident enough on a moment’s thought, but the last
two deserve special mention.
The behaviour of planarians on meeting solid bodies in
their path in the course of movement is entirely made up of
reactions to mechanical stimuli. The behaviour in detail is
as follows :—If a gliding specimen meets squarely head-on an
obstruction of considerable size, so that it cannot glide over
it without changing to some extent the position of its long
axis, it will stop an instant, raise the head, let it drop down
till it touches the obstruction again, and then glide directly
up on to and over the solid body. ‘This behaviour is invari-
able, so far as my observations go, if the worm meets the
obstruction squarely. It is at once seen to be merely a
special case of the usual reaction to a weak mechanical
stimulus, characterised by the raising of the head. ‘The
behaviour is evidently purposeful in the long run, because it
will take the organism up on to food material just as well as
indifferent bodies, If the gliding worm meets the obstruc-
tion obliquely the behaviour depends in large part on the
physical nature of the cbject. If it is food material, or some-
thing else of a rather soft and yielding texture—as, for
example, another planarian,—the worm will immediately raise
the head, turn it towards the object, and crawl up over it.
This behaviour is evidently the typical positive reaction to a
weak mechanical stimulus. A special and rather curious
case of this positive reaction, which I have twice observed,
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 621
appeared when two specimens eliding along, with the anterior
ends shghtly raised in the normal manner, met head-on,
Both were simultaneously stimulated to the positive reaction
and raised the anterior ends, and then let them drop again.
As they came down the two ventral surfaces were brought
squarely together in the way shown in Fig. 24; then each
started gliding up the ventral surface of the other. In a
movement as a result of the constantly changing form of the
body, the ventral surfaces slipped off from one another and
the two worms went on their way. When the obstruction is
a hard body, asa piece of glass, the specimen meeting it
obliquely usually turns the head away slightly at the first
contact (negative reaction), and then glides along parallel to
the edge of the body fora distance. If it happens to touch
if again with the side of the head, it frequently gives the
negative reaction and turns away again. After the solid body
Fie, 24.—Side view of two planarians starting to glide up on the
ventral surfaces of each other.
has been touched several times, however, the positive reaction
is usually given, and the worm passes at once up on to the
solid body. This behaviour is shown in Fig. 25. The precise
form of the behaviour on meeting obliquely a solid body in
the path varies considerably with the general physiological
condition of the individual. In case it is much excited, the
first touch will induce a strong negative reaction, and the
individual will turn away and pass out of the neighbourhood.
In the cases where the final positive reaction is preceded by
two or three negative ones, it would seem as if repetition of
what must be an almost identical stimuius causes it to be-
come in effect weaker. Leaving aside all variations in the
exact character of the behaviour on meeting a solid, the
important point to be brought out is that all this behaviour is
based on the simple reactions to mechanical stimuli. The
VOL. 46, PART 4,—NEW SERIES. Ss
622 RAYMOND PEARL.
exact behaviour in any given case depends on _ several
different factors. These are the position of the animal with
reference to the obstruction, the physical nature of the
obstruction, and the physiological condition, whether of
ereater or less excitation.
So, again, with reference to the habit of the animal of
moving about on the surface film, a problem is presented.
When a specimen, gliding up the side of a dish, touches its
anterior end to the surface film at the point where the latter
joins the glass, it immediately gives a characteristic positive
reaction, precisely like that in response to any other weak
mechanical stimulus. The head is raised and turned towards
the side from which the stimulus came, and then dropped
5 6
Fic. 25.—1, 2, 3, 4, 5, and 6 are successive stages in the reactions of
Planaria on meeting obliquely an obstacle in its path. The heavy
straight line represents the obstacle.
again. As a consequence of this reaction, the head end
comes to rest on the under side of the surface film at a point
some little distance out from the side of the dish. The
ventral surface of the anterior end of the body flattens out on
the surface film, and the animal glides out on to the film,
following the direction determined by the reaction of the ante-
riorend. Thus itis seen that the going on to the surface film is
only aspecial case of a response to a weak mechanical stimulus,
i.e. the positive reaction, the film itself acting as the stimulant.
The leaving of the surface film and passing down the side of
the dish is evidently also due to the same positive reaction.
ra
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 623
There are a number of other points in the general behaviour
which are directly related to the reactions to mechanical
stimuli, which will be taken up later in connection with the
other reactions.
jf. Summary.—Before passing on to a discussion of the
next subject, it may be well to summarise briefly the chief
findings with reference to the effect of mechanical stimuli on
planarians.
It has been shown that the planarian responds in a well-
nigh perfect manner to the localisation and intensity of
mechanical stimuli. It turns away from strong stimuli (in
the long run harmful) applied to the side of the body ; turns
towards weak stimuli (in the long run beneficial, almost never
harmful) ; it crawls rapidly away from strong stimuli applied
to the posterior end; backs and turns away from similar
strong stimuli applied at the anterior end.
It has been shown, further, that these reactions have all
the characteristics of reflex actions, complex, it is true, but
still reflexes.
The mechanisms of the reactions to unilateral stimulation
are unilateral, and lie in the side stimulated.
Discussion of the implications of these results on
mechanical stimulation, with reference to the psychology of
the organism and the general theories regarding the reactions
of organisms to stimuli, is deferred till the results from other
sorts of stimuli are in hand.
Il. Reactions to Food and Chemical Stimuli.
Evidently one of the most important factors in the sum
total of the activities of any aquatic organism is its reactions
to chemical substances. Its ability to receive chemical
stimuli and react to them must be of prime importance in its
struggle for existence, for in its natural habitat such an
aquatic organism must be almost constantly encountering
different chemical substances. Some of these may be harm-
ful and some beneficial, and it would seem that if a species is
624 RAYMOND PEARL.
to survive, its individuals must have some sort of reaction
whereby they may avoid the harmful and take advantage of
the beneficial. In the case of planarians, the reactions to
chemicals seem to be of about equal importance with the re-
actions to contact stimuli in the general activities. Since the
reactions to food substances are a special case of the reactions
to chemicals in general, they may be discussed first.
a. Food Reactions.—The nature of the things used as
food by fresh-water planarians has been discussed already in
the section on ‘‘ Natural History,’
us here,
A typical case of the food reactions to a bit of crushed
> and hence need not detain
Fie. 26.—Diagram showing the successive stages in the normal food
reaction of Planaria. A represents a small bit of meat.
mollusc may first be described, to serve as a basis for the
account.!. Ifa piece of the body of Physa which has just
been extracted from the shell and crushed between the points
of a pair of forceps is placed in a small dish containing a
number of active planarians, it will result from chance alone
that some of the flat-worms must in course of time pass near
the food material. For a very short time after the food has
1 The food reactions of Planaria have been briefly described by Bardeen
(:01, a).
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 625
been placed in the dish specimens may pass very near it—
within two or three millimetres—without being affected in
any way. They simply glide straight by as if there were no
food there. After a few minutes have passed, however, it
will be found that a worm coming near the food is affected
in a very characteristic manner. Its behaviour is as
follows: —When within about three or four millimetres of
the piece of meat (Fig. 26, a) it stops abruptly, raises the
head, and turns it towards the food (Fig. 26, b). As the head
is raised and turned the gliding is resumed, and the head
being almost immediately lowered, the movement is directly
towards the food. ‘Thus far the reaction is evidently
precisely like the positive reaction to weak mechanical
stimuli, and so we may speak of it as the positive reaction to
food, the reaction being the same in the two cases, though
the stimulus differs. When the anterior end of the head
eee eA
Fie. 27.—Diagrammatic side view of Planaria to show the
*‘oripping” of a bit of food, A.
touches the food it flattens down upon it, and, if the con-
figuration is such as to make it possible, “grips” ib
(Fig. 26,c). The details of this “ gripping ” (shown in side
view in Fig. 27) are as follows:—The anterior end closes
down over the very edge of the piece of food, or over the
whole piece provided it is small enough, and then
apparently squeezes it by contraction of the longitudinal
muscles on the ventral surface of the head. The action is
very characteristic, and evidently forms an integral part of
the normal food reaction. Its probable function will be
brought out later. While it is taking place the worm as a
whole stops its progressive movement and remains quiet.
After the “ gripping” has continued for some time the worm
starts gliding ahead up on to the food. It passes forward
till the point where the opening for the extrusion of the
pharynx is located is approximately over the place pre-
626 RAYMOND PEARL.
viously “gripped” (Fig. 26,d). Then the pharynx is ex-
truded and feeding begins (Fig. 26,¢). After a time the
worm voluntarily leaves the food and glides off over the
bottom.
Having described the typical case of a food reaction, we
may take up some of the more important variations from the
type, and describe the various phases in the reaction in
vreater detail.
Starting with the very beginning of the reaction, it may
be said that the distance from the food at which any effect
on the planarian is produced varies greatly, as is to be
expected. This distance, of course, depends on the extent
which the juices or chemicals of the food have diffused from
it. When a piece of meat is first put into the water
specimens will pass very close to it without being stimulated.
In fact, if a specimen finds a piece of food within three or
four minutes after it is put into the dish, it wil usually have
done so as a result of accidentally coming in contact with it.
As has been brought out above, when a gliding worm
touches anything of a rather yielding texture, like food, it
immediately gives the positive reaction and passes up over
it. This plays an important part in the getting of food,
because, as I have found in experiments, unless the food is
crushed and pressed with forceps the juices diffuse rather
slowly, and for some time specimens will not give the
positive reaction unless they actually touch the food. On
the other hand, after the food has been in the water for
some time, so that diffusion has taken place, the distance at
which specimens may be affected becomes quite considerable.
I have seen specimens gliding by a small piece of meat at
a distance of 1} cm. from it give the positive reaction and
turn towards it. At greater distances than this food is not
effective, according to my observations. ‘The distance from
food at which a given specimen will give the positive
reaction and go towards it depends also on the physiological
condition of the individual. Specimens in a state of general
excitation will, as I have frequently observed, go closely by
MOVEMEN'S, ETC., OF FRESH-WATER PLANARIANS. 627
a piece of food without turning towards it, while other
Specimens in a more normal condition will give the positive
reaction some distance from it.
After the first specimen has begun feeding on a piece of
material the zone of influence of that piece becomes almost
immediately widened appreciably. As the number of feed-
ing specimens increases the area in the surrounding water
which affects others becomes correspondingly greater. ‘This
phenomenon is very striking in many cases, as an illustration
will indicate. Several pieces of crushed snail were put in a
dish with a number of planarians. In a short time a
specimen in gliding about the dish had come near to one
of these pieces, had given the positive reaction and begun
feeding. At almost the same time another of the pieces of
food had “attracted ”’ another specimen. The other bits of
food were quite similar in every way to these two, and lay in
the dish not far from them. Yet at the end of fifteen
minutes the two pieces by which the first two worms had
been affected were completely covered with feeding
specimens, while the remaining pieces of food, with a single
exception,! did not have a specimen on them. ‘This increase
in the effectiveness of the food as a stimulus must be due
to the diffusion of more chemical substance from it.
Apparently the increase is due either to some secretion of
the feeding animals or to some change which they induce in
the food. It is probably due to a combination of these two
factors. That a digestive secretion is poured out through
the pharynx of the feeding worm is well known, and clearly
shown by the appearance of a piece of food on which a
specimen has been feeding. ‘The surface of the meat is
turned white, and rendered very soft and almost flocculent.
It is probable that this digestive secretion acts as a positive
chemotactic stimulus to other worms, and that coupled with
this there is an increased diffusion of juices from the food
itself caused by the changes which it is undergoing.
The reaction which is caused by this chemical stimulus
’ One piece farthest removed from the others hada single specimen on it.
628 RAYMOND PEARL
from the food is evidently essentially the same thing as the
positive reaction given to weak mechanical stimuli. It con-
sists in a turning of the anterior end of the body towards
the source of the stimulus. There is no reason for supposing
that its mechanism is in any way different from that of the
same reaction to mechanical stimuli, and hence this need not
be further discussed here. A question of prime importance
with regard to this positive reaction in response to chemical
stimuli, which was not taken up before, is— how well localised,
with reference to the stimulus, is the reaction ? or, in other
words, how precisely does the anterior end point towards the
source of the stimulus,—in this case food? Have we here a
clear-cut orienting response? In answer to this problem it
may be said that when the worm is only a short distance
from the food the response is very precise. The anterior
end is brought by the first positive reaction so as to point
exactly towards the meat, and as the worm glides ahead it
never misses it. This is true where the specimen is near
enough (usually within three quarters of its own length), so
that the stimulus which reaches it is a fairly strong one.
In case the worm is stimulated near the edge of a large
diffusion area when the stimulus is very weak, the first
reaction may not suffice to direct the animal straight towards
the food. In this case the behaviour is usually like that
shown in Fig. 28, in which the line B, B, B, represents the
effective margin of the diffusion area of the piece of food A.
(By “ effective margin” is meant the line outside of which no
effect is produced by the food on passing specimens.) The
first reaction which the worm gives on reaching this diffu-
sion area (Fig. 28, 1 and 2) is a weak positive one. It then
proceeds on the new path into this area, but not directly
towards the food. After a short time, however (Fig. 28, 3),
it is again stimulated to a positive reaction (4). ‘This time
both the stimulus and the reaction are stronger than before,
and the worm is directed more nearly towards the centre of
diffusion, but still not exactly. When it gets opposite the
food again (5) another positive reaction (6) is given, and this
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 629
time, since the stimulus is a rather strong one, the reaction
is a very precise one, and the subsequent movement carries
the animal directly to the food (Fig. 28, 7). This behaviour
is typical for this sort of stimulation, but may vary in its
component phases, depending on the relative strength of the
stimulus—the distance from the food when first stimulated.
et ee
~_
~
_-
- ~
~
3S
Fig. 28.—Diagram showing the reactions of Planaria to food (A)
from which juices have been diffusing into the water for some time.
B, B, B, represent the effective margin of the diffusion area of the
food A. 1, 2, 3, 4,5, 6, and 7 are successive positions taken by
the organism.
Thus either two or as many as four positive reactions may be
necessary to bring the animal to the food. This shows
clearly that with reference to chemical stimuli, the precision
of localisation of the positive reaction decreases as the in-
tensity of the stimulus diminishes. Indeed, I have observed
what is evidently an unlocalised positive reaction, although
RAYMOND PEARL.
this seems paradoxical. The behaviour was as follows :—A
large diffusion avea had been formed, and a specimen was
stimulated to a weak positive reaction at a distance of about
twice its own length from the food (Fig. 29, 1). It passed
into the diffusion area, but did not give another positive reac-
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Fic. 29.—Showing the reaction of a planarian to a very weak food
stimulus. Letters as in Fig, 28.
tion when opposite the food, but instead glided by and away
from it. When it had gone some distance in this direction
it stopped and gave a very clear and characteristic positive
reaction, so far as the form of the reaction indicated, but
with the turn away from instead of towards the centre of
MOVEMENTS, E'TC., OF FRESH-WATER PLANARIANS. 631
diffusion. ‘There was no doubt of the character of the
reaction; the head was raised and the body turned in the
usual manner of the positive reaction, which one can never
mistake after once having become familiar with it. The
specimen kept on in the path determined by this last
reaction (Fig. 29, 4), and passed entirely out of the region of
the food. Evidently in this the worm was stimulated very
weakly by a chemical, and the stimulus was nearly as strong
on one side of the body as on the other, and when the reflex
was set off it was on the wrong side of the body. This is
not the usual result of weak stimulation, and has been
observed in only two cases, but it serves very well to show
the decrease of the power of localisation when the stimulus
is very weak.
When, as frequently happens, the worm approaches the
food exactly head-on, the reaction usually consists merely
of that portion of the reflex expressed in the raising of the |
head, while the worm keeps on in its straight path till it reaches
the food. The head may be waved from side to side slightly,
but the general direction of motion is not changed. The
action evidently corresponds to the positive reaction following
weak mechanical stimulation of the dorsal surface of the head
in the middle line, as described above. In some cases, how-
ever, I have observed very active and hungry specimens of
Dendrocelum, sp., which were going straight towards the
food, give a complete positive reaction and turn to one side
and start off in a new direction away from the food. This,
however, of course brought the specimen at once into a
position where the stimulus was acting unilaterally, and it
again gave a positive reaction, this time heading it again
for the food, which it usually reached without further
reaction. But in some cases I have observed the specimen
give so strong a reaction as to be taken almost directly
away from the food by the subsequent movement, and,
passing out of the area of diffusion, fail to reach it at all.
Specimens behaving in this way were “‘wild” in their
general reactions. ‘I'he responses were very vigorous, but
632 RAYMOND PEARL.
not localised with reference to the stimulus with the usual
precision.
The “ gripping” of the food substance by the anterior part
of the worm is a very characteristic feature of the normal
food reaction. Its exact form depends on the configuration
of the food or other body “ gripped.” In its most typical
form, where the food material 1s in the form of a cylinder, or
approximately such, the action reminds one of the action of
the human hand in grasping a stick. The tip of the head
closes over the material in the same way that the fingers do,
while the region just behind the auricles bears the same
relation as does the proximal part of the palm, just in front
of the wrist, in grasping. After the head has been placed
over the material in this way it can be seen to contract
rather strongly, and thus literally squeeze the food. In case
the surface contour of the food does not admit of this reflex
being carried out in its typical form, as close an approxima-
tion to this is made as possible. ‘To compare again with the
human hand, when the surface is flat, or forms the surface of
a cylinder of large radius, the ventral surface of the head is
pressed closely to it, the tip attempting to dip in, as it were,
below the surface, in just the same way that a man “ claws”
with his finger tips in attempting to obtaim a hold on a
similarly configured body, too large for complete grasping.
While the “ gripping” is in general a very characteristic
feature of the food reaction, it may be omitted in rather
exceptional cases. he cause for the omission where it
occurs, or any laws governing the matter, I have not been
able to discover. A necessary accompaniment of the
> of the food is the cessation of the forward
movement of the animal as a whole. This pause when the
“ eripping’
food is first touched by the anterior end and before the worm
passes up on to it, occurs in practically every case, whether
the gripping accompanies it or not. The length of the pause
is, of course, considerably greater when the “ gripping ”
occurs than when it is absent.
The function of the “ gripping” of the food material before
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 6383
feeding begins is not immediately apparent, but I am inclined
to think its purpose is to intimately test the substance with
regard to its availability as food. Some evidence on this
point and further discussion regarding it will be introduced
later.
After the preliminary pause and “ gripping” of the food
the worm glides up on to it to begin active feeding. The
position taken by the worm brings out a very nice correlation
in reflexes. In a very large number of cases (certainly over
75 per cent., so far as my observations haye gone) the worm
advances over the food until the pharyngeal opening is
exactly over the place where the first “ gripping” occurred,
and there the pharynx is extruded and feeding begins.
Fic. 30.—Diagram showing great extension of the pharynx. The
stippled area represents food substance on which the planarian
1s resting.
When the worm reaches this position the posterior part of
the body relaxes and takes on the appearance character-
istic of the resting specimen. The pharynx is thrust out,
and becomes attached very quickly. As it passes out through
the opening in the body-wall it becomes usually considerably
extended, and its diameter becomes correspondingly smaller
than when it is in the pharyngeal sac. It may or may not
attach to the food directly beneath the body. When con-
ditions are favourable it usually does, and consequently
canuot be seen on looking down on the animal from above.
On the other hand, I have frequently seen it stretched out
and attached some little distance to one side of the body, as
shown in Fig. 30. The stimulus, causing the extrusion of the
634. RAYMOND PEARL.
pharynx, is the contact of food or other solid body with the
pharyngeal region of the ventral surface, together with an
appropriate chemical stimulus. The pharynx is not extruded
until the animal gets up on to the food so that the opening of
the pharyngeal sac is in direct contact with it. This can be
demonstrated by direct observation by the use of a very
small piece of food material and a plane mirror placed
beneath the glass dish in which the specimen is moving. By
lifting gently the posterior end of the body on a needle it can
also be seen that the pharynx is not extruded before it is over
the food. The most striking illustration of the correlation in
the reaction which brings about the extrusion of the pharynx
when it is just over the food, is to be seen when a specimen
of the nemertean Stichostemma asensoriatum is used as
food, and the long axis of the planarian and of the nemertean
are at right angles to each other. After first “ gripping”
—@ ee,
a x b
Fig. 31.—Diagrammatic longitudinal section of a planarian feeding
on a nemertean (shown in cross-section at x ).
the nemertean the planarian glides along over it until the
pharyngeal opening is just above it, and then pauses, and the
pharynx is extruded and attached (a and b, Fig. 31). ‘hese
facts strongly indicate that the effective stimulus for pharyn-
geal extrusion is received, at least in part, in the pharyn-
geal region itself. That it is necessary for both contact
and chemical stimuli to act to produce the extrusion of
the pharynx may be shown by experiments on specimens
gliding on the surface film ventral side uppermost. If, with
such a specimen, a chemical known to produce under other
conditions extrusion of the pharynx, is allowed to come in
contact with the pharyngeal region, there is no result. Of
course in performing this experiment proper precautions
were taken not to disturb the animal by allowing the solution
todrop uponit. Another demonstration of the same fact that
a chemical stimulus alone does not suffice to cause extrusion
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 635
of the pharynx is that specimens immersed in favourable
solutions, such as sugar solutions, do not show this phe-
nomenon. ‘That mechanical stimulation alone does not suffice
is demonstrated by the fact that planarians pass over and rest
on other planarians without extruding the pharynx, although
the consistency of their bodies is evidently much the same as
that of the animals used as food. In fact, they will be
used as food frequently if they are wounded so as to afford
the proper chemical stimulus. ‘The stimulation of the anterior
end of the body by the food seems also to be necessary before
pharyngeal extrusion takes place. The data on this point will
be presented later in connection with operation experiments.
The appearance of the body on the food is quite charac-
teristic. As mentioned above, when the pharynx is extruded
forward, movement stops, and the posterior part of the body
becomes more or less relaxed. The anterior third of the
body, however, keeps in movement during a considerable
part of the time the specimen is feeding. The head is waved
about from side to side, and touched to the food or the
bottom of the dish here and there. It keeps its character-
istic extended form to a greater or less degree. A favourite
position is for the anterior third or half of the body to lie on
the bottom and move about, while the posterior part lies up
on the food. This is the position most frequently seen in
specimens feeding on a rather small piece of meat. When
the anterior end gets on the bottom it gives every appear-
ance, in many cases, of attempting to glide away, and being
only restrained by the attachment of the pharynx to the
food. In other cases, however, the anterior end remains
quiet. The importance of the attempted movement will be
brought out later. As has been mentioned above, the flat-
worm is able to move off and drag the food still attached to
the extruded pharynx along behind it. In the fastening of
the food to the body in this case the sticky slime undoubtedly
assists the pharynx.
After the food has been softened by the digestive juices,
it is taken into the body through the pharynx.
636 RAYMOND PEARL.
After the worm has been feeding for a certain length of
time it will detach the pharynx and spontaneously move off
from the food, the pharynx being withdrawn again into its
sac. The length of time after the beginning of the feeding
at which this takes place varies very greatly in different
cases. I have observed a specimen which fed on a piece of
molluse for as long as an hour and thirty minutes, while in
other cases the worm may stay on the food only ten minutes,
or even less. Judging from the rate at which food is taken
up while the animal is feeding during the day, and from the
fact that pieces of meat left in the dish overnight are almost
entirely consumed by morning, it would appear that much of
the time during the mght is spent in feeding when any
material available for the purpose is at hand. While the
anterior end of the feeding worm retains its normal sensi-
tiveness to stimuh, it nevertheless requires considerable
stimulation to induce a feeding worm to leave the food.
Shaking of the dish, which would ordinarily set all resting
specimens into rapid movement, has little or no effect on
feeding specimens. If a worm is suddenly pulled off a piece
of meat on which it is feeding a very good view of the
extruded pharynx may usually be had, as this organ is
retracted somewhat slowly when torn from food in this way.
So far as I have been able to discover, the presence of
food in the immediate neighbourhood of a resting planarian
has no effect upon it. Apparently the stimulus afforded by
crushed meat is not sufficiently strong to produce a response
from such an individual. ‘The following experiment copied
from my notes will show this.
May 14th, 1901, 3.10 p.m.—A piece of freshly crushed
snail was placed 1 mm. distant from the anterior end of a
resting specimen. No reaction or other effect produced.
3.30 p.m.—Worm in same position as before.
4.5 p.m.—No change. (At this time the worm was acci-
dentally started into movement and the experiment conse-
quently ended.)
This lack of effect of food on resting specimens may be
-
MOVEMENTS, E'C., OF FRESH-WATER PLANARIANS. 637
the reason for the statement of Bardeen (loc. cit., p 522) “ that
worms which had been kept in pure rain water for a week or
two, and were thus in a hungry condition, would remain
unmoved by the presence close by their side of a piece of
fresh snail, a food much prized by them.”
1. Food Reactions of Specimens after Opera-
tions.—For the purpose of throwing light on the general
mechanism of the food reaction, experiments were tried on
specimens cut in different ways. It is unfortunately very
different from practical reasons to get many certain results
from these experiments. Many of the results are negative,
and hence not entirely conclusive. Since, however, some
important facts have been brought out by these experiments,
they will be described.
The first operation which will be discussed is that of
cutting the animal in two transversely. If such a cut is
made in the region in front of the pharynx, the anterior
resulting piece, after it has recovered somewhat from the
shock effect of the operation, will show the following reac-
tion. On coming into the zone of diffusion about a piece of
meat it gives the positive reaction just as a normal worm
does, and turns towards the food. On reaching the edge of
the meat its behaviour is again like that of the normal
animal; it stops, usually “grips” the food, and then passes
on over it. At this point appears the striking difference
between the behaviour of this anterior piece, which, it must
be remembered, has no pharynx, and the behaviour of the
entire worm. The anterior piece after gripping the food
glides up over it, and without the slightest change, even in
the rate of gliding, passes down off of it on the other side.
There is not the slightest indication of any stopping for the
pharynx to be extruded.
If the transverse cut is made farther back, so that the
pharynx is included in the anterior piece, this will then
behave with reference to food quite as a normal animal does.
It will stop on the food and extrude the pharynx.
The posterior pieces resulting from transverse cuts do not
VOL. 46, PART 4,—NEW SERIES, gy
388 RAYMOND PEARL.
give any definite food reaction, so far as I have been able to
ascertain, until they have been regenerated to some con-
siderable extent. Posterior pieces from which only the head
has been cut will glide by pieces of snail on which other
worms are feeding, without giving the slightest reaction.!
In experiments so arranged that the gliding posterior piece
would just touch with its anterior end the edge of a piece of
food, it gave no reaction. This same arrangement with a
normal worm practically never fails to call forth the positive
reaction and bring the worm up on to the food. Posterior
pieces placed gently on pieces of food material do not extrude
the pharynx and start feeding, but immediately e@lide down
froin it and over the bottom of the dish. These experiments
with posterior pieces have been tried many times and under
varied conditions, in the hope that some sort of positive
results might be obtained, but never with success. This is
true for three days after the operation. After a new head
has been fairly well formed the animal will react to food
again. ‘The behaviour of one of these posterior pieces on
touching with the anterior end a piece of food is very
strikingly different from that of a normal animal. The cut
piece, if it touches with the front or sides of the anterior end
the smallest shred of food material, or any other substance,
gives a well-marked negative reaction, and goes in a new
direction away from the obstruction. It does not, as a rule,
crawl up over anything which it meets squarely “ head-on,”
but instead turns away.
Thinking that possibly the pharynx might play a more or
less independent part in the normal food reaction, i. e., that
it might have a set of reflexes of its own, not determined by
the rest of the body, I tried experiments with the isolated
pharynx removed entire from the body. Such an isolated
pharynx will remain alive for a considerable period, and
respond to stimulation. When first removed from the body
' Bardeen (:01, a) has shown that if the transverse cut is in the region in
front of the eyes the posterior piece (comprising in this case nearly the whole
worm) will react normally to food,
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 639
it contracts rhythmically in a longitudinal direction for a
time, and then comes to rest at about its normal leneth when
in the body. Mechanical stimulation causes merely longi-
tudinal contraction, while the presence of food near it has no
effect whatever. Freshly crushed snail meat placed within a
millimetre of such an isolated pharynx had no effect upon it
in the course of an hour. I have tried laying the isolated
pharynx directly on pieces of meat to see if there would be
any tendency for the end of the organ to attach itself as it
normally does. This was not done, nor was any other
definite reaction produced.
These operation experiments show, so far as they go, that—
(1) The presence of the pharynx in the body (i.e., the
functional] ability to take food) has nothing to do with deter-
mining the reaction of the anterior end of the body to food
stimuli. The anterior part of the body gives the same re-
action to food in every case, without regard to whether so
doing actually puts the animal ina position to get food or
not. The reaction is only purposive under certain circum-
stances; when changed conditions make it no longer purpo-
sive, no adaptive change in the behaviour of the anterior end
occurs. This shows clearly how little basis there is for con-
sidering the behaviour towards food as anything of the
nature of intelligent behaviour,
(2) The stopping of the worm on the food under normal
circumstances is due to the posterior half of the body, not the
anterior. The behaviour of the anterior cut piece in gliding
directly over the food is what one might be led to expect from
the behaviour of the same part of the body under normal
circumstances. As described aboye, it was seen that the
anterior end of the normal individual gives every appearance
of attempting to continue moving forward while the posterior
part is feeding, and is only prevented from doing this by the
mechanical hindrance of the attached pharynx. In a sense,
we may consider that in a large degree the work of the
anterior end of the body with reference to feeding is over
when it gets the animal up on to the food.
640 RAYMOND PFARI.
(3) The reception of the food stimulus is a function of the
head. In other words, the head is the only part of the body
capable of receiving very weak chemical stimuli.
(4) Decapitated specimens do not extrude the pharynx, so
far as my observations go, even though the proper normal
stimuli are given the pharyngeal region. Presumably the
brain is the necessary organ in this connection, as we have
already seen that the sense organs concerned with the act of ex-
trusion are not those of the head, but of the pharyngeal region.
Bardeen (: 01, a, p. 178) states that ‘the simple reflexes of
extending the pharynx and of swallowing are preserved after
removal of the head. I found, by repeated trials, that one of
the headless pieces could usually be made to eat if it was
placed on its back on a shide in a small drop of water. Under
the conditions mentioned the pharynx is usually protruded,
and will engulf bits of food placed in the mouth.” Regard-
ing this conclusion, I can only say that in a large number of
experiments with decapitated specimens I have never been
able to induce extrusion of the pharynx, under conditions
approximating as closely as possible to the normal. I do not
wish to affirm that the decapitated planarian cannot extrude
the pharynx, but merely that it does not when placed in
situations which normally produce pharynx extrusion.
(5) The pharynx is not an independent organ in its reactions,
since, when separated from the body, it does not react with
reference to the localisation of the stimulus, as it does when
normally connected with the remainder of the body.!
2, Summary of Food Reactions.—It is shown above
that planarians have a very definite and characteristic set of
reactions to food substances which enable them to become
aware of the presence of food, and find it. The importance .
of these reactions in the life of the individual can hardly be
over-estimated. While planarians, like many other lower
organisms, can live for a considerable time without food, yet
in the long run they must, of course, have it. The question
1 Evidence on this latter point will be brought forward in connection with
the reaction to chemicals,
MOVEMEN''S, ETC., OF FRESH-WATER PLANARIANS. 641
of how a lower organism gets its food, taking advantage of
the good and rejecting the bad, and thus apparently choosing
one thing from several, is one of the most interesting and
important in comparative psychology.
The food reaction of planarians consists of an extremely
well co-ordinated set of reflexes, which may be set into action
by stimuli of two sorts,—first, chemical; and second,
mechanical. Both sorts of stimuli are, of course, given by
the food. he first and most important of all the reflexes in
the food reaction is the turning of the head towards the
source of stimulation, followed by movement in that direction.
This is the reaction which enables the animal to find food.
Evidently it is the same thing exactly as what has been
described as the positive reaction to mechanical stimuli; or,
in other words, the positive reaction to mechanical stimuli is
only a special case of the general food reaction. Its primary
function is evidently the getting of food, whatever the stimulus
which calls it forth. The reason for a food response following
mechanical stimulation is to be found in the fact that it most
frequently happens that many things (e. g., whole animals)
which are available for food are not emitting chemical sub-
stances into the water in sufficient quantity to cause an
effective stimulus. If the planarian did not give a positive
reaction after contact with such bodies they would be missed,
and no advantage taken of them as food. By reacting
positively to weak mechanical stimuli the animal is in a
position to take advantage of the presence of food of all sorts,
whether it is in condition such as to diffuse chemical sub-
stances through the water or not. This fact that the animals
react to food substances as a result of mechanical stimulation
affords a possible explanation of the “ gripping” phase of the
general response. ‘The purpose of this ‘ gripping” may be
to bring the sense organs of the head, which are sensitive to
chemical stimuli, into very close contact with the substance
in order to determine whether it possesses the chemical
characteristics of food. In other words, this reaction is a
2
“tasting ” reaction, which is made necessary by the fact that
642 RAYMOND PEARL.
the organism turns toward all bodies of a certain physical
texture under most circumstances. The active squeezing of
the material in the “gripping” undoubtedly helps to press
out to the surface any juices which may be in the material.
In closing the section on food reactions it may be well to
give a sort of general picture of the whole behaviour of
fresh-water planarians towards food. The method by which
the planarian finds material suitable for food is as follows :
1. Chemical substances diffusing from food come in con-
tact with the sensitive head region of the planarian ; or—
‘The moving animal touches with the head some soft sub-
stance, and as a result of either of these two sorts of
stimulation—
2. The organism gives a positive reaction, i.e. turns
towards the source of the stimulus. This reaction is very
precisely localised in most cases, and is the most essential
part of the whole food behaviour. Its mechanism has been
previously described (v. sup., p. 619).
3. When the anterior end squarely touches the food as a
result of this reaction it typically closes tightly over it,
giving what I have called the “gripping” reaction. This
reaction is evidently a very much specialised feeling move-
ment for the purpose of closely testing the chemical nature
of material. It is produced by a contraction of the ventral
longitudinal muscles of the head region, While it is taking
place progressive motion ceases.
4. After this pause the worm glides over the piece of food
till the opening of the pharyngeal sac lies over or nearly
over the place “ gripped,” and there the posterior part stops
and the pharynx is extruded and attached to the food. 'The
factors determining the place where the pharynx shall be
extruded are (a) the stimulation of the ventral surface of
the body in the pharyngeal region of the food (pure reflex
factor), and (b) the presence of the brain, which probably
acts as a co-ordinating centre for this reaction,
5. A digestive fluid is poured out through the pharynx,
and the food is partly digested before being taken up.
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 645
6. The softened food is taken into the body through the
pharynx.
7. The animal spontaneously stops feeding after a certain
time.
The question now arises, if the normal process of getting
food is at bottom in the majority of cases a reaction to a
chemical stimulus, what is the nature of the chemical sub-
stance causing it? Can the same response be induced by
the use of different inorganic and organic chemicals? Is
there any relation between chemical composition and the
intensity or form of the reaction? ‘To answer these and a
number of other questions arising out of them recourse must
be had to experiments in which the nature and concentra-
tion of the chemicals affecting the organisms may be con-
trolled. All the experiments of this kind I will group
together under the heading—
b. Reactions to Chemical Stimuli—Chemotaxis.
1. Reactions to Localised Chemical Stimuli.—
This particular phase of the general subject of the effects of
chemicals may be considered first, since it is most closely
related to what has preceded on the food reactions. The
plan of the experiments was to try the effect of a series of
substances when applied to restricted areas of the body. A
sufficiently large number of chemicals were used to include
representatives from each of the main groups of substances
which have been found to have marked effects on organisms.
a. Methods.—The method which was found to give the
most satisfactory results in the application of localised
chemical stimuli was the use of a capillary tube filled with
the solution whose effects it was desired to test. The form
of the tube used is shown in Fig. 32. The tubes were 10 to
15 cm. long, and were made from glass tubing of about
2°5 mm. internal diameter. Hach end was drawn to capillary
fineness, and then broken off so as to give an opening of the
desired size. ‘The opening at the upper end was made
644. RAYMOND PEARL.
slightly larger than that at the lower, which was used in
giving the stimulus. The tube was filled with solution by
suction. ‘lhe rate of diffusion can be regulated by changing
the sizes of the openings, and can be determined for each
tube from the rate at which the fluid sinks at the upper end
of the tube. Considerable experimenting is necessary in
order to get the best rate of diffusion for work on planarians.
Since the animal is moving rather rapidly while the stimulus
is being applied it is necessary to have reasonably rapid
diffusion or the worm will not react at all, or not for so long
a time after the stimulation has begun that one cannot be
certain of the results. It is easily possible to get the
capillary so fine that no results can be obtained. On the
other hand, when it is too large the solution affects too large
a portion of the body at one time, and furthermore, as will
be shown later, may cause a rheotactic reaction of the
organism. ‘lhis, of course, introduces a possible source of
Vic. 32.—Glass tube used in giving localised chemical stimuli.
serious error. It can be avoided by frequent and proper
control experiments.
It will be well to describe in advance the conduct of a
typical experiment and the precautions taken, so that it may
not be necessary to repeat these details in the account of
each experiment. Six to ten normal active planarians were
taken from the aquarium dish and put in a Petri dish of
about LO cm. diameter, in freshly drawn, filtered tap water.
Kuough water was put in the dish to give a depth of about
lcm. ‘lwo or three of the capillary tubes with different
sized openings were filled with the test solution. ‘These
tubes were all tested before a final experimental series was
begun, and usually only one which had been found to allow
diffusion at the satisfactory rate was used. In some cases,
however, varying degrees of sensitiveness among the
different specimens made it necessary to use for some in-
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 645
dividuals capillaries of faster or slower rates than what may
be called the standard. After preliminary experiments to
determine the relative sensitiveness of the different parts of
the body to chemicals, attention was devoted almost entirely
to stimulation of the head region, and consequently in the
experiments which will be reported first the stimulus was
applied only to the head, unless otherwise stated. The
method of applying the stimulus was to place the point of
the capillary tube a short distance (about 2 mm.) from the
place on the body to be stimulated. ‘The animal was stimu-
lated as it was gliding along in the normal way, and hence
it was necessary to move the capillary tube at the same rate
the animal moved in order to keep it opposite the same point
in case the reaction was not given at the instant the capillary
was put into place, which, of course, almost never happens.
With a little practice one can move the tube along as the
worm glides so as to keep the relative position of the two
almost identically the same. Just as soon as a reaction had
been obtained with a given specimen the capillary tube was
removed from the water, so as to permit as little as possible
of the chemical to get into the water surrounding the
organism. After a series with any substance, the worms
were transferred at once to a dish of fresh water before
beginning another series. Further, in any long series, when
for any reason it might be supposed that the water was
becoming contaminated with the chemical to an extent
sufficient to affect the results, the worms were transferred to
another dish of fresh water. All through the course of an
experiment frequent control tests were made by trying the
effect on the worms of the water surrounding them when
diffusing out from the same tube used previously for the
chemical. After each experiment the tubes were thoroughly
rinsed by drawing distilled water back and forth through
them many times. ‘lhe tubes were also frequently discarded
and new ones substituted.
646 RAYMOND PEARL.
The following substances were used in the experiments :
(esas
Mineral acids . +» Hydrochloric
| gakphat:
(eae
Citric
Formic
Organic acids
{Sodium hydrate
Alkahes : ; ;
| Sodium carbonate
Salts of heavy metals | COPPer SEAS
: | Zine sulphate
Bane chloride
Sodium bromide
Other salts . rer : ‘
Potassium chloride
Magnesium chloride
Cane-sugar.
Distilled water.
Since distilled water was found to have a decided effect in
producing a reaction, the solutions were prepared in both
distilled water and in filtered tap water. In case of any
doubt, as with very dilute solutions, the effects of the solu-
tions prepared in each sort of water were tested and
compared.
Since only qualitative results were desired, and for the
practical reason of greater convenience, percentage rather
than molecular solutions were used.
(3. Results.—The results are, in a way, so remarkable that
they will be presented in some detail.
Mineral Acids.
Nitric (sp. gr. 1:42), + per cent.—This solution causes
strong negative reaction, If applied to the head region the
animal turns away from the side stimulated immediately, and
strongly. If the stimulus is long continued the animal
writhes and twists about violently.
Stimulation of the posterior region causes the part where
MOVEMENTS, E'TC., OF FRESH-WATER PLANARIANS. 647
the solution strikes to contract very violently, and the whole
animal to start crawling ahead rapidly. This concentration
is very injurious, and if its action is continued, quickly kills
the individual. It will be noted that its effects are the same
essentially as those of strong mechanical stimuli applied to
the same parts of the body.
ify per cent. and 54, per cent.—Results the same as in +
per cent. The animal is not as quickly and extensively
injured by these solutions as by the former. It is to be
noted that with these comparatively strong solutions the
reaction time after stimulation of the posterior end of the
body is so slow that this part of the body is permanently
injured or destroyed before the animal gets away.
zy per cent.—In some cases a well-marked positive re-
action was caused by stimulation of the head region with
this solution. The head would turn towards the mouth
of the pipette in the characteristic fashion of the food reaction,
or the reaction to weak mechanical stimuli. In other in-
dividuals the reaction given was weakly negative, while still
other specimens were indifferent. In cases where there was
an indifferent reaction there was a local contraction of the
side of the head stimulated.
sy per cent.—Clearly marked positive reaction in large
majority of cases after the stimulus has acted for some
time. ‘This solution never caused the negative reaction. Some
individuals were, in a few cases, indifferent to this solution.
This solution is too weak to start a resting specimen into
movement.
chy per cent. and weaker.—Indifferent reactions or weak
positive.
This acid appears to be a very strong stimulus for the
negative reaction in conceutrations down to 34, per cent.,
while below that it is a rather ineffective stimulus, and the
reaction when induced is positive.
Hydrochloric, ;4, per cent.—Strong negative reaction.
There is noticeable in some cases a teudency for some
individuals to turn very slightly towards the source of
648 RAYMOND PEARL.
stimulation before giving the strong negative reaction.
Stimulation of the anterior end of a decapitated specimen
caused a slow negative reaction with long reaction time.
This solution causes the change from the glide to the crawl
when applied to the posterior end of a normal worm.
zy per cent.—Negative reaction; rather weaker than with
preceding solution. With this solution one specimen would
turn towards the source of the stimulus until the head came
into the strong acid near the mouth of the pipette, and then
give the sharp negative reaction.
qo per cent.—Specimen A gave positive reaction in every
case ; specimen B in about 50 per cent. of all cases, while
in the remainder of trials gave weak negative. Other
specimens negative reaction.
sy per cent.—Specimen A as in preceding case. Specimen
B gave positive reaction in about 90 per cent. of all trials.
Other specimens weakly negative reactions.
tuo per cent.—All specimens give well-marked positive
reaction. They glide up to the end of the capillary and
“orip”’? if with the anterior end as in the food reaction.
After holding on for a moment they let go and give a sharp
negative reaction, indicating that the stimulus is still too
strong when continued. ‘his behaviour will indicate the
machine-like character of the positive reaction.
sxo per cent.—In the majority of cases indifferent re-
action. Remainder positive.
to give an idea of the dependence of the reactions to
chemicals on the physiological condition of the organism, the
following series of experiments with HCl in solutions of >45
per cent. and weaker concentrations may be described. It is
to be understood that these experiments were carried out
on different animals from those just given.
teo per cent.—No sharp positive reaction. Specimens
will give a weak negative reaction if the opening of the
capillary is held very near the head. In most cases reactions
are indifferent.
gives positive reaction and
gzo per cent.—One specimen g
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 649
goes through whole food reaction on the end of tube.
The remainder still give weak negative reactions.
six per cent.—Reactions essentially the same as in zt
per cent.
At this point this series was discontinued. It shows that
0
any absolute concentration for a chemical solution which will
cause al] planarians to give the positive reaction cannot be
assigned. How a given individual will react to a given
concentration of chemical depends almost, if not quite as
much, on the individual as it does upon the solution.
Sulphuric, =, per cent. and 1, per cent.—Caused imme-
diate and violent reaction. Decapitated worm reacts like
normal. This is evidently a very strong stimulus,
qi per cent.— Caused strong negative reaction in majority
of cases. One specimen reacted as follows :—the capillary
tube being held some distance away from the head, it
first gave a well-marked positive reaction. On coming
into the stronger solution near the mouth of the tube it
began strong convulsive contractions (evidently on account
of too strong stimulation). It remained, however, at the
same spot, and after a few minutes extruded the pharynx
and swept it about over the bottom. ‘The specimen re-
mained this way for some time. The tube was, of course,
removed immediately after the first positive reaction was
given. A decapitated specimen in one case gave a very
distinct positive reaction to this solution, the tube being
held some distance away from the specimen.
zy per cent.—Negative reaction. Decapitated specimen
gave positive reaction once. ‘This solution, applied to
the posterior end of the body, induces the crawling move-
ment.
xia per cent.—Negative reaction. Isolated pharynx con-
tracts into a ball when stimulated with this solution.
xia per cent.—Positive reaction in one case. Remainder
negative. Same result with pharynx as in ;4, per cent.
gin per cent., zs, per cent., and 5,4,5 per cent.—With
these solutions the reactions were for the most part negative.
650 RAYMOND PEARL.
In a few cases positive responses were produced, but not
recularly.
_1__ yer cent. — Positive reaction in all cases. The
5120
whole food response was produced in case the end of the
ce a)
tube was left in position. The worms “ gripped” it, glided
up on toit,and extruded the pharynx, in many cases running
the latter up into the lumen of the tube. Anterior piece,
resulting from cutting animal in two transversely, acts like
whole worm (positive reaction), but less strongly. Decapitated
worm gave no response. In order to make sure that in this
ease it was the extremely diluted acid which was producing
the result, numerous controls with distilled water and culture
water and fresh tap water were tried on the same speci-
mens, in alternation with trials with the acid. With tap
water and culture water the specimens were indifferent ;
but with the acid solution (34,5 per cent.) mixed in either
tap water or distilled water they gave a well-marked positive
reaction. This showed clearly that the results were due to
the acid.
Summary.—With the three mineral acids tested it was
found that to concentrations above a certain point the speci-
mens always gaye the negative reaction, while to concentra-
tions below this point the positive reaction was given. The
absolute value of this “critical point” varies widely with
different individuals. The behaviour is essentially the
same as that in response to mechanical stimulation, viz. to
strong stimuli the negative reaction is given, to weak the
positive.
Organic Acids.
Oxalic, + per cent. and ;4, per cent.—Sharp negative
reaction. his solution affords a very strong stimulus
and quickly kills the specimen. ‘The negative reaction
is very violent when once induced, but several specimens
were killed before they turned away. ‘There was notice-
able a shght tendency to turn towards the stimulus the
instant it was perceived, and before this could be replaced by
MOVEMENTS, ETC,, OF FRESH-WATER PLANARIANS. 651
the negative reaction the specimens were nearly or quite
killed.
zy per cent.—Convulsive negative reaction in the great
majority of cases. In one case stimulation was followed
by sharp positive reaction, succeeded by extrusion of the
pharynx.
> per cent. and 4, per cent.—A few specimens on some
trials give positive reaction, and then go into convulsive
twisting movements as they get into stronger solution.
Remainder negative.
sia per cent. and =4, per cent.—Positive and weak nega-
tive reactions about equally divided.
giv per cent. and 5,4, per cent.—Positive reactions
becoming proportionately more numerous. Negative re-
actions are very weak when given in response to these
solutions. In the cases where there is a positive reaction
the full response is not given; the specimens go up to
the mouth of the tube, but do not grip it nor extrude the
pharynx.
seu per cent.—With this solution all but one specimen
give the positive reaction. Specimens will follow the end of
the pipette about the dish if itis moved slowly. his is done
by a series of positive reactions. Specimens will give the
complete food reaction on the end of the tube.
Citric, 2 per cent.—Strong negative reactions.
1 per cent.—Less marked negative reactions. ‘l'endency
to positive in some cases.
;'; per cent.—Positive reactions in nearly all cases. Re-
mainder indifferent.
=, per cent.—Indifferent.
Citric acid in weak solutions seems to be a very ineffective
sort of stimulus, not causing pronounced reactions of any kind.
Formic, + per cent. and ;', per cent.—Prompt and de-
cided negative reaction. Causes a resting worm to give
a weak negative reaction of the anterior end, but does not
start the whole animal into movement, provided the tube is
withdrawn after the first reaction is obtained.
652 RAYMOND PEARL.
si; per cent.—Negative reaction, but decidedly less pro-
nounced than with preceding concentrations. Does not
cause any movement whatever in resting specimen.
4, per cent.—Negative reaction, less strong than in pre-
vious cases. In some cases positive reaction. Noticeable
tendency to give slight positive reaction just before the
definite negative response.
=i, per cent.—Well-marked positive response.
Summary.—The same conclusions are to be drawn from
the experiments on organic acids as from those on mineral
acids, viz. that to strong concentrations of a given substance
the negative reaction is given, while weak concentrations
cause a positive response. Oxalic acid is rather peculiar in
that it appears to furnish in all concentrations a stimulus of
the proper quality to induce the positive response, but is at
the same time excessively harmful in any above the weakest
solutions.
Alkalies.
Sodium Hydrate, + per cent., ;4, per cent.,and 54 per
cent. — Immediate strong negative response. Specimens
turn away very sharply. In ; per cent. the reaction is
slightly weaker than in the other two.
zi, per cent.—Negative reaction. Stimulus applied to pos-
terior end of body is sufficiently strong to cause crawling
movement.
, per cent.— Weaker negative reaction. Sufficiently
strong to start resting animal into movement.
xiv per cent.—Weak negative reaction. Ineffective on
resting worm and on posterior end of body of moving
specimen.
zi; per cent.—Very weak negative response. In one
specimen sharp positive reaction; performs whole food re-
action on the end of the tube.
aiy per cent.—Positive reactions from all specimens. The
complete food reaction is given.
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 653
To solutions below this concentration the organisms are
either indifferent or, in a few cases, weakly positive.
Sodium Carbonate, + per cent.—Rather weak negative
reaction.
zg per cent.—Majority of all reactions positive. Remainder
weakly negative.
zg per cent.—Well-marked positive reaction in all cases.
The specimens can be led around the dish by moving the tube
slowly.
Below this concentration the reactions were either in-
different or weakly positive.
Summary.—A caustic alkali (NaOH) and a salt of strong
alkaline reaction (Na,CO;) produce essentially the same
results as the acids. In strong solutions they cause negative
reactions ; in weak, positive.
Salts of Heavy Metals.
Copper Sulphate, ~, per cent.—At the very first
trials the animals all turned sharply and immediately towards
the stimulus (positive reaction), but the solution was so
strong as to throw the animal into convulsions, when the
head came very near the mouth of the tube. Subsequent
trials produced the negative response.
zy and =, per cent.—All specimens give positive reaction.
The head is brought up to the tube, and the worm glides up
over the latter.
With all concentrations of CuSO, there is a very well-
marked local contraction of that part of the body which is
stimulated.
In another series of experiments with this same substance,
the strongest concentration which would produce the positive
reaction in all cases was 74,5 per cent.
Zine Sulphate, + and 54, per cent.—Sharp and imme-
diate negative reaction.
zs per cent.—Negative reaction, but less pronounced than
in former cases.
zy and ,!, per cent.—Specimens stop when stimulated,
vou. 46, part 4,
NEW SERIES, UU
654 RAYMOND PEARL.
and wave the anterior end about in the water, first
away from and then towards the source of stimulation.
As the head comes nearer to the end of the tube, where the
solution is strongest, it is more strongly stimulated, and
gives a definite negative reaction. As it gets out into the
weaker zone again it is stimulated to a positive reaction
once more. If the tube is now removed the specimen will,
in some cases, after a short time turn sharply towards the
place where it was, and move in that direction. In other
cases the negative reaction finally predominates. It not in-
frequently happens that in the earlier part of this reaction the
anterior end only moves very slightly towards, or very
slightly away from the stimulus, so that the body seems, at
first sight, to be fixed in one position. The planarian, in this
strenuous reaction, probably comes as near to the hypothe-
cated behaviour of the famous “ Buridan’s ass” as anything
is ever likely to in actual practice.
=i; per cent.—One specimen gave clearly marked positive
reaction in every case. Others as in the preceding solutions
(1; per cent. and 4, per cent.).
xi; per cent.—Well-marked positive reaction. Specimens
give complete typical food reaction.
In one case, with a small worm, I was able to produce
crawling ina backward direction by continuous stimulation
of the anterior end in the middle line of the body with
wy per cent. ZnSO,.
Summary.—The results from solutions of salts of two
heavy metals are in accord with those obtained with other
chemicals.
Other Salts.
Sodium Chloride, + per cent. and ;4, per cent.—Nega-
tive reaction; distinct, but not as strongly marked as the
negative reaction to strong acids.
si; per cent.—Weak negative reactions and weak positive
reactions in about equal numbers. Many of the trials produce
no response whatever.
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 655
ga per cent.—Weak positive reactions in nearly every
case. No negative reactions. The typical, complete food
reflex I have not been able to induce with sodium chloride.
Concentrations below this do not produce any definite re-
action.
In general, NaCl is a very ineffective stimulus to pla-
narians, either to the positive or the negative reaction. Dis-
tilled water is a considerably stronger stimulus to the positive
reaction.
Sodium Bromide, 2 per cent.—Weak but distinct
negative reaction in all cases.
2 per cent.—Well-marked positive reaction in all cases.
Complete normal food reaction is produced.
Potassium Chloride, 2 per cent.—The animals usually
react in a peculiar way to this and stronger solutions of KCl.
When stimulated they stop, turn the anterior end either
shightly towards or slightly away from the source of stimula-
tion, and then stay in the same place and squirm and twist
the body. In some cases there is a well-marked negative
reaction.
4+ per cent.—Some specimens give negative reactions
in the first few trials; afterwards give definite positive
responses, as do other specimens in all cases. In one case
the specimen gave marked positive reaction, and after the
head was turned towards the stimulus, remained quiet in the
same position as long as the chemical acted.
zg per cent.—All specimens give positive reaction or
are indifferent. ‘The whole food reaction took place on the
end of the tube. In this experiment it could be clearly
demonstrated that the pharynx is positively chemotactic to
this substance. It is probably positively chemotactic to all
substances which induce the preceding portions of the feed-
ing reaction. If, after the pharynx had been extruded, the
tube was turned about so that the ventral surface of the
animal could be seen, and the posterior part of the body was
moved with a needle, so as to change the position of the
pharynx with reference to the mouth of the tube, it could be
656 RAYMOND PEARL.
seen that this organ bent directly towards the mouth of the
capillary. ‘The pharynx oriented itself with reference to the
issuing chemical.
The cases in which specimens were “indifferent” to this
solution (i.e. did not give either the positive or negative re-
action) were evidently not due to the fact that the animal
was not stimulated, but, on the contrary, that it was stimu-
lated about equally to negative and positive responses. This
was indicated by their restless behaviour when “ indifferent.”
While the animal as a whole moves in a straight line, the
head constantly moves slightly towards and away from the
stimulus. Evidently the solution is not quite strong enough
to induce a definite negative reaction, nor quite weak enough
to cause a clear positive response.
zy per cent., z45 per cent., and 4, per cent.—Distinct posi-
tive reaction in all cases.
ziz per cent.—Positive reactions in some cases, mainly
indifferent. The ‘ indifference”? is now due to lack of
stimulation.
Below ;4, per cent. I have been unable to get definite
responses of any sort with KCl.
Magnesium Chloride, 4 per cent.—Usually sharp
negative reaction. In some cases a slight turn towards the
stimulus preceded the negative response, and in some few
other trials the animal was indifferent.
|; per cent.—Weaker negative reaction. In one case
clear positive reaction. No local contraction of the region
stimulated is caused by this chemical.
sly per cent.—Positive reaction in all cases. Complete
food reaction could be induced.
zi; per cent.— Weak positive reaction or indifferent.
Summary.—To the salts NaCl, NaBr, KCl, and Me(l,
the planarians react as to other chemicals, by giving the
negative response to strong concentrations and the positive
to weak.
Cane-sugar.—Sugar solutions, in all concentrations
above ;'; per cent., so far as I have been able to discover,
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 657
sause well-marked positive reactions in all cases. his is
the only chemical which I have found that causes only one of
the reactions.
Distilled Water.—To distilled water applied by the
capillary method the organisms give a well-marked positive
reaction in all cases. That the reactions to very dilute solu-
tions of chemicals were not due to the distilled water in cases
where this was used as the solvent, rather than to the chemical
itself, was proven in the following way :—Parallel experiments
were performed, using tap water as a solvent, and in every
case the same reaction was given to the tap-water solution as
to that in distilled water. At the same time the specimens
would not react to clear tap water applied in the same way by
the same tube.
2. General Summary.—Putting all the results on the
effects of localised chemical stimuli together, we are forced to
the somewhat remarkable conclusion that practically all sub-
stances are both “ attractive ” and ‘ repellent ” to planarians.
HKvidently, then, the chemical composition of a substance is
not of the first importance in determining how the individuals
shall react to it; but, on the contrary, its concentration is the
important matter. To weak solutions of any chemical the
animals give positive responses, while to strong solutions they
give negative.
Between the behaviour towards chemical stimuli and
towards mechanical stimuli there is a very close parallelism,
or, perhaps better, identity, which is evidently something of
fundamental importance. In order to bring this out more
clearly it may be well to arrange in tabular form the results
of the study of the reactions to these two stimuli.
658 RAYMOND PEARL.
Mechanical Stimuli. Chemical Stimuli.
Strong. Weak. Strong. | Weak.
| Unilateral stimula- | Negative re- | Positive | Negative re- | Positive
tion of head region | action reaction | action reaction.
|
ene zie Py
Stimulation of head Hithera very | Positive | Strong nega- | Positive |
region on median strong ne- | reaction tive reac- | reaction.
line | gative reac- tion, or
tion, or crawling
crawling backwards |
backwards
Stimulation of middle | Essentially the same as | The same as for stimula-
region of body for stimulation of the] — tion of the head, except
head that the sensitivity is
much less, and dimin-
ishes more rapidly pos-
teriorly than in case of
mechanical stimuli.
Stimulation of pos- | Crawling) Local con-]Crawling | No effect,
terior region of| ahead traction ahead or slight
body local con-
traction.
From this close parallelism we must conclude, I think, that
in the behaviour of planarians the qualitative character of a
stimulus is of little importance in comparison with its quanti-
tative relations. Or, to express it differently, to all stimuh
which are of low intensity the flat-worm gives the positive
reaction, while to stimuli which are of high intensity it gives
a negative response. ‘his sort of behaviour will at once be
seen to be, in the long run, purposive, and is, further, of a
kind which might very well have been developed by the
action of natural selection. In the long run the planarian’s
reactions will take it away from injurious substances and
into favourable surroundings.
These results on chemicals are interesting in connection
with the work so much done in recent times on the specific
MOVEMENTS, E'TC., OF FRESH-WATER PLANARIANS. 6059
effects of ions and the conclusions based on very fine quanti-
tative results with chemicals. Two such series of experi-
ments as those quoted above from HCl and CuSO, indicate
what would be the worth of the assignment of an absolute
value for the concentration of either of these two substances
which would produce the positive reaction in planarians.
Such instances might be multiplied, and they serve to bring
out the fact, apparently so frequently lost sight of, that what
an organism will do when stimulated is quite as much a
function of the physiological condition of the organism itself
at the time as it is of the stimulus.
A. comparison of these results with those of Yerkes (: 02)
on the reactions of Gonionemus is of much interest. ‘This
author finds that though there is a well-marked and
characteristic food reaction, which is given in response to
food substances, whether in solid or liquid form, yet this
reaction cannot be induced by other chemicals. It is stated
that a number of chemicals were tried in all concentrations
for the special purpose of determining whether the food
reaction might not depend upon intensity rather than quality
of stimulus. ‘his was not found to be the case. We must,
then, conclude that Gonionemus 1s a stage farther along in
its psychic development than is the flat-worm, for the medusa
reacts with reference to the quality as well as to the in-
tensity and location of the stimulus, while with the flat-worm
the intensity and location of the stimulus are by far the most
important factors. It is necessary in the case of the flat-
worm, to be sure, that there be mechanical and chemical
stimuli acting together in order to produce the complex of
reflexes forming the complete food reaction, thus indicating
some relation to quality of stimulus. But for the production
of what is, in one sense, the most important phase of the re-
action, the turning towards the source of stimulation, the
quality of the stimulus is not significant.
With an understanding of the method of reaction to
localised chemical stimuli, a number of interesting special
problems present themselves. While it will not be possible
660 RAYMOND ‘PEARL.
to take up all of them in this paper, a few of the specially
important ones may be considered.
One such important general question which arises is the
problem of orientation to diffusing chemicals. Do planarians
orient themselves along radial lines of diffusion and proceed
towards the centre of diffusion? It would seem that in the
case of such a perfectly bilaterally symmetrical organism as
Planaria, if anywhere, Loeb’s theory of orientation ought to
hold good. ‘This theory accounts for orientation by sup-
posing that when an organism is stimulated unilaterally its
motor organs are caused to act either more strongly or more
weakly, as the case may be, on that side than on the other.
This results in bringing the long axis of the body parallel
with the lines of action of the stimulus; and then, since
symmetrical points on either side of the body must be equally
stimulated, the organism moves in a straight line towards or
away from the stimulus. Jennings has shown (: 01) that for
most stimuli this theory of orientation does not hold in the
case of the Infusoria.
From the account of the reactions of planarians to
chemical stimuli which has been given, it will be at once
seen that there is in this case, to some degree at least,
an orienting reaction. With weak chemical stimuli the
head turns towards the stimulus in such a way as to point
the anterior end very directly towards the source of stimula-
tion. It mght be thought that this marked a pure orienta-
tion, but it must be remembered that the organisms turn the
head just as precisely towards the point from which a weak
mechanical stimulus comes. ‘The two reactions are evidently
exactly the same thing. However, a single mechanical
stimulus can hardly be considered a directive stimulus of
the sort which induces an orientation, such as, for example,
the electric current. The orientation of unicellular organisms
to the constant current is the purest type of an orienting
response, however, and the most characteristic thing about it
is that the organism, after having the anterior end turned
towards one of the poles, keeps the long axis of the
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 661
body parallel to the lines of action of the stimu-
lus. This movement of the animal in a constant relation to
a constantly acting stimulus is, as I understand it, the funda-
mental criterion of an orientation according to the theory
above mentioned. Now it we find, as has been shown above
to be the case, that the organism gives precisely the same
reaction to a chemical unilaterally applied as it does to a
single weak mechanical stimulus similarly applied, it seems
doubtful whether we can consider that there is such an
orientation in the case of the chemical, even though the head
is directed very precisely towards it. On the contrary, it
seems apparent that we are dealing here with a well co-
ordinated motor reflex only—such as, for example, the reflex
of a frog’s hind leg, which brings its foot very exactly to the
point stimulated on the side of the body.
A crucial test of this point may be obtained by submitting
the animals to the action of some chemical to which they are
known to give the positive reaction when it is applied locally,
only arranging the experiment so that it is diffusing over a
large area. Under these conditions, if the organism shows
positive orientation, it ought to move along the lines of diffu-
sion straight up to the source of diffusion. To test this mattcr
I constructed a trough of the form shown in Fig. 33, I.
On a plate of glass A was fastened the trough B, which was
cut froma block of paraffin. The internal dimensions of this
trough were 50mm. x 50mm.-x 5mm. Only the sides were
of paraffin, the glass plate serving as the bottom. A hollow
was cut in one end of the trough, and a glass tube D, about
4 cm. long, was fastened into it in an upright position. Then
from the point a on the inside of the trough a fine needle was
thrust through the paraffin till it came out into the hollow
previously cut in the wall. A sectional view of this part of
the device is shown in Fig. 33, II. When it was desired to
use the apparatus the trough was filled with filtered tap
water and a number of planarians placed in it. Then into
the tube D was introduced a certain amount of the solution
whose effects were to be tested. By varying the amount of
662 RAYMOND PEARL.
the solution introduced, the rate of its diffusion through z
into the water could be very nicely controlled. This matter
was thoroughly tested, and the apparatus in a_ sense
calibrated by the use of coloured solutions before the actual
experiments were begun.
A considerable number of experiments were tried with this
diffusion trough, with the following results :—In no case was
there any observable orientation of the organisms. A typical
experiment will illustrate what actually took place. A
‘per cent. solution of Na,CO;, which by the capillary
20
method always produces a sharp positive reaction, was put
Fie. 33.—I. Diffusion trough used for testing the reactions of planarians
to diffusing chemicals. A, A. Glass base plate. 5B, B, B. Paraffin
trough. «. Point of opening of diffusion tube. C. Cavity of
trough in which the specimens are placed. D. Tube in which the
solution to be tested is placed. If. Enlarged sectional view of the
end of the trough bearing the diffusion tube. Lettering as in I.
into the tube D in sufficient quantity to give a diffusion of
moderate rate. After it had been diffusing for some time (by
test with coloured solutions long enough to reach the middle
of the trough) specimens were introduced at the end C. They
started gliding about in random directions at once. Some
passed diagonally up to the end D; others remained nearer
the end C; while still others went up on the paraffin sides to
the end D. None went straight towards « after they had
come into the region where the chemical had diffused. No
reaction of any sort was given in the course of the passage
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 663
towards the end Din the majority of cases. In some few
instances an individual would give a weak positive reaction
(i.e. turn slightly towards x) at some point in its course, but
this was so small in amount that it did not in most cases
turn the animal directly towards a. Further, the direction
of movement was frequently changed considerably, and turned
away from «# after this» weak positive response. In other
words, the animals moved about in the trough practically at
random, giving only slight reactions in a few cases while in
the area of diffusion. Many of the individuals, after reaching
end D of the trough, turned around and went back to the
other end again, just as they would have done provided no
chemical had been present. Other specimens would glide
across the trough on the paraffin of the end D. Only these
specimens showed any definite response to the chemical.
When they came within the length of their own bodies from
the opening w they gave a well-marked positive reaction and
went to a Having arrived there, they explored and
“oripped” the edge of the hole with the head, and then
extruded the pharynx. The pharynx was usually stretched
up into the diffusion opening, and the worm proceeded to
feed for a time on Na,COs,.
These experiments were repeated many times with a
variety of chemicals of various concentrations, and diffusing at
various rates. It was very certain in all cases that there was
no definite orientation along lines of diffusing ions. When
the organism by chance came near the diffusion opening ~, it
would give a positive reaction if the solution was of the proper
concentration, and then proceed to give the complete food
reaction over the hole, but there was no continued orientation.
There was a similar absence of a negative orienting
response when strong solutions of acids were used. In this
case the animals stayed at end C of the trough, but this was
because when, in the course of their random movements, they
struck the diffusing chemical where it was of sufficient con-
centration, they gave the usual negative reaction, turning the
anterior ends about 30° away, and starting off on the courses
664 RAYMOND PEARL.
so defined. If they came in contact with the strong solution
again they repeated the reaction. In no case did they turn
squarely arcund with their heads directly away from w and
the long axis parallel to the lines of diffusion.
It would be unprofitable to further multiply accounts of
these experiments, since all led to the same result. No
definite orientation occurred, but only the positive and nega-
tive motor reflexes coupled with random movements.
Whether, as some maintain, we have in these positive and
negative reactions the “ Dinge an sich” of orientations is a
question for the metaphysician rather than the physiologist
to decide. The objective reality of the matter is that in the
behaviour of planarians towards chemicals there is no orien-
tation in the lines of diffusing ions, i.e. no phenomenon lke
the orientation of Paramecium to the electric current.
Another problem of importance in connection with the re-
actions of the organisms to chemicals has to do with the
formation of collections of individuals. Are collections
formed in certain chemicals, as is the case with certain of the
Infusoria as described by Jennings? As this author has set
forth, Paramecia will form dense aggregations in drops of
various chemicals, particularly weak acids, introduced into
the culture water. The method by which this is done is as
follows :—Individuals swimming about at random strike the
drop of acid by chance and pass into it without giving
any reaction; when, however, they come to the opposite
side of the drop, and start to pass from it to the water again,
they are stimulated and give their characteristic motor re-
action (jerk back and turn towards the aboral side). This
reaction turus them back into the drop, which forms, as it
were, a trap for all that enter it. In a short time a dense
ageregation is formed. ‘This is almost the only method of
active reaction, known aside from orientation, which will
produce collections of organisms in chemicals. Its essential
feature is not the getting of the organisms into the chemical,
this being purely a matter of chance, but the holding of
them in the chemical after they have entered it, by what
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 665
amounts to a negative reaction to the surrounding water.
The question, then, is, can we get any such formation of
collections by the retention of those specimens which have
entered an area by chance in the case of. Planaria ?
This problem was attacked in a number of different ways,
but the clearest results could be obtained by the “ two-drop ”
method of Massart. ‘Two drops of fluid of equal size are
placed near each other on a slide, and a narrow connecting
band is made between the two by drawing some of the fluid
across with a needle. One of them was usually of culture
water, while the other was of the solution to be tested. Now
evidently, if the animals form collections by the ‘ motor
reflex ”’
without any reaction, but when they attempt to pass back
into the water drop they should be stimulated to a negative
reaction and thus turned back.
An experiment with a solution to which the animal gives a
sharp positive reaction may first be reported. One of the
drops was tap water, and the other was 1 per cent. sugar
solution, to which the specimens gave a strong positive re-
action. Several small planarians were put into the water
method, they ought to pass into the drop of solution
drop. They glided rapidly about this drop, and soon one
came up to the bridge connecting the water with the sugar.
It was headed straight for the sugar drop, and passed over
into 1t without any reaction whatever. Up to this point the
behaviour is like that of the Infusoria towards the acid drop.
This specimen circled about in the sugar drop, and after a
time became directed towards the connection between the
sugar and water, and passed back into the water drop with-
out giving the faintest trace of a reaction of any sort. All
the specimens passed back and forth between the two drops
without giving any reaction, except in some cases a weak
positive one. ‘lhe conditions under which a positive reaction
is given are that a specimen should come more or less trans-
versely across one end of the connecting bridge, as shown in
Fig. 34. It then usually gives a weak positive reaction and
turns slightly towards the other drop. It may do this on
666 RAYMOND PEARL.
passing either from the water to the sugar or vice versa.
When in sugar solution it gives a positive reaction to tap
water, whether applied by the capillary tube method or as just
described. It is evident, from this experiment, that collec-
tions are not formed by planarians in the same way that they
are by Infusoria. The animals are not negative to the
surrounding water after they have been in the solution, ‘To
test and verify this conclusion the experiment was repeated
with solutions of different substances. It was found that in
case of all substances in concentrations to which the animals
eave a positive response when stimulated by the capillary
method, the specimens would pass back and forth from water
to solution and vice versa, indifferently. If solutions were
used in concentrations to which a negative reaction was given
Fic. 34.—Diagram showing the arrangement of “ two-drop ”’ experi-
ment with chemicals.
when stimulation was by the capillary method, the specimens
merely stayed in the water drop. When they came to the
boundary line of the strong solution they gave the negative
reaction, and hence stayed in the water. This immediately
raises the question, why would there not be a permanent
collection of the planarians formed in a drop of a substance
to which they give the positive reaction, provided they were
first put in a drop of some substance to which they were
strongly negative ? There is evidently no theoretical reason
why this should not take place, but there is an important
practical one. ‘This is that any solution which would cause a
negative reaction, under these circumstances, will, so far as I
have found, also seriously modify the animals’ movements, if
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 667
they are immersed in it. They will simply squirm about and
make no progressive movements, and hence not get into
the drop of substance to which they are positive. But it is
quite possible that by making a long enough series of experi-
ments on this point, one unght get a solution just strong
enough to cause a negative reaction, and in which the
organisms would still move well. We would then get a
collection in the positive drop. The important thing, how-
ever, is that to the water in which they live the animals do
not, under any circumstances, give a negative reaction, and
hence under normal conditions no collections can be found by
a “motor reflex”? method.
It may be well, before leaving this subject, to point out the
fundamental physiological difference between the Infusoria
and the planarians, on which the difference in the behaviour
towards chemicals is based. It is that in the case of the
Infusoria there is but one form of reaction (the ‘‘ motor reflex”’
turn towards a structurally defined side) regardless of whether
the stimulus is strong or weak, while in the case of the
planarian there is a qualitatively different reaction to strong
stimuli from that which is given to weak. When the in-
fusorian passes into the drop of acid it is apparently not
stimulated at all (for what reason we do not know). When
it attempts to pass from acid to water it is given a stimulus
which must be in the nature of things a rather weak one, yet
it responds with the only reaction it has, and is, as a
consequence, kept in the acid. With the planarian any slight
change in environmental conditions gives a weak stimulus,
and the specimen turns towards the source of stimulation.
This serves, together with random movements, to get it into
the drop of solution; but when it strikes again the water,
which again must furnish a weak stimulus, it gives the same
positive reaction and passes out into the water. The ability
to differentiate in the reactions between the strong and weak
stimuli gives the organism a far greater range in_ its
activities.
Another problem which is of interest in connection with
668 RAYMOND PBRARL.
food and chemical reactions is the relation of the condition
of the organism as regards hunger to its reactions to stimuli.
It might be supposed thatan individual which had not had food
for some time would be more apt to give the positive reaction
to a given stimulus than one which had just fed.
To test this point parallel experiments were instituted with
specimens allowed to feed till they left the food spontaneously
about three hours before the experiments, and specimens
which had been kept for three weeks in a dish of clear water.
NaBr was used as the stimulating solution, and was applied
by the capillary method. ‘The specimens chosen were of the
same species, P. dorotocephala, and as nearly as possible
of the same size. The only difference which could be detected
between the fed and the unfed animals in their behaviour
towards a 2 per cent. solution of NaBr was that the unfed
animals gave the whole food reaction on the end of the
capillary tube, while the recently fed specimens only went so
far as to give the positive reaction, and touched the end of
the tube with the anterior end of the head. ‘They did not
“orip’’ it and pass up on to it, as did the others. In the
main point at which I was working, namely, the giving of
the definite positive reaction, there was no discoverable
difference between the fed and unfed specimens. One set
cave the reaction just as promptly and decidedly as did the
other. Next a weaker solution, ;4, per cent., was tried.
With this solution about 50 per cent. of the specimens in
ordinary condition give a weak positive reaction, and 50 per
cent. are indifferent. This concentration, being about on the
border line between that which affords no stimulus at all and
that which is a definite stimulus for the positive reaction,
ought to bring out any differences which may exist between
fed and unfed individuals in the sensitivity to stimuli for the
positive reaction. As a matter of fact, no difference in the
behaviour of the two sets was to be observed. One gave a
well-marked positive reaction in as many cases as did the
other. In some instances the reaction time of the fed
specimens seemed to be slightly greater than that of the
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 669
unfed, but this was neither marked nor of general occur-
rence. This experiment was afterwards repeated with other
specimens, and with sugar as the stimulus, with essentially
the same results. I have also repeatedly tried stimulating
with various solutions specimens which had just ceased
feeding, and in these cases found no certain difference
between their behaviour and that of specimens which had
not been fed for some time, with regard to the giving of the
positive reaction. It would appear, then, that so far as the
giving of the positive response to weak stimuli is concerned, the
amount of food the animal has previously had is of very little
consequence. The failure of fully fed specimens to give the
full feeding reaction on the end of the capillary tube indicates
that the physiological changes induced by recent feeding
affect the performance of the food-taking rather than the
food-seeking reflexes.
3. Unlocalised Action of Chemicals.—An extensive
Fic. 35.—Diagram showing the form of crawling movement exhibited
by Planaria when placed in 10 per cent. NaCl.
series of experiments on the effects of immersing planarians
in various solutions was performed, but as the results threw
but comparatively little light on the general nature of the
behaviour, they will be reported only briefly. Immersion in
any strong solution causes marked changes in the move-
ments. The gliding is made very much slower or entirely
disappears. In 10 per cent. NaCl a peculiar form of crawling
appears. Very pronounced contraction waves pass over the
body longitudinally, giving it the appearance shown in
Fig. 35. In 2 per, cent. CuSO, the animals make no pro-
gressive movements, but wave the head violently from side to
side. In strong solutions of acids the worms squirm violently
without making any effective progressive movements. In all
these strong solutions the sensitiveness to all stimuli is
vot. 46, PART 4,—NEW SERIES. sas
670 RAYMOND PERARL.
greatly diminished. This can best be shown with mechanical
stimulation. In strong solutions of NaCl (10 per cent.) the
animals make no attempt to right themselves if placed with
their dorsal surfaces down. Another peculiar effect of strong
solutions of NaCl is to cause the extrusion of the pharynx.
This organ is thrust out of the body and extended to a much
greater length than is usual. Immersion of the animal in
weak solutions that cause the positive reaction—as, for
example, 1 per cent. sugar—has no definite effect on the
movements, but when in these solutions the animals will give
the positive reaction to tap water when the latter is applied
by the capillary tube method. Under such circumstances
contact with water is a slight environmental change, and acts
as a weak stimulus.
Ill. Thigmotaxis and the Righting Reaction.
a. Thigmotaxis.—lf a specimen of Planaria is turned
over and placed dorsal side down on the bottom, it will
immediately right itself. This is done by a very characteristic
reaction, and is one of the first things to attract the attention
of one studying the behaviour of the organism. Loeb (94, pp.
251—252) held that the righting reaction in the polyclad
Thysanozoon was due to the negative and positive thig-
motaxis (stereotropism”’) of the dorsal and ventral surfaces
respectively. The evidence offered for this view was that
when the thigmotactic relations of these two surfaces were
reversed, the animal reacted strongly, and that this result
could not be due to any effect of gravitation, since the animal
assumed all possible relations to gravity, and kept them for
considerable periods. of time. It seemed to me desirable to
get, if possible, some further evidence on this subject, and to
work out the mechanism of the righting reaction.
That the dorsal surface of the animal is negatively thigmo-
tactic 1s certain, and can be shown in other ways than by
laying the animal on its dorsal surface. For example, if a
piece of cover-glass be gently laid on the dorsal surface of
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 671
either a resting or a moving specimen, it will very promptly
move out from under it. Further, if crevices are arranged of
this form (—) by supporting cover-glasses at two corners,
and letting the two opposite corners rest on the bottom of the
dish, specimens will not go into them. ‘The moment the
dorsal surface touches the cover-glass above, the worm
begins to react violently, changing its direction of movement,
and goes out from under the cover.
With the existence of an apparent negative thigmotaxis of
the dorsal surface established, however, there still arises the
question as to whether this is the sole cause which induces the
inverted animal to right itself. The following experiment
throws light on this point :—A specimen is placed ventral
side up on a dry spatula in the air, and then the spatula is
placed just beneath the surface of the water in a tall jar or
large test-tube and quickly pulled out from under the worm,
so that the latter starts falling through the water in an in-
verted position. Another way in which the worm may be
started falling ventral side up is by holding it on a scalpel
point above the water, and then dropping it beneath the
surface in the desired position. Before the worm has
dropped any great distance it will give the characteristic
righting reaction, and turn itself over so as to bring the
ventral side down again. ‘This is done in precisely the same
way as when the animal is inverted on the bottom (to be
described later). After the falling animal has thus righted
itself it may again give the same reaction, and thus turn
itself over so that the dorsal side is down again. In a few
cases I have seen a worm after righting itself the first time
keep right side up during the remainder of the fall. The
most usual behaviour is for the animal to keep giving the
righting reaction all the time that it is falling, although this
does not, of course, keep it all the time with the same side
uppermost. JI have performed a large number of these
dropping experiments in which the animals were started in
both upright and inverted positions, and in all cases they
gave the righting reacting one or more (usually more) times
672 RAYMOND PEARL.
before reaching the bottom, provided the distance through
which the drop was made was greater than 7—10cm. ‘This
result seems to indicate that there is something more con-
cerned in the righting reaction than the negative thigmo-
taxis of the dorsal surface for the following reasons :—(1) the
dorsal surface is not in contact with any solid of this experi-
ment; (2) it is in contact with water only, just as is normally
the case when the animal is right side up. It may be
objected that the experiment is not conclusive, because, as a
result of the falling, there is an increased water-pressure on
the dorsal surface, and this may act as a _ thigmotactic
stimulus. This objection is met by two different facts.
First, the animal gives the righting response in some cases
while falling ventral side down, under which circumstances
there can be no increased pressure on the dorsal surface.
Second, if a stream of water from a pipette is directly
squarely against the dorsal surface of a worm normally
gliding about on the bottom the righting reaction is not
induced, regardless of the force of the stream. LHvidently
this stream of water against the dorsal surface produces a
pressure on the dorsal surface similar to that when the
animal is falling, and if the righting reaction in the falling
is due to increase of pressure on the dorsal surface, we might
suppose that some indication of it would be produced in this
case. As a matter of fact it is not. We must conclude,
then, that the righting reaction is due, at least in very large
part, to some other cause than the negative thigmotaxis of
the dorsal surface. This is indicated also by the fact that
when solid bodies are laid on the back of a specimen in its
normal position, the reaction which is caused is not the
righting action, as would be expected if the latter were
due solely to the negative thigmotaxis of the dorsal surface.
The righting reaction is clearly not due to gravitation, since
the flat-worms move on the surface film with the dorsal
surface downward. This leaves, as the only factor te which
the reaction can be due, the positive thigmotaxis of the
ventral surface. Iam convinced that it is to this factor that
MOVEMENTS, EBTC., OF FRESH-WATER PLANARIANS. 673
the reaction is chiefly due. While the negative thigmotaxis
of the dorsal surface plays some part in the reaction, it is, as
the experiments described above show, a comparatively un-
important factor. ‘The specific relation of these two factors
to the definite righting reaction will be brought out in the
next section, in which the form and mechanism of this
reaction will be set forth.
b. The Righting Reaction.—The righting reaction is
a very characteristic piece of behaviour, and can best be
described in a single phrase by saying that when the animal
is placed on its back it throws itself into a spiral in such a
way that the ventral surface of the head comes into contact
with the bottom. This ventral surface then attaches itself to
the bottom by means of the mucous secretion, and starts
eliding ahead. As it goes forward it unwinds the remainder
of the spiral, as each successive posterior part of the ventral
Fre. 36.—Showing the form taken by Planaria in the righting reaction.
surface comes into full contact with the bottom. ‘he form
of this spiral just after the ventral surface of the head has
come into contact with the bottom is shown in Fig. 36. The
spiral is thrown very quickly after the dorsal surface touches
the bottom, and usually includes the whole length of the
body at once. However, by observing a specimen in which
it takes place a little more slowly than usual, it can be seen
that the movement is started at the anterior end. Beginning
with, for example, the right side of the head, this is turned
under, while at the same time the left side is raised. ‘This,
of course, brings the ventral surface of the head region down,
and at the same time makes a twist in the body, just back of
the head. In some cases this is the only twist that is made,
while in otliers another similar twist is thrown in the body
farther back. As the anterior end after it is righted elides
674 RAYMOND PEARL.
ahead, the spiral is unwound by the raised edge of each
twist dropping down and attaching to the bottom as soon as
it is in a position where this is possible. Thus, of course,
when the animal has traversed a distance equal to its own
length it will have come entirely into the normal position
again. The reaction is really a rotation of the body on its
long axis through 180°. The mechanism of the turning is
such that only a part of the body rotates at a time,—first
the anterior end, then the portion next behind that, and so
on, till the whole animal has turned over. This rotation by
sections, as it were, causes the spiral form which the animal
takes on in the reaction,
The number of turns into which the body is thrown in
forming the spiral varies with the length of the individual,
and apparently to some extent with its physiological con-
dition. ‘There may be only a half-turn in the whole body, or
there may be one complete turn; or, again, one and a half
turns; or, finally, as many as two complete turns in the body.
One complete or nearly complete turn, as shown in Fig. 36,
is the usual form of the reaction. In large individuals
more twisting is frequently seen. Hvidently all the twisting
that is absolutely essential for the righting of the specimen
is the half-turn given by the turning of the anterior end
ventral side down.
The determination of the direction in which the spiral is
thrown, or, in other words, the side of the body towards
which the anterior end turns in order to get right side up,
was for some time a very puzzling problem. A collection of
statistics on the matter showed that the anterior end twisted
towards the right and towards the left! in an approximately
number of cases. This is precisely the result which would
be expected if the matter were due to chance only, but the
reaction did not give the appearance of being a chance
matter. Finally, the determining factor was found to be the
relation of the dorsal surface to the bottom. A cross-section
' In the figure (Fig. 36) the worm is represented with the spiral thrown
towards the left,
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 675
of the body of Planaria has the form shown in Fig. 37. It
is convex in outline on the dorsal side, and nearly straight on
the ventral, As a consequence of this shape of the dorsal
surface the animal when placed in an inverted position very
seldom lies exactly on the mid-dorsal line, and if it does at
first it almost immediately tips over to one side or the other,
so that its cross-section has the relation to the bottom shown
in Fig. 37, Band C. It is then found that the side of the
body which is in contact with the bottom determines in
which direction the spiral shall be thrown. If the right
side of the dorsal surface is down the right side of the head
will turn under towards the left and the left side will be
raised up over towards the right, or, in other words, the
head as a whole will rotate from right to left, i.e. in a
Dorsat
LIES
VENTRAL
A
ath BE BEA ESB 0
B C
Fig. 87.—Diagrammatic cross-section of Planaria to show the contact
relations of the dorsal surface of the body to the substrate in the
case of a specimen in an inverted position.
counter-clockwise direction. If the left side of the dorsal
surface of the body is down at the beginning, the head will
rotate from left to right. This relation may be made out
easily by direct observation in all cases where the reaction is
not too rapid.
The righting reaction is a fairly rapid one. The head is
turned over and the spiral thrown in the case of a normal in-
dividual almost immediately when the dorsal surface touches
the solid. The length of time which it takes a specimen to
eet completely righted evidently depends on the length of
the body, because the longer spiral which must be unwound,
the more the time which must be taken. The following
figures will bring out this relation between the size of the in-
dividual and the time taken in righting. In ten trials with
676 RAYMOND PEARL.
an active but large specimen (about 12 mm. long) of
P. dorotocephala the average time taken to regain com-
pletely the normal position after being inverted was 8°68
seconds. With a small specimen (5°5 mm. long) the average
time taken in righting in ten trials was 5°22 seconds. The
time taken in the reaction also depends, of course, on
the general physiological condition of the animal. Thus in
ten trials with a sluggish specimen, approximately 9 mm.
long (thus shorter than the first specimen mentioned), the
average time taken in regainine the normal position was
10:90 seconds.
The thigmotactic irritability may be modified or reduced
in several ways, and, as a consequence, the righting reaction
will disappear entirely or in part. One of these cases has
been mentioned above (p. 670) where it was shown that a
specimen placed on its back in a 10 per cent. solution of
NaCl makes no attempt to right itself. Sinmularly a specimen
put in an inverted position on a dry surface, care being taken
that no water surrounds the animal, will not give the righting
reaction. In both of these cases the specimens are able to
move.
The Mechanism of the Reaction.—It is a very
difficult matter to determine exactly the muscular mechanism
of this righting reaction, since it is such a complicated move-
ment, and is ordinarily done in its most essential feature—
the formation of the spiral—so very quickly. Furthermore,
as will appear from the operation experiments to be described,
it is almost impossible to devise crucial experiments of a
character which will demonstrate what the mechanism is.
What I shall do, then, will be to present a tentative explana-
tion of the mechanism of the reaction, together with the
evidence for it which I have been able to obtain. I may say
that the view to be presented is the result of a long and
careful study of the phenomena both in normal and operated
worms, and I believe that it is a correct explanation.
The mechanism of the righting reaction is probably as
follows :—The half of the body of an inverted specimen which
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 677
is in contact with the bottom extends (by the mechanism
previously described, pp. 556,557) in response to the stimulus
given by the contact of the dorsal surface of that side of the
body with the bottom. At the same time the opposite half of
the body, by active muscular contraction, keeps its length
the same. Thus any bending of the body away from the
side stimulated as in the ordinary negative reaction is pre-
vented, or, in other. words, the long axis is kept straight by
the opposite side maintaining actively its normal length.
Now the necessary mechanical result of keeping one side of
a flexible system at a constant length while the other side
lengthens must be that the lengthening side will be thrown
into a series of waves. In other words, it is mechanically
impossible for the lengthening side to keep its whole edge in
the same plane. Furthermore, if in such a system it is
possible for rotation about a longitudinal axis to occur, the
system will be thrown into a spiral of the form which the
planarian takes in the righting reaction. Again, as soon as
one side of such a system under elongating stress changes its
level with reference to the remainder of the system, and thus
starts the formation of the spiral, the lone axis of the
system (i.e. the centre of the spiral) will keep itself straight.
Any further force elongating one side will merely throw the
spiral into tighter coils without having any tendency to bend
its long axis. This fact is of importance in the case of the
planarian where the maintenance of the initial straightness
of the long axis is done by the opposite side of the body. Of
course, a symmetrical spiral cannot be formed unless the two
edges are of equal length, but the moment the spiral of the
planarian is started all necessity for one side keeping a con-
stant length ceases. It must be kept in mind, however, as
has been indicated above, that the force which produces the
spiral must act on one side only, and hence the side of the
planarian opposite that initiating the movement must be
moved passively by the other in the spiral formation after
this has once begun. The direction in which the spiral shall
turn will evidently not be determined by the mere lengthen-
678 RAYMOND PEARL.
ing of one side of the body. The determinant of this is
evidently a difference of tension on the upper and lower sides,
the spiral turning towards.the side of greatest tension.’ This
ereatest tension is evidently, then, in the normal reaction on
the dorsal surface, as we should expect on a priori grounds,
since that is the part directly stimulated.
To sum up, the spiral righting reaction of the planarian,
as I have worked it out, is due to an elongation of that side
of the body whose dorsal surface is in contact with the solid,
while the opposite side of the body actively maintains its
original length. As the elongation occurs the various parts
of the body rotate freely about its long axis, and hence the
whole worm takes on the spiral form. The spiral turns
towards the dorsal surface in every case (i.e. so as to bring
the ventral surface of the head down), as a result of the
ereater tension of the dorsal musculature on the elongating
side.
The reaction is thus seen to be of almost the same cha-
racter as the ordinary negative reaction to strong mechanical
stimuli, in that the primary reaction is an extension of the
side stimulated. The difference between the two is that in
one case there is a bending of the longitudinal axis of the
body, while in the other there is a rotation about this axis.
On the view just given of the mechanism of the righting
reaction the specific parts played by the positive and negative
thigmotaxis of the ventral and dorsal surfaces are evident.
The positive thigmotaxis of the ventral surface is the primary
cause of the whole reaction, and is evidently the stronger
factor of the two, as shown by the experiments of laying
solid bodies on the dorsal surface of the animal when in a
normal position. It will be recalled that such treatment does
not call forth the specific righting reaction. Further
evidence of this same thing is found in the fact that speci-
' The statements as to the mechanical principles of a spiral have been veri-
fied with different sorts of models, including plastic clay, rubber bands, ete.
Lack of space will not permit the enumeration of these experiments in detail, but
anyone can verify for himself the various statements with very little trouble,
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 679
mens will remain in the normal position on the bottom of a
dish when there isa layer of plant débris a half-centimetre
in thickness above them, and necessarily in contact with the
dorsal surface. The negative thigmotaxis of the dorsal
surface plays its part in the righting reaction in determining
in which direction the turning shall take place.
It has so far been shown that the view of the mechanism
of the righting reaction presented is in accord with all the
mechanical principles necessary to produce the observed
results. The attention may now be turned to an examination
of the evidence that this mechanism is the one which actually
brings about the reaction. ‘This evidence is obtained from
experiments with worms on which operations have been per-
formed. Obviously, if the mechanism described is the one
by which the reaction is produced, any operation which
destroys or throws out of working order any essential part
of the mechanism will cause the typical reaction to disappear,
or be greatly modified.
We may first consider the reactions of the pieces resulting
from cutting the animal in two transversely in the middle of
the body. It is found that each of the pieces resulting from
such a cut will perform the righting reaction in the typical
manner. ‘The spiral is formed, but there is usually only one
half-turn of the body, i. e. just enough to bring the anterior
end ventral side down. This then attaches itself to the
bottom and starts gliding, unwinding the spiral just as under
normal circumstances. There is observable the same rela-
tion between the side of the body, which is in contact with
the bottom and the direction of the turn as in the normal in-
dividual. The only striking difference in the behaviour of
the anterior and posterior pieces is that the reaction time of
the former is much shorter than that of the latter. The
anterior piece rights itself practically as quickly as does the
normal animal, while the posterior piece took in one series of
experiments | minute and 38°1 seconds (average of ten trials)
for complete righting. This slower righting reaction is
another expression of the generally lowered tonus of. such
680 RAYMOND PEARL.
posterior pieces. By varying the position of the cuts, seg-
ments of the body of various lengths may be obtained. All
of these, which are about 14 mm. in length, will usually right
themselves by as close an approximation to the typical spiral
reaction as is possible under the circumstances. ‘The side of
the body which is lowest can be seen to elongate in these
very short pieces, and just enough of a twist is found to
bring the ventral surface of one corner of the anterior end
into contact with the bottom. Of course, no complete spiral
‘an be found in such short pieces. Their reaction time is
very slow.
Next, experiments were tried with the pieces resulting
from splitting longitudinally anterior halves of worms in the
middle line. These pieces had the form shown in Fig. 38.
Evidently such pieces have only a half of the mechanism
necessary for the performance of the spiral righting reaction,
Fie. 38.—Operation diagram (see text).
according to the view given above, and therefore should not
be able to give the typical response. They have one com-
plete side which may elongate, but they have no other side
to keep the middle line straight, and so make the elongation
effective in forming a spiral. Such pieces, when placed with the
dorsal surface down, reacted immediately by bending strongly
towards the cut side, i. e., so that the concavity was on the
cut side, This was kept up for a time, the animal squirming
about violently, but it was finally replaced by another reaction.
The ventral longitudinal muscles contracted strongly, and
‘aised the anterior end of the piece well up from the bottom
(shown in side view in Fig. 39,a). After a strong raising con-
traction the piece would extend and settle back again. Then
after a time the raising was repeated, and it soon became
noticeable that the piece was rising higher each time and
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 681
settling back less after each trial. Successive stages of this
rising are shown in Fig. 39, b, c,d. Finally, it worked up till
it stood directly on the posterior end (e), and then the next
contraction caused it to fall over of its own weight and come
down right side up (f). The sticky mucous secretion at the
posterior end was undoubtedly what held the piece up after
each successive trial. This behaviour, as described, was
uniform in all the trials.
The behaviour of these pieces brings out several points of
importance. First, it is to be noticed that no trace of the
typical spiral righting reaction is to be seen; yet, on the other
hand, we find the pieces bending strongly towards the cut
side when first inverted, which is just the effect which would
2 f
Pie. 89.—Diagram showing the method of righting adopted by one
of the pieces shown in Fig. 38.
be produced by the lengthening of the stimulated side in the
normal righting reaction, provided, as actually obtains in this
case, there was no opposite side to keep the long axis of
the piece straight. Thus we get precisely the result
which would be expected if the view given of the mechanism
of the reaction is the correct one. Another fact that is
brought out by this experiment is the apparent adaptation
shown. Whenthe animal is unable to give the usual reaction
for righting itself it very quickly reacts in an entirely
different way, but attains the same end result.
A worm was cut so as to give a piece of the form shown at
A in Fig. 40. This piece was placed in an inverted position
682 RAYMOND PEARL.
and its reactions observed. Evidently, so far as injury of
the mechanism by the operation is concerned, such a piece
is in essentially the same condition as the pieces described
in the previous experiment. It has only one complete side
of the body. The piece when inverted squirmed about consider-
ably at first, but gave no indication whatever of the normal
spiral reaction. In a short time the violent movements ceased,
and a notch was noticed in about the middle of the uncut
edge (cf. Fig. 40, b). This soon grew larger, and extended
more and more towards the ends of the piece, as shown in
e oe f
Vic. 40.—a. Operation diagram. Heavy lines indicate the cuts. 4, ¢,
and d. Successive stages in the righting reaction of the piece A of
diagram a. e and /. Cross-sections through A at two successive
stages in the righting process. See text for further explanation.
cand d. By close observation the cause of this appearance
was found to be that the thin mobile edge was folding
under and attaching its ventral surface along the bottom, A
cross-section through the worm at this stage had the outline
shown ine. As soon as a considerable portion of the edge
had so folded wnder and become attached, the piece gave
a series of strong contractions and literally “ flopped” over
the attached edge and came down right side up. A stage in
this process is shown in cross-section in f. ‘This behaviour
was so peculiar, and at the same time precise, that the
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 683
experiment was repeated many times on this piece and on
others cut in the same way. The same method of righting
was always observed. After the first few times the turn
is made in this way; it is done more quickly at each succes-
sive trial.
This experiment leads to the same conclusion regarding
the mechanism of the righting reaction as did the previous
one. It affords another and more striking example of regu-
lation in reactions. The piece attains the end (normal
position) by a reaction which it undoubtedly never had
occasion to practise before.
Tsolated longitudinal halves of the body react in the same
way as did the piece described in the preceding experiment.
They right themselves by folding under the edge, and then,
by violent contraction, drawing the rest of the body up over
it. There is no trace of the spiral righting reaction.
A specimen cut in the manner shown in Fig. 41 shows a
Fie. 41.—Operation diagram (see text).
very peculiar righting reaction. When placed dorsal side
down the portion posterior to the median longitudinal
slit immediately gives the spiral righting reaction, and drags
the two passive anterior pieces over. ‘I'he process is slow but
very characteristic, so that there is no doubt of the nature of
the reaction. This shows that in that part of a single piece
of a worm where the necessary mechanism is present we get
the spiral righting reaction, while in other parts it does not
appear.
The same point can be brought out by sphtting a worm
longitudinally from the posterior end up to a poimt near the
head. The complete anterior part of such specimens gives
the normal spiral reaction, while the posterior parts remain
passive so far as this reaction is concerned.
A considerable number of different experiments were per-
684 RAYMOND PEARL.
formed for the purpose of testing the righting reactions after
operations, but since none of them bring out anything
different in principle from the results already given, they will
not be reported here. But it may be said in general, that all
the experiments gave the same results with reference to the
mechanism of the reaction, namely, that so long as the
mechanism described above was intact the typical spiral re-
action was given; when this mechanism was destroyed or
injured the reaction was not given, but the animal, if it
righted itself at all, did it by a different method.
When the animal falls freely in the water the righting re-
action is induced because the ventral surface is no longer in
contact with a solid. There is no reason for thinking that
the mechanism of the reaction in this case is any different
from what it is when the animal is placed in an inverted
position on the bottom. The direction in which the spiral is
thrown in the case of the falling animal is probably deter-
mined by slight differences of pressure on the two sides of the
body.
c. Summary.—The flat-worm is positively thigmotactic on
its ventral surface, and negatively thigmotactic on its dorsal
surface. As a result of this it gives a characteristic righting
reaction whenever the normal relations of either surface are
changed. This righting reaction consists in throwing the
body into a spiral in such a way as to bring the ventral sur-
face of the anterior end down into contact with a solid (in all
vases except when the animal is dropped into free water).
The anterior end starts glding and unwinds the spiral,
thus righting the whole body. The thigmotactic reaction
may be modified by chemical and other stimuli. movements. It is then
as possible, and makes ‘feeling’
withdrawn, and the animal curls up again. After the drying
has proceeded for some time the most characteristic feature
of the whole reaction appears. This is a lengthening of the
posterior part of the body to its fullest extent. The posterior
end then attaches itself to the surface, and strong waves of
contraction, like those in the crawling movement, pass over
the body from the posterior end forward. No progressive
movement is made, but backward crawling is evidently
attempted, and is only prevented by the dry surface which
the animalis on. There may be considerable variation in the
first part of the reaction with regard to the curling up; this
may appear or may not, but the attempted backward crawl-
Fic. 49.—Diagram showing the reaction of Planar ia to desiccation.
ine movement of the posterior part of the body I have found
to be a constant feature in the experiments which I have
performed. When the dorsal surface of the worm becomes
dry all movement ceases. If quickly put back into the water
the worm will usually recover completely, even though all
movement has ceased in the air.
If the worm is put on a slide in the centre of a small area
which has been wet, but on which there is no standing water,
it will squirm about and extend the head frequently, as in
the last experiment. If the head goes outside the wet area
it is very quickly jerked back, and the specimen gives the
negative reaction, i.e. turns away from the side stimulated.
The attempted backward crawling occurs in this case just as
in the others, a short time before the dorsal surface dries off.
It is to be noted that there is never any actual progressive
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 697
movement of a specimen in the air. If a specimen is placed
on very wet filter-paper it is not able to progress unless
water 1s kept constantly dropping on it from above, so that it
is at any time surrounded by a layer of water. On account
of this lack of ability to move when out of water, there is no
true hydrotaxis in the sense of movement towards water.
As has been mentioned before, specimens placed on a dry
surface dorsal side down do not show the righting reaction.
To sum up, it is found that planarians, when removed
from the water and subjected to a process of drying, are
unable to make progressive movements. At a certain stage
in the drying process they attempt to crawl backwards—a
form of movement which, under certain circumstances, might
get the animal back into water. On meeting a dry surface
with the anterior end the animals give a well-marked negative
reaction, The animal does not give the righting reaction on
being inverted on a dry surface.
On the whole, the general behaviour when subjected to
drying is purposeful; that is, it would tend to prevent the
animal ever becoming dried up under natural conditions.
There is nothing im the behaviour of planarians to indicate
how the change from aquatic to terrestrial life could be
brought about. The fresh-water Triclads, so far as I have
observed them, never leave the water and crawl up into the
air above the surface film as some other forms do.
VI. Rheotaxis.
A large number of experiments were performed early in
the course of the work with various sorts of devices to deter-
mine whether the animal showed any distinct reaction to
currents in the water, but without success. Streams of water
from a pipette, currents made by filling the tube of the
diffusion apparatus described above (pp. 661, 662) with water
and blowing into it, and other methods gave no results. If the
currents were made with sufficient force to threaten dislode-
ment of the animal from its hold on the bottom it would stop
moving and contract longitudinally, and thus attach itself
698 RAYMOND PEARL.
more firmly to the substrate. Weaker currents caused no
effect whatever. I was inclined to believe that the longitu-
dinal contraction and the gripping of the bottom were the
only rheotactic reactions which the organism exhibited. It
was found later, however, that there was a very precise
rheotactic reaction of a different character. In the course
of the experiments on reactions to localised chemical stimuli
by the capillary tube method, it was discovered that by using
a tube with a relatively large opening (from 4 to } mm, in—
diameter) and lettmg the ordinary tap-water in which the
animals were flow out of it, by its own weight, a current of
just the right intensity to cause a positive reaction could be
produced. The animals would turn very sharply towards the
source of such a current, the reaction being evidently the
same as that given to other weak stimuli (chemical and
mechanical). This reaction is localised m the same way as
the usual positive reaction. It is given only when the current
is directed against the head or anterior part of the body.
It is thus seen that the planarian is positively rheotactic to
very weak currents, the form of the reaction being precisely
the same as that given to other weak stimul. It seems very
doubtful if this reaction is of any importance in the normal
activity of the animal.
G. GENERAL SUMMARY AND Discussion oF ReEsvUts.
As was stated earlier in the paper, the problem with which
this study deals is the analysis of the behaviour of the
common fresh-water planarian. The movements and reac-
tions to all the more important stimuli, with the exception of
light and heat, have been described and analysed into their
component factors in the body of the paper. It is believed
that it is of the greatest importance to have as complete and
detailed an account of the various activities as possible, and
as a consequence full details have been given in the case of
each subject treated. Since this method of treatment
necessarily makes the account of considerable length, it has
a tendency to obscure the general and significant results in a
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 699
mass of detail. It is desirable, then, to state clearly at the
end the important general facts which have been brought out
by this study, and to discuss to some extent their significance.
In this place I shall state the results in a categorical manner,
making no attempt to indicate the evidence on which the
conclusions are based. This will avoid needless repetition.
1. The locomotor movements of Planaria are of two sorts,
eliding and crawling. The gliding movement is produced
by the beating of the cia on the ventral surface of the
organism. It is by far the most usual method of locomotion.
For its production it is necessary that there be a layer of
sticky, mucous slime between the ventral surface of the body
and the substrate. In this slimy secretion the cilia beat and
so propel the animal (cf. pp. 544 and 545). The organism
never moves freely through the water without some sort of
mechanical support. The rate of the gliding is changed by
the action of various agents, such as light, chemicals, elec-
tricity, etc. Its direction is always forward.
The crawling movement is produced by strong longitudinal
waves of muscular contraction passing over the body from
the anterior to the posterior end. It is more rapid in rate
than the gliding. It appears only after strong stimulation
of the organism, and its purpose is evidently to get the
animal quickly away from harmful stimuli. Its direction
may be either forward or backward.
Periods of movement alternate with periods of rest in the
course of the animal’s daily activity. When at rest the flat-
worm is in a condition of relaxation and generally lowered
tonus, corresponding to the condition of a higher organism
in sleep. The causes which induce the coming to rest are—
(a) a more or less fatigued condition of the organism. This
is the primary cause; without it the other causes are ineffec-
tive. (b) A relatively low intensity of hght. (¢) Roughness of
the substrate. This brings the body into a position such that
its different parts form angles with one another, and causes
the animal to come to rest as the result of a reaction which
I have called goniotaxis (p. 562). (d) Certain chemical con-
700 RAYMOND PEARL.
ditions. Asa result of the action of some one or all of these
above-mentioned factors, collections or groups of planarians
are frequently formed.
Planarians which have been injured by operative procedure
move comparatively little during the course of regeneration,
thus showing a sort of regulation or correlation between
behaviour and morphogenetic processes (pp. 573, 574).
2. There are two principal qualitatively different reactions
to stimuli, the positive and negative reactions.
The negative reaction is given in response to strong
unilateral stimulation of the anterior portion of the body.
It consists essentially in a turning of the head away from
the side stimulated. It is brought about by the extension of
the body on the side stimulated. This extension is produced
by a contraetion of the circular, dorso-ventral, and transverse
systems of muscle-fibres. The purpose of the negative
reaction is evidently to get the organism away from harmful
stimull.
The positive reaction is given only in response to weak uni-
lateral stimulation of the anterior portion of the body. It is
essentially a turning of the head towards the source of the
stimulus. This reaction is one of considerable precision,
bringing the anterior end into such a position that it points in
most cases exactly towards the source of the stimulus. The
turning is brought about by the contraction of the longi-
tudinal muscle-fibres of the side stimulated. The evident
purpose of the positive reaction is to get the animal into
regions of beneficial stimuli.
3. Whether the negative or the positive reaction shall be
oiven in response to a particular stimulus depends primarily
on the intensity of the stimulus, and secondarily on its loca-
tion. Neither reaction is given unless some part of the body
in front of the pharyngeal region is stimulated. The negative
reaction is given only im response to stimuli above a
certain intensity (strong stimuli). This relation between
intensity of stimulus and form of reaction holds for both
mechanical and chemical stimuh.
4
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 701
4, The reactions of Planaria to a variety of chemicals, in-
cluding representatives of several of the most important
chemical groups, were studied. It was found that to a weak
solution of any substance, regardless of its chemical composi-
tion, the organism gave a positive reaction identical with the
positive reaction to mechanical stimul. To strong solutions
of the same substances (with a single exception, see p. 657)
the organisms responded by a negative reaction identical with
that caused by strong mechanical stimuh.
Planaria does not orient itself to a diffusing chemical in
such a way that the longitudinal axis of the body is parallel
to the lines of diffusing ions. Its reactions to chemicals are
motor reflexes identical with those to mechanical stimuli. The
positive reaction is an orienting reaction in the sense that it
directs the anterior end of the body towards the source of the
stimulus with considerable precision, but it does not bring
about an orientation of the sort defined above.
5. Several important features in the normal behaviour of
the flat-worm are found upon analysis to have their explana-
tion in the positive and negative reactions to mechanical and
chemical stimuli.
The method by which the organism gets its food is simply
a special case of the positive reaction. From substances
which serve as food for the planarians, various juices diffuse
into the surrounding water. When the planarian meets any
of these diffusing substances it gives the positive reaction,—
that is, turns in the direction from which the stimulus comes.
The food substance acts as a weak chemical stimulus, to
which the animal reacts in the same way as to all other weak
chemicals.
The direction of the planarian’s movement, and its behaviour
with reference to obstacles in its path, are usually deter-
mined by its reactions to mechanical stimuli.
The behaviour of the organism with reference to the
surface film is determined by its reactions to mechanical
stimuli.
6. Strong stimulation—either mechanical or chemical
of
vou. 46, parr 4.—NEW SERIES. ZZ
702 RAYMOND PEARL.
the posterior portions of the body induces the crawling move-
ment. This is to be regarded as the specific reaction of this
portion of the body. Weak stimulation of the same region
causes local contraction at the point stimulated in the case of
mechanical stimuli, while weak chemical stimuli apphed to
this region are ineffective.
7. The ventral surface of the body of Planaria is strongly
positively thigmotactic, and the dorsal surface is negatively
thigmotactic.
8. When the organism is placed in an inverted position it
performs the righting reaction. This reaction consists im a
turning of successive parts of the body about the longitudinal
axis through 180°. During the process the animal takes the
form of a spiral. The anterior end is brought into the up-
right position first. On analysis the righting reaction is
found to be a special case of the reaction to strong stimuli
(the negative reaction). It is brought about by an extension
of one side of the body, while the other side maintains its
original length (pp. 676—679). The reaction is given when-
ever the ventral surface is removed froma solid or the surface
film of the water.
9. To the constant electric current Planaria reacts by
turning the anterior end towards the kathode. Complete
orientation and movement towards the kathode may occur.
The turning towards the kathode is brought about by an
extension of the anode side of the body. The current causes
a contraction of muscular elements whose long axes are
parallel to the direction of the current (pp. 690—693). The
current very quickly paralyses planarians on which it acts.
The rhabdocele Stenostoma leucops orients to the
current with the anterior end towards the kathode, and
moves towards this pole. This orientation is brought about
by changes in the positions and consequent effective beat of
the cilia, exactly like those which occur in the case of the
ciliate Infusoria. Cilia, on the portions of the body directed
towards the kathode pole, take on reversed positions.
10. All the normal reactions to stimuli are of the nature of
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS, 705
reflexes, more or less complex. What the animal will do
after a given stimulus, or in a given situation, can be predicted
with reasonable certainty. There is, however, some variation
in the behaviour, depending on the physiological or tonic
condition of the individual at the time of stimulation. Thus
a stimulus sufficiently weak to induce the positive reaction in
one specimen may cause the negative reaction in another; or
at different times the same individual may show different re-
actions—either the positive or negative—to the same stimulus.
11. Psychological Position of Planaria.
tive psychological position of any organism is evidently deter-
The objec-
mined by the relative simplicity or complexity of what it
does. With a view of determining what the position of
Planaria in the psychological scale is, it may be well to
make a catalogue of the things which it does in the course
of its ordinary existence.
The animal performs the following acts :
a. It moves progressively by two methods, a ciliary motion
and a muscular motion.
b. It turns, by a complex of simple reflex acts, towards all
weak stimuli investigated.
c. It turns, by another set of simple reflex acts, away from
_all strong stimuli investigated.
‘d. It comes to rest in certain definite environmental
situations.
e. When stimulated in a certain way it extends the pharynx
and feeds.
f. When its ventral surface is removed from contact with
a solid body (or the surface film), a reflex of essentially the
same character as that of ¢ brings this surface again into
contact with the solid.
From these essential factors is composed a behaviour
whose complexity one has only to study to realise.
The behaviour is thus seen to be, in the main, what may
be characterised as reflex. It is very simple to say that an
animal’s activity is composed of a series of invariable reflex
acts in response to stimuli, but I doubt whether the full
704 RAYMOND PEARI.
significance of such a condition is always realised. It implies
that the animal as an individual “ does” nothing in the sense
that a man “does” things. It is moved about from place to
place by its locomotor organs; it is put into certain definite
and invariable relations to its surroundings by its reflex
mechanisms. Considered as a whole, such an organism is a
sort of shell to hold a series of mechanisms, each of which is
independently capable of doing a certain thing, and in the
doing produces some effect on the shell as a whole. We may
perhaps get a clearer picture of what such a reflex existence
means by considering for a moment what would be the effect
if all a man’s activities were composed of invariable reflexes,
to be set off by the appropriate stimuli. Under such circum-
stances, whenever a man saw or smelled food he would have
to go to it and eat it. Whenever anything touched him he
would have to move in a new direction very closely related
to the position of the object which touched him. Whenever
he touched water he would have to take a bath, or perhaps
drink till he could hold no more. During the day he would
have to move always in a definite direction with reference to
the sun, and so on ad infinitum. All he did would be
definitely fixed and, in a sense, predetermined by the things
about him.
It is apparent that the behaviour of Planaria is not thus
entirely and purely reflex, because there is a certain amount
of variation in it. As has been brought out im several places
in the body of the paper, and in paragraph 10 of these
conclusions, this variation in the behaviour is the result of
the physiological condition of the individual. ‘To put this in
a more concrete form, we may say that a fatigued animal
or an animal in a state of great excitation does not always
react to a certain stimulus by the same set of reflexes as that
by which a normal animal would react. Furthermore, there
is a variation in the intensity of the negative reaction
dependent upon the intensity of the stimulus producing it.
Another point in which the reactions of Planaria differ
from what would obtain in the case of an organism whose
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 705
behaviour was composed of invariable reflexes is found in the
behaviour following repeated strong stimuh applied to the
anterior end (vide pp. 580, 581). In this case the organism
shows an evident modifiability in reaction, for after giving
for some time the ordinary negative reaction, and not thereby
getting away from the stimulus, it finally turns directly
towards the source of the stimulus. Again, in the righting
reactions of pieces of the body we see entirely new forms of
reaction appearing (pp. 680—683).
In order to give a concrete idea of the psychological
position of Planaria it may be well to present in parallel
columns the principal factors which make for simplicity in
the behaviour on the one hand, and for complexity on the
other hand.
Factors which tend to make Factors which tend to make
the Behaviour Simple. the Behaviour Complex.
A. Mssential reflex character at the A’. Comparatively large number of
basis of all the reactions. qualitatively different general
reactions.
B. General lack of modifiability of B’. Marked qualitatively different re-
reactions. actions to differing intensities of
stimulus,
C. Comparatively small number of C’. Definite relations of reactions to
qualitatively different reflexes location of stimulus.
composing the general reac-
tions.
D’. Rather close dependence of reac-
tions on the physiological condi-
tion of the individual. This brings
about variation in the reactions.
The behaviour of Planaria is evidently much more com-
plex than that of the Infusoria, as described by Jennings
(loc. cit.). In the case of the Infusoria, all the factors A’, B’,
C’, D’, which make the behaviour of Planaria so complicated,
are nearly or quite absent; and in respect to C these organ-
isms are at a much lower stage than Planaria. The
Infusoria have practically but one purely reflex reaction to
nearly all stimuli, and this reaction is not localised with
706 RAYMOND PEARL.
reference to the location of the stimulus. Again, the Infu-
soria do not show qualitatively different reactions to differing
intensities of stimuli, as does Planaria to a marked degree.
We thus see that Planaria stands considerably higher in
the psychological scale than the Infusoria, and that the
development is taking place along two main lines: (a) the
higher organism reacts differentially with reference to the
location and intensity of the stimulus; and (b) the physio-
logical balance in the higher organism is much more deli-
cately adjusted than in the lower, and as a consequence we
see much more variation in the physiological condition.
These variations in the physiological condition bring about
variability im the reactions.
In the case of the ctenophore Mnemiopsis Leidyi we
have an intermediate stage between the Infusoria and
Planaria. Here the animal reacts with reference to the
position, but not the intensity of the stimulus. This condi-
tion, in which an organism reacts with relation to the position
of a stimulus, and not to its imtensity, must be for the indi-
vidual a precarious one, because the animal must either go
towards or away from all stimuli alike, whether good or
harmful. Chances are theoretically equal that after each
stimulus it may get a toothsome morsel of food, or, on the
contrary, serve in that capacity itself. Further development
beyond the point in the behaviour series where Planaria
stands must be in the line of further differential reactions
with reference to quality of stimulus. A beginning along
this line is made by the planarian, and the process is carried
a step farther in the case of Gonionemus, as recently
described by Yerkes (loc. cit.).
12. Relation of Behaviour and Structure.—tThe reac-
tions of organisms are evidently, in any case, very closely de-
pendent on the structural relations of the given organism, and
on the conditions under which it lives, i.e. its environment
in the broadest sense. Thus we find the asymmetrical Infu-
soria, which live freely in the water and move about by means
of cilia, all reacting in the same way, and the determinative
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 707
factor in the reaction is the asymmetry of the body (cf.
Jennings, :00). Now Jennings has further found! that
certain rotifers, which live freely in the water and move
about by the activity of cilia in a similar way, and further-
more are asymmetrical in fundamentally the same way that
the Infusoria are, react in essentially the same manner as do
the Infusoria. Similarly, I believe that the general reactions
method of the planarians may be found to be in the main the
method by which all organisms presenting the same general
structural relations and mode of life react. Only one
example on which this conviction is based may be given
here. In the case of such fresh-water molluscs as Physa it
is apparent that the actual locomotor and sensory organisa-
tion is symmetrical in form, and furthermore these forms live
in fresh water on the surface of solid bodies just as do
planarians. Now I have found, in a series of observations
not yet published, that in the case of several of these molluscs
the fundamental scheme of reaction is like that in the
planarian. They react in the same way with reference to
the location and intensity of the stimulus, and these are the
fundamental things. In fact, the general behaviour is
strikingly alike in the two widely separated groups.
13. Purposive Character of Reflexes.—A fact which
is strongly impressed on one working on the behaviour of an
organism whose activities are largely reflex is the purposive
character of these reflexes. They are so adjusted that in the
long run they keep the animal out of danger, and get it into
favourable conditions. Inthe flat-worm these two things are
very well done in general by the negative and_ positive
reactions. Of these two reactions it 1s easy to see that the
positive is the more highly developed, in particular in the
fact that it is much more precisely localised with reference to
the position of the stimulus. We can see a reason for this in
the fact that under the conditions of the planarian’s life the
1 Complete observations not yet published. or preliminary account see
‘Science,’ N.8., vol. xv, pp. 524 and 525 ; and Jennings, : 01, in bibliography
at the end of this paper.
708 RAYMOND PEARL.
eetting of food is of far more importance in the struggle for
existence than the avoidance of danger. This point has,
however, been discussed earlier in the paper, and need not
detain us here. The real problem is presented in the attempt
to discover how any of the purposive reflex acts in the
organisms arose. I see no reason for denying that many of
them—such as, for example, the positive reaction which gets
the animal its food—were developed by natural selection.
There are other evidently purposeful reactions, however, with
whose development it hardly seems as if natural selection
could have had anything to do, since they cannot themselves
be of selective value. This pomt has been well brought out
in a recent paper by Morgan (:02, p. 281). I think a pos-
sible explanation of some of these may be found in their
analysis into component factors, when it may appear that
only a very few simple reflexes had to be formed by natural
selection, and then all the reactions are built up from these.
An example will make my meaning clearer. In the righting
reaction of the planarian we have a fairly complex reaction
which is evidently immediately purposeful. Yet we find on
analysis that this reaction is at bottom nothing but a slight
modification of the ordinary negative reaction, which might
very well have been developed by natural selection. And
thus it is with other reactions and pieces of behaviour. They
are for the most part built up from a very few simple
purposive reflexes. If we can get them subdivided and
spread out, as it were, so that we can see what goes to
compose them, we may find that our problem has diminished
very much, and we shall have to deal with only a few factors
where before there appeared to be many.
A difficult problem in purposeful behaviour presents itself
when we find that new methods of reaction appear at once if
the usual reaction is prevented. The best examples of this
are found in the righting reaction of cut pieces of planarians.
Here we find pieces of the body, in which the normal
mechanism of the reaction has been destroyed, immediately
reaching a certain end (the righting) by a method differing
MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 709
entirely from any that planarians ever used before to attain
the same end, so far as we have evidence. These phenomena
have a considerable resemblance to such phenomena as the
well-known regeneration of the lens from the iris in some
Amphibia. It is not easy to see how such behaviour comes
about, and natural selection helps us very little. The matter
belongs apparently to the same class of phenomena as
morphological regulations, and probably has ultimately the
same explanation. What this explanation is we do not know.
14, Functions of the Nervous System.—The most im-
portant function of the brain is the preservation of the tonus
of the organism. After its removal the general tonus rapidly
diminishes, and on this account the positive reaction—which
depends rather closely on the physiological condition—can
be obtained only with great difficulty im such decapitated
specimens. There is no evidence of the presence of special
centres in the brain. ‘he nervous system, as a whole, has
its main function in the rapid conduction of impulses.
15. Subjective Psychic Attributes.—One of the
principal questions which forever recurs with regard to work
on animal behaviour is, does the animal possess conscious-
ness? Now although it has been shown what the component
parts of the activities of the planarian are, yet it cannot be
said, as it seems to me, that the planarian does not, or, on the
other hand, that it does, possess consciousness. Al] that any
such an organism ever has done in the past, or ever will do
in the future, cannot tell us whether it was conscious in the
doing or not. Any “ objective criterion” of consciousness
does not exist. Furthermore, whether consciousness is or is
not present in any given case is not, in any event, the greatest
concern of the physiologist, who rests content with the objec-
tive explanation of how results are brought about, regardless
of what the animal is thinking about the matter. On this
subject Claparéde (: 01, p. 24), in concluding an interesting
and valuable discussion, has said, “A la question ; les
animaux sont-ils conscients ? la physiologie—et méme la
psychologie en tant que cette science est explicative—doivent
710 RAYMOND PEARL:
3
done répondre non seulement, ‘Je Vignore, mais encore,
‘“Peu m’importe’!” With this standpoint I am in thorough
accord.
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714. RAYMOND PARI.
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ON THE DIPLOCHORDA. 715
On the Diplochorda.'
IV. On the Central Complex of Cephalodiscus
dodecalophus, MclI.
By
A. T. Masterman, M.A., D.Se.,
Lecturer on Zoology, School of Medicine, Kdinburgh.
With Plates 32 and 33.
INTRODUCTION.
Iv the following description a number of the organs involved
already possess a plurality of names, arising from the fact
that various observers have recognised differing homologies,
and emphasised themin the nomenclature. ‘Thus the “ buecal
shield” of Cephalodiscus is also commonly termed the
d d)
In the case of
the two canals opening from the cavity of the buccal shield to
“ epistume,” “oral disc,” and “ pre-oral lobe.
the exterior there is the same difficulty. From homology with
Balanoglossus they are termed tle ‘ proboscis pores,”
although the buccal shield has never been termed the proboscis;
and they are more than pores, being of the nature of definite
canals. As this work indicates an even closer structural
resem blance to Balanoglossus than heretofore recognised,
it would be well to retain as far as possible the nomenclature
indicating this relationship; hence the terms collar cavities
and trunk cavities are retained, whilst the term pre-oral canal
is used as a synonym of proboscis pore with its attendant
canal. The term subneural gland is retained for the ‘ noto-
' Read before the Royal Society of Edinburgh, May, 1901.
716 A. T. MASTERMAN.
chord” of Harmer, with its homology to the similarly named
structure in Balanoglossus. Lastly, the terms pericardial
sac, glomerulus, and ectodermal pit are adopted for organs
in Cephalodiscus which seem to be homologous with simi-
larly named organs in Balanoglossus.
In the region lying between the buccal shield and the
collar of Cephalodiscus are several important organs, the
exact relationships of which have not previously been fully
determined. This area may be described as the central com-
plex, a convenient term already applied to the same region in
Balanoglossus. In this region there are externaily the
ectodermal pit, the pre-oral pores, and the central nerve-mass ;
internally are the subneural gland, certain important blood-
vessels, the pericardial sac, and the mesenteric walls of the
pre-oral and collar cavities, together with the glomerulus and
muscular strands. ‘lhe general outlines of the subneural
gland and the pre-oral and collar cavities have been indicated
by previous workers (McIntoshs, Harmers) and by myself,
but a detailed examination by carefully orientated and serial
sections has brought to hght some interesting new facts.
We may describe the organs under the following headings :
1. Kcetoderm and nervous system, ectodermal pit, pre-oral
pores.
2. Subneural gland and pharynx.
3. Pericardial sac, pre-oral cavity, pre-oral canals, glome-
rulus, collar cavities, and blood-vascular system.
1. Hetoderm.—Figs. 1—8 all illustrate the condition of
the ectoderm in this region. Ventrally the ectoderm on the
buccal shield consists of long narrow epithelial cells with
numerous unicellular glands, which form a buccal gland as
described by McIntosh (6). This epithelium is not shown
here. Dorsally it consists of columnar epithelial cells with
a very definite cuticle. In the region of the central nerve-
mass the inner ends of the cells are seen to pass downwards
as delicate fibres, terminating in peculiar conical ganglion-
cells (figs. 1 and 2). At their base these ganglion-cells give
off other delicate fibres running forwards and backwards.
ON THE DIPLOCHORDA. rally)
These fibres make up the main nerve-mass lying over the sub-
neural gland; they are seen in transverse section in figs. 6—
8, and in longitudinal section in figs. 1—5. Forwards they
run along the dorsal surface of the buccal shield, and back-
wards they branch outwards to form the two lateral cords;
hence, in longitudinal sections such as figs. 1—5, the nerve-
fibres appear to terminate abruptly backwards against the
wall of the pharynx, which here is in contact with the dorsal
ectoderm. The same remark applies to the dorsal blood-
sinus.
Immediately in front of the central nerve-mass is seen a
pit or depression, the ectodermal pit (fig. 1). This pit lies
exactly over the apex of the subneural gland, and extends
transversely in a slightly crescentic form. At the outer ends
of the crescent open the pre-oral pores, which are situated
at the posterior termination of the pre-oral cavity, at the
level of the central complex. The ectodermal pit, therefore,
represents the line of division dorsally between the head or
buccal shield and the collar, as do the “‘epidermistache”
of Balanoglossus and the epiblastic pit of Actinotrocha
(formerly termed the neuropore).
It is well to notice in dealing with this part that the
nerve-mass is co-extensive with the collar, as this part is
considerably narrower in the dorsal region; and further, that
the subneural gland les entirely in the collar area. At first
sight one is inclined to suppose, reasoning from prior know-
ledge of Balanoglossus, that the two collar cavities are
produced forwards into the pre-oral cavity, but such is not
the case. The buccal shield is produced backwards ventrally,
but the subneural gland lies in its primitive position in the
collar, and is in no way produced into the pre-oral cavity.
It is a backward ventral extension of the buccal shield
which makes the subneural gland lie in front of the mouth,—
not, as in Balanoglossus, a forward median extension of
the subneural gland into the pre-oral cavity.
2. The Subneural Gland and Pharynx.—The sub-
neural gland is an elongated cecal tube or prolongation of
vot. 46, PART 4,—NEW SERIES. Aas
718 A. T. MASTERMAN.
the anterior wall of the pharynx. Its total length is usually
about ‘14 mm., and its breadth about ‘02 mm. It usually
has for about four fifths of its length a central lumen, which
opens posteriorly into the pharynx, and terminates anteriorly
in a variety of ways. Fig. 1 shows the subneural gland cut
throughout nearly its whole length. ‘The lumen usually, as in
this case, contains a rod of glandular secretion of the nature
of mucus. At its apex the gland is bent dorsally. Throughout
the greater part of its extent its wall is composed of a simple
glandular epithelium, but at its distal extremity the cells
show a chordoid modification. The cells become vacuolated
and reticular, producing the well-known chordoid structure of
the “notochord” of Balanoglossus, Actinotrocha, and the
Vertebrata. ‘his is well seen in figs. 8 and 9. The extent
of this chordoid modification varies immensely, and it is only
the largest (oldest) individuals which show such a complete
chordoid apex as in figs. 8 and 9. This specimen also shows
a not uncommon feature in the complication of the central
lumen. In the apex it forks out into two lateral canals as
well as the median central canal (fig. 7)—a character also
found in some Enteropneusta.
The relationships of the subneural gland to the pharynx
have been already described elsewhere (10), and its connection
with dorsal pharyngeal and peripharyngeal grooves has been
demonstrated. In fig. 5 the commencement of the dorsal
and peripharyngeal grooves is shown with their numerous
nnicellular glands. The commencement of the pleurochord
is seen in fig. 5. It is important to notice the relationships
of the subneural gland to the pre-oral and collar cavities.
It is bounded laterally throughout its extent, except at the
apex, by the walls of the two collar cavities, and ventrally by
the wall of the pre-oral cavity. Above it the two collar
walls form a median dorsal mesentery (fig. 2), and then
diverge under the ectoderm to form the dorsal blood-sinus.
At its distal end or apex the subneural gland reaches just
beyond the collar walls, and plugs up the mouth of the heart,
as described below (figs. 1 and 2),
ON THE DIPLOCHORDA. 719
3. The Pericardial Sac and Heart, Pre-oral Cavity,
Pre-oral Canals, Glomerulus, Collar Cavities, and
Blood-vascular System.—The pericardial sac lies ante-
riorly to the distal extremity of the subneural gland.
In most specimens it is nearly square in cross-section,
but may be compressed at its base as in fig. 6. Roughly
its cross-section is about ‘05 mm., and its length about
‘(08 mm. It appears to be a closed sac formed of very
delicate endothelium; its posterior wall is invaginated to
form the heart. This inner wall is thickened, and has
numerous muscular fibres stretching across the cavity of the
sac to its outer wall (figs. 6 and 2). It is doubtless con-
tractile, and the shape of the pericardial sac varies greatly
according to its state of contraction. On its ventral wall
there is a fairly constant transverse groove (fig. 1). The sac
lies in the blood-space or cavity between the walls of the
pre-oral and collar cavities, and its walls do not differ except
in their extreme delicacy from those of these cavities. In
transverse sections it is seen that the pericardial sac is bent
over the apex of the subneural gland dorsally and ventrally
(figs. 7 and 8). laterally it is bounded by the wall of the
pre-oral cavity, which is thickened into an epithelial lining of
the pre-oral canal. In fig. 6 both pre-oral canals are clearly
seen, and the right pre-oral canal is cut throughout its length
from the pore at the base of the ectodermal pit to the mner
opening on the wall of the pericardial sac. The canal is
lined by a delicate columnar epithelium, apparently ciliated.
In this connection we may note the statement of Ehlers (2)
that the “ proboscis canals”? of Cephalodiscus end in blind
sacs. ‘here can be no doubt whatever that McIntosh and
Harmer were perfectly correct in stating that they open
freely into the pre-oral cavity, though in a specimen examined
as a transparent object the pre-oral canal might appear to
terminate in the pericardial sac.
I have elsewhere (7) described the blood-vascular system
of Cephalodiscus, and we have here to notice that, as
indicated by Harmer (4), the organ I first took to be the
720 A. T. MASTERMAN.
heart now proves to be a pericardial sac, containing the true
heart! within it. The dorsal sinus can be seen running along
immediately under the ectoderm and above the dorsal collar
mesentery (figs. 1—4). Anteriorly it terminates against the
posterior wall of the pericardial sac (which in a large number
of specimens is ruptured). Here it is also joined on each
side by a branchial vessel coming from the branchial plumes
(fig. 7). Further, the anterior end of the dorsal sinus is
continued into the cavity of the heart by paired lateral
canals, the relationships of which are not easy to find nor to
describe. If we could pull the apex of the subneural gland
backwards from the mouth of the heart it is clear that the
dorsal sinus would communicate directly with the heart. In
the normal condition, however, this wide aperture of the
heart is almost completely plugged up by the apex of the
subneural gland. Dorsally and ventrally (figs. 7 and 8) this
organ rests closely up against the pericardial wall, but
laterally a small canal remains running downwards from
dorsal sinus to heart (fig. 4). This canal is bounded pos-
teriorly and laterally by the wall of the collar cavity, and
anteriorly by the wall of the pre-oral canal (pre-oral cavity).
It is doubtless through this paired canal that the blood finds
its way from the dorsal sinus to the heart.
Below the subneural gland is a well-defined ventral sinus,
which passes backwards to the level of the mouth and round
it on either side. It is wide and large posteriorly, but passes
forwards, getting narrower and narrower till it is lost in the
olomerulus (fies. 7 and 8). Ventrally it is bounded by the wall
of the pre-oral cavity, which also extends ventrally, laterally,
and anteriorly to the pericardial sac. Various parts of this
wall (pre-oral cavity) are thrown out into cecal prolongations
into the cavity, with thickened protoplasmic walls. The
cavities of these czeca are in direct communication with the
blood-sinuses. They produce an appearance closely similar
to that of the glomerulus or pericardial gland of Balano-
glossus, with the exception that the walls are simple and
'The “ pre-oral sac’ of my previous work.
ON THE DIPLOCHORDA. Fea |
not of a definite cellular structure. There is usually a paired
patch of this glomerular tissue on the antero-lateral surfaces
of the pericardial sac (figs. 1 and 5) in close proximity to the
internal apertures of the pre-oral canals. Further, the wall
of the ventral sinus shows a similar structure (figs. 3, 4,
and 7). In many cases the glomerular tissue of the ventral
sinus is also paired, and the ventral sinus is then almost
constricted into two paired sinuses.
There is little doubt that this glomerular tissue is homolo-
gous with the pericardial gland or glomerulus of Balano-
glossus. Antero-dorsally to the pericardial sac we may
notice a pre-oral sinus bringing blood back from the buccal
shield (fig. 2) to the glomerulus.
We may now briefly run over the figures given here, noting
the special points of each. Figs. 1 to 5 are selected from a
series of very nearly sagittal orientation. In fig. 1 the sub-
neural gland is cut almost throughout its length, its opening
into the pharynx being more to the right. The right collar
cavity is cut just at the apex of the gland, so that the sinus
is rather more to the right anteriorly than posteriorly. The
right glomerulus is also seen, whilst the cavity of the heart is
spacious, although not at its largest (in the median line).
The dorsal sinus is cut throughout its length, and two oral
grooves may be recognised. In fig. 2 the collar mesentery is
cut for some portion of its extent, and the glomerulus of the
ventral sinus is coming into view; the left dorsal groove is
also just appearing. In fig. 3 the pericardial sac is cut in
the median line; the peculiar shape of the heart is noticeable.
Further back the left collar cavity is alone seen, the dorsal
sinus is restricted, and the subneural gland is interrupted.
The left peripharyngeal and dorsal grooves are differentiated.
In fig. 4 the heart is no longer visible, but the left canal from
dorsal sinus into heart is seen. The posterior portion is still
more to the left, showing the grooves as before. Fig. 5 is
eight sections further to the left. In following the sinus one
notices the left pre-oral canal becoming gradually more pro-
minent, first laterally and then dorsally; the left glomerulus
1422 A. T. MASTERMAN.
appears, and the left collar cavity increases greatly in size.
Posteriorly the first trace of the left pleurochord is seen,
lying laterally to the dorsal groove. In this section the
ganglion-cells are no longer seen, and the nerve is inter-
rupted at the pre-oral canal.
In figs. 6 to 8 we have selected sections from a transverse
series. The right side of the figures is slightly posterior to
the left. Thus in fig. 6 the right pre-oral canal (on the left)
is cut throughout its length, but the left only in part. In
this section the pericardial sac is cut transversely, and the
heart is seen in its greatest size. Fig. 7 is a few sections
further back. Here the apex of the subneural gland is cut
through, and shows three internal canals and a chordoid
structure. Dorsally the pericardial sac is still cut, and below
the ventral part is the glomerulus of the ventral vessel. On
the right is seen the left branchial sinus leading out from the
dorsal sinus, and a wider right branchial sinus opposite. The
two horns of the ectodermal pit are also seen. Fig. 8 is still
further back. The pericardial sac is no longer seen dorsally,
but is still cut ventrally. The walls of the two collar cavities
are approximately in the middle line, and behind the sub-
neural gland will form a dorsal mesentery. The lateral nerves
of the post-oral ring are seen in this section.
In fig. 9 the chief features here described are reproduced
in a semi-diagrammatic median section of the entire animal.
I have also shown the pharyngeal structure formerly described,
i. e. the pleurochords, the dorsal and ventral grooves, and the
oral grooves.
The facts above described must inevitably tend to bring
Cephalodiscus into even closer union with Balanoglossus
than heretofore. Not only is every organ in the central
complex of the former to be directly compared to its homo-
logue in the latter, but the latter has no organ in this region
which does not occur in the former. The only essential
difference is one which several years ago appeared to me to
be of fundamental importance, but which must now be regarded
as of secondary value. In Balanoglossus the pericardial
ON THE DIPLOGHORDA. i235
sac, glomerulus, and subneural gland protrude forwards
into the pre-oral cavity, and hence are covered dorsally as
well as ventrally; but in Cephalodiscus they protrude
upwards between pre-oral cavity and collar cavities,
and they are therefore dorsal and posterior to the former.
In this way the pericardial sac lies in contact with the dorsal
ectoderm, and the subneural gland is only separated there-
from by the dorsal sinus. ‘This difference cannot be regarded
as fundamental in view of the anatomical resemblance, and
we have seen above that it is due to the forward protrusion
into the pre-oral cavity of the subneural gland in Balano-
glossus, whereas in Cephalodiscus it remains in the
collar.
Of other points the homology of the subneural gland is a
most important question. It appears desirable to adhere to
this term, firstly, because it is unquestionably glandular in
function; secondly, because it has precisely the same re-
lationship to a system of dorsal and ventral grooves in
the pharynx as is the case with the similarly-named organ
in Tunicata (10); and thirdly, because its anatomical position
is exactly under the main nerve-mass. These and other
facts led me to doubt its homology with the “ Hicheldarm”’
of Balanoglossus, but its relationships to perivardial sac
and glomerulus and the chordoid structure of its apex
appear to me to be conclusive in favour of accepting Har-
mer’s original comparison. I would extend the appellation
of subneural gland to the organ in Balanoglossus, for, as
in so many other features, Cephalodiscus would appear to
show us amore primitive condition of the organ than Balano-
glossus. In making this comparison it appears to me to be
questionable how far the subneural gland is at all comparable
to the notochord of the Vertebrata. As indicated elsewhere
(11, p. 412), a chordoid histological structure by itself
cannot be regarded as an absolute criterion of homology, and
the occurrence of chordoid organs of the same nature as, but
not homologous with, the Vertebrate notochord is to be
expected in these low chordates. The view of Willey that
724 A. T. MASTERMAN.
Cephalodiscus is to be regarded as a degenerate ally of
Balanoglossus has not much to commend itself; the conse-
quent assumption that the former has lost numerous gill-
slits perforating its anatomy in all directions, not to mention
numerous other organs, has no justification in fact. We may
with Lang (5) suppose that Cephalodiscus has undergone
certain important modifications due to a semi-sedentary habit,
but the assumption that its proximate ancestors had many
pharyngeal clefts and gonads has nothing to recommend it
but its necessity for Willey’s theory. I would prefer to
regard Cephalodiscus as the more primitive form, as its
want of metameric segmentation and its primitive method
of feeding would imply (9). On this basis the “ Hichel-
darm” of the Enteropneusta must be regarded as a glan-
dular specialisation of the anterior end of the pharynx, to
be termed the subneural gland, owing to its functions and
structural relationships.
In Cephalodiscus its distal end often exhibits a com-
mencing degeneration into chordoid tissue (which, by its
development in Actinotrocha, is clearly an arrested form of
glandular epithelium), whilst it is still functionally active as
wu gland. In Balanoglossus, with a specialised burrow-
ing habit, the original function has been largely lost (though
the “ Hicheldarm” of Balanoglossus is unquestionably
glandular), and the chordoid tissue with supporting function
becomes still more in evidence. The organ to which the
name of subneural gland was given in Actinotrocha
occupies exactly the same position as in Cephalodiscus,
but as it is only embryonic its walls would hardly be expected
to be of a definitely glandular nature.
The pericardial sac of Cephalodiscus and its contained
heart are so similar to the pericardium (Herzblase) and
heart respectively of Balanoglossus, and so different from
any structures found elsewhere, that the homology need not
be insisted upon. In a similar manner the mutual relations
of the ectodermal pit, the pre-oral canals, the pericardial sac,
and the surrounding blood-sinuses speak for themselves.
ON THE DIPLOCHORDA. pa)
Lastly, there can be little question that we have in the
glomerulus a homologue of the proboscis gland of Balano-
glossus. Each is a proliferation of the pre-oral ccelomic
endothelium in the neighbourhood of the pericardial sac and
pore canals, consisting of czcal vascular processes.
It is evident that in the study of the budding processes
(10) the origin of the pericardial sac must have been over-
looked ; but as we do not yet know how this organ arises in
the demersal larva of Balanoglossus, nor even with
certainty in ‘l'omaria, this is not surprising. From certain
indications it appears that in Cephalodiscus it is a portion
of the pre-oral cavity constricted off from its posterior end,
and therefore ccelomic in origin.
During the progress of my work on Cephalodiscus Cole
(1) has published a short paper upon the bulbous termina-
tions of the twelve branchial plumes. His results appear to
indicate that the migration of oval lens-like bodies out of the
epithelial cells to the exterior is to be regarded as a normal
process, and that McIntosh’s view (6) that these organs are
masses of unicellular glands is correct. Assuming that the
migration might be an abnormality, I had suggested that “it
seems most reasonable to regard them tentatively as primitive
eyes,” a view I had already abandoned before the unexpected
appearance of Cole’s work. Cole further finds that the
glandular bodies break up to form rhabdites, which I think
quite probable, especially as I had already found and described
indications of ‘‘ one or more areas iu the centre (of the bodies)
staining more deeply than the rest (7).” I cannot agree with
Cole’s description of the epithelium in these terminal knobs
as normally correct, as such a vacuolated swollen mass with
little or no cuticle occurs commonly in other parts of the
body, and seems to be an abnormality ; the vacuolated con-
dition of the bulbs is undoubtedly present, especially in
older specimens. Cole denies the existence of a cuticle, of
pigment, and of nerve-endings in the cells. In respect to the
cuticle [am hardly prepared to inaugurate a discussion upon
the line of distinction between a “ peripheral deeply-staining
726 A. 'l. MASTERMAN.
membrane and a cuticle. ‘There is lttle doubt that the in-
tracellular bodies under discussion arise in the young form
in close contact with this limiting membrane, but it is
possible that they do not actually arise from it. I am still of
the opinion that fine brown pigment granules are scattered
throughout the cells (McIntosh [6] previously remarked upon
the “deep yellowish tint” of this region) ; and I still believe
that the inner end of each cell “ tapers to a fibre-like thread,
which I believe to have in some cases traced into the main
nerve of the plume” (7, p. 344). Indeed, it is rather difficult
to understand otherwise in what region the very evident nerve
down each plume terminates. None of these features are
opposed to the “battery” function as suggested by Cole,
though I have not as yet seen the rhabdites, which appear to
require special staining. If their presence is corroborated
it would form by no means the least interesting feature of
Cephalodiscus. Cole, as a critic of the work of his prede-
cessors, might perhaps make a somewhat more sharp distinc-
tion between a tentative suggestion and a definite statement
of fact; but leaving this apart we may regard his work as
confirming McIntosh’s previous interpretation of the bulbous
endings as masses of unicellular glands, the glandular se-
cretion being extended to the exterior through the surface
of the cells. Further, there is every reason to believe that,
according to Cole, some at least of the glandular masses
break up into rods.
LITERATURE.
Cotz, F. J. ‘Journ. Linnean Soc.,’ vol. xxvii, No. 175.
Turers. ‘ Abb. k. Gesellsch. Wissensch. Gottingen,’ Bd. xxxvi (1890).
Harmer, 8. F.—Appendix to “ Challenger” Report, vol. xx.
d ‘Zool. Anzeiger,’ No. 545.
“Take, A.—‘ Jenaische Zeitschrift,’ xxv (1890).
McIntosu, W. C.—*On Cephalodiscus dodecalophus,” ‘ “ Chal-
lenger” Report,’ vol. xx.
DAP OP
ON THE DIPLOCHORDA. TPA |
7. Mastermay, A. T.— On the Diplochorda,” part ii, ‘Q. J. M.8.,’ Aug.,
1897.
8. 7. “On the Notochord of Cephalodiscus,” ‘Zool.
Auz.,’ No. 545, 1897.
9. - *©On the Origin of Vertebrate Notochord and Pha-
ryngeal Clefts,” ‘ Rep. Brit. Assoc.,’ Sept., 1898.
10. = On Further Anat. and Budding Processes of
Cephalodiscus,” ‘Trans. Roy. Soc. Edinb.,’
vol. xxxix, pt. ill, No. 17.
15 s “Qn the Diplochorda,” part ii, ‘Q. J. M. S.,
vol. xlili, pt. il.
DESCRIPTION OF PLATES 32 & 33,
Illustrating Mr. A. T. Masterman’s paper “On the
Diplochorda.”
Fres. 1—5.—Selected sections (1, 3, 5, 7, 15) from a longitudinal sagittal
series through Cephalodiscus dodecalophus (Zeiss, obj. 7, eyep. 1).
Figs. 6—8.—Selected sections (1, 4, 6) from a transverse series through
Cephalodiscus dodecalophus (Zeiss, obj. 7, eyep. 1).
Fic. 9.—A semi-diagrammatic right half of a polyp.
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HYPURGON SKEATI.
On Hypurgon Skeati, a New Genus and Species
of Compound Ascidians.'
By
Igerna B. J. Sollas, B.Sec.Lond.,
Bathurst Student of Newnham College.
With Plates 34 and 35.
Amone the marine sponges from the Malay Peninsula
collected by Mr. R. Evans, of Oxford, at present Curator
of the Government Museum in Georgetown, Demerara, and
very kindly handed over to me by Dr. Harmer for description,
there were included two specimens of the new genus of
Synascida Didemnida, which I have endeavoured to describe
below.
The locality named on the collector’s label in the case of
each of the two specimens is Pulau Bidang.
The association of the Tunicate with a sponge was merely
fortuitous, and due solely to participation in the same surface
of support.
The colony forms a thin sheet, little over 1 mm. in thick-
ness, adherent to the substratum. The colour in spirit is a
dirty yellowish brown.
The appearance of the colony when examined by reflected
light under a low power of a binocular microscope is repre-
11 take the Greek troupydc, and by lengthening the 6 get irovpydr,
meaning a place where things are made serviceable.
730 IGERNA-B. J. SOLLAS.
sented in Pl. 34, fig. 2. This external view shows at once
the character to which the generic name alludes, namely, the
presence in the test of numerous ovoid fecal pellets. These
are seen through the transparent substance of the test, and
now appear of an opaque cream-white colour. Clusters of
calcareous asters (fig. 3) mark out the oral siphons, since
they make a conspicuous snow-white patch around each
siphonal aperture. These white spots are visible also with
the naked eye.
The arrangement of the ascidiozoids is irregular. A large
number of them share the same atrium, the atria being
shallow but extensive cavities with but few and small
siphons. The siphons are not visible in surface view, but in
section it is seen that their lps are formed of transparent
test-tissue destitute of spicules.
The bulk of the common test, which consists of actual
tunicin, is small, its substance being excavated by numerous
oval spaces, in which the fecal pellets he. To reach this
position the pellets must, after being ejected into the atrium,
sink through the excessively thin epithehal wall of that
cavity. The cellular elements in the test are of the usual
types; bladder-cells are specially abundant near both upper
and under surfaces, and round the oral siphons. Spicules
occur in small numbers, chiefly aggregated round the oral
siphons and in the neighbourhood of the branchial sac.
They may be isolated or packed in dense clusters (Pl. 34,
fio. 3). Finally the renal vesicles, described presently, are to
be reckoned among the structures included in the test. The
ascidiozoids, as is common among Didemnida, have a sharp
constriction between the branchial region of the body and
the abdomen.
The number of lobes round the oral siphons varies from
four to six. The tentacles are twenty-four in number ;
twelve long ones alternate with twelve short. The branchial
sac has four rows of five stigmata on each side. Connectives
(Hancock; trabecule, Yves Delage) are absent. The
dorsal languets are long and median in position. The sub-
HYPURGON SKEATI. Tar
neural gland has a simple opening with a swollen lower lip
(fig. 4, d. ¢.).
Through the narrow aperture of communication between
the two regions of the body the cesophagus descends to open
into the stomach, while the intestine passes upwards into the
rectum, which lies above the constriction, so that the anal
opening is close to the base of the branchial chamber.
The walls of the stomach are raised up round the termina-
tion of the cesophagus ; or, in other words, the cesophagus
has its opening deep in the cavity of the stomach; the ter-
minal part of the cesophagus is richly ciliated. 'The intestine
of a young bud is frequently found attached at both ends to
the cesophagus, to which it owes its origin. When this is
the case the thoracic portion of the same bud is to be seen
lying in the test at the opposite side of the cesophagus. The
budding is thus of the type known as pyloric (Giard),
and found among Didemnidee in the tribe Didemnine (Y.
Delage).
The walls of the stomach are smooth; seen en face from
the outside they show a beautiful reticulum formed of the
more deeply staining protoplasm which surrounds and con-
nects the nuclei of the cells of the gastric epithelium.
The intestine as it leaves the stomach is richly ciliated; in
passing thence to the anus its walls become continually
thinner, the walls of the rectum being almost membranous.
The anus has thickened lips. The alimentary canal is bathed
by blood-sinuses along its whole course.
The heart in its pericardium runs more or less vertically
between the upper and lower walls of the abdominal
cavity. Its lower end abuts against and sends a large vessel
into a prominence of the test, the sides of which are covered
by a patch of specially large cells of the mantle which form
the glandular part of the renal organ (r. gl., figs. 5 and 7).
The excreta of these glandular cells appear to be picked up
by wandering cells—presumably corpuscles of the blood con-
tained in neighbouring vessels or sinuses. These cells would
then migrate into the test, carrying their burden with them.
732 IGERNA B. J. SOLLAS.
Large numbers of vesicular cells contaming concretions are
to be found embedded on each of the above-mentioned
prominences of the test, while in older kidneys there may be
a relatively enormous rounded mass of such vesicles more
deeply situated in the test substance (fig. 7, k.). Some such
masses may be found in the basal layers of the test at a
distance from the abdominal cavity of any zooid; these have
evidently been left behind, the zooid to which they belonged
having shifted upwards as the floor of the cloacal cavity was
raised by the continual addition of fresh pellets.
Thus the excretory organs of Hypurgon agree with the
simple type of excretory organ found in Botryllus, in that
the urinary concretions are stored in the cavities of single
vesicular cells; but apart from this particular they are of a
type unlike any yet described (Dahlgriin, ‘ Archiv fiir mikr.
Anat.,’ vol. lviii, 1901) among Tunicates, and are far less
simple than any known in other Synascida.
The reproductive organs lie in shallow depressions of the
wall of the abdominal cavity (fig. 9). The testis is oval,
and the vas deferens makes four or five turns of a spiral
around it. The ovary has membranous walls, and contains a
string of eggs of successive ages. I have not seen an oviduct.
Any mature ova that I have seen have sunk deep into the
test, and so have come to lie in a great recess of the abdomi-
nal cavity (fig. 10), communicating with it by a narrow
aperture. The material contains but one larva, which was
developing in a completely closed cavity in the test (fig. 11).
This may or may not be the normal course taken by the
developing eggs. Hggs are not to be seen being sheltered
by any other part of the organism than the test, though eggs
of all ages were found in the ovaries.
The feecal pellets, which contribute so largely to the
formation of the test, show a very remarkable degree of
coherence. If a piece of the colony be boiled in sulphuric
acid, the residue consists of faecal pellets which retain their
form perfectly, and continue to do so even if the boiling be
much prolonged. Even thin sections of pellets, isolated by
HYPURGON SKEATI. 733
boiling microtome sections of the colony in sulphuric acid,
may still be mounted whole after this treatment. Boiling in
aqua regia and boiling in fuming nitric acid are equally
ineffectual in disintegrating the pellets; when these latter
reagents are used the test naturally forms part of the residue,
since they are not capable of dissolving tunicin.
When isolated by means of sulphuric acid the pellets have
a black colour, due to the action of the acid on the organic
matter contained in them. These blackened pellets may next
be washed and calcined, and though raised repeatedly to
cherry heat they still remain intact, and are now opaque
white when examined by reflected light. Mounted in oil, or
passed through oil into balsam, they become transparent.
Calcined pellets dissolve completely in hydrofluoric acid.
Prolonged boiling in a strong solution (nearly saturated) of
caustic soda resulted in the dissolution of calcined pellets.
It seems, then, that the strong coherence of the pellets must
be due either (1) solely to cohesion and adhesion between the
foreign particles contained in them, or (2) to a deposition of
silica between these particles. The siliceous nature of the
greater part of this foreign matter makes it impossible to
determine between these two alternatives. It naturally
suggests itself that this property of coherence of the pellets
is an adaptation to enable the aninal to utilise waste organic
matter with impunity. But it must be mentioned that the
pellets are porous, taking stains readily both before the
treatment described above, and also at every stage during it.
It is curious that the pellets are also highly fragile; they
crumble at once under pressure of the cover-slip.
Melicerta tubes were boiled in avid for comparison: the
form of the component pellets was lost almost immediately—
as soon as the cementing substance between neighbouring
pellets disappeared.
A parasitic crustacean was found in one ascidiozoid,
occupying a large part of its branchial chamber. ‘The body
of the parasite is a mere sac filled with ova in an advanced
state of segmentation. There appear to be six pairs of
von. 46, PART 4.—NEW SERIES. BBB
734 IGERNA B. J. SOLLAS.
appendages belonging to the anterior region of the body,
besides one foremost pair which serves as an organ of
attachment, and is inserted into the tissues of the host.
The systematic position and diagnosis of the genus may
be stated as follows:—Synascida Didemnida Didemnina,
(Y. Delage). Colony thin; ascidiozoids with four rows of
branchial shts and twenty-four tentacles; vas deferens
spirally coiled round the testis ; feecal pellets included in the
test, in which organ the renal vesicles are likewise contained.
In conclusion, it gives me much pleasure to take this
opportunity of expressing my thanks to Mr. Graham, Kerr
for kind help and advice.
LITERATURE CONSULTED.
Herpman.— Voyage of H.M.S. Challenger,’ Tunicata 11.
Yves Detacre.—‘ Zoologie concrete,’ vill.
Dantertn, W.—‘ Archiv fiir nat. Mikr.,’ lviii, 1901.
Granp.—‘ Arch. d. Z. expér.,’ i, 1872.
EXPLANATION OF PLATES 34 & 35,
[llustratine Teerna B. J. Sollas’s paper “On Hypurgon
Skeati, a New Genus and Species of Compound
Ascidians.”
as. Caleareous spicule. aé.s. Atrial siphon. éd.c. Blood-corpuscle. d/. e.
Vesicular cell of test. 02.8. Blood-sinus. d./. Dorsal languet. d. ¢. Dorsal
tubercle. exd. Mndostyle. ££ Fusiform cell. g. Nerve ganglion. 4. Heart.
int. Intestine. ¢. Larva. x.éc. Notochord. @. Cisophagus. ov. Ova. p.
Fecal pellet. p.e. Pericardium. rect. Rectum. 7. Renal organ. 7. gl.
Glandular cells of renal organ. 7.¢c. Renal concretion. sf. Stomach. ¢.
Testis. v.app. Vase. appendage. v.d. Vas deferens.
HYPURGON SKEATI. 735
PLATE 34.
Vig. 1.—A piece of a colony of Hypurgon Skeati, slightly larger than
natural size.
Fic. 2.—A portion of the surface of a colony seen under a binocular micro-
scope. X 75.
Iie. 3.—Caleareous spicules from the test of Hypurgon Skeati. a@and d
from one colony ; ¢, e, and ffrom a second. /, acluster of spicules.
Vie. 4.—A vertical section through a part of a colony of Hypurgon
Skeati, showing the branchial sac and parts of the abdominal cavity of one
zooid (slightly reconstructed from neighbouring sections). x SO.
Fig. 5.—Vertical section of an abdominal cavity.
Fic. 6.—Diagrammatic reconstruction of a slice of acolony of Hypurgon
Skeati, showing one zooid from the left side and one from the dorsal surface.
Drawn as though it were transparent.
PLATE 35.
Fie. 7.—Section of a renal organ of Hypurgon Skeati which has been
functioning long enough to form the considerable accumulation of concre-
tions K.
Fic. 8.—Portion of the test of H. Skeaticontaining renal vesicles, more
highly magnified.
Fic. 9.—Section of an abdominal cavity of zooid of H. Skeati, to show
reproductive organs.
Fic. 10.—Section of alarge ovum of H. Skeati ina recess of the abdominal
cavity.
Fig. 11.—Section of a tailed larva of H. Skeati developing in a closed
cavity in the test.
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ANATOMY OF ARENICOLA ASSIMILIS.
The Anatomy of Arenicola assimilis, Ehlers,
and of a New Variety of the Species, with
some Observations on the Post-larval Stages.
By
J. H. Ashworth, D.Se.,
Lecturer on Invertebrate Zoology in the University of Ndinburgh.
With Plates 36 and 37.
ConTENTS.
PAGE
I. Introduction . ;
II. Arenicola eect Ehlers. : Se 4
III. Specimens of Arenicola from New Zenon 75
IV. Systematic Position of A. assimilis and of the Specimens Frey
New Zealand . 760
V. Post-larval Stages of Arenicola on the Falkland Telomtte . 164
VI. Adult Specimens of Arenicola from the Falkland Islands . 768
VII. Distribution of Arenicola assimilis , : mee ike:
VIII. Specific Characters of the Caudate Arenicolide 775
IX. Summary of Results. : : ' Bae AT
itt)
X. Literature
I. Introduction.
In response to my inquiry regarding the occurrence of
Arenicola on the shores of New Zealand, Professor Benham
kindly sent to me three specimens of this worm from Otago
Harbour, and one from the Macquarie Islands.
‘The specimens were caudate Arenicolide resembling A.
marina, Linn., and A. claparedii, Levinsen, in external
form. A rapid examination of the grosser anatomical
features of one of the Otago specimens seemed to point to its
738 J. H. ASHWORTH.
close affinity with the latter species, for it was at once seen
that the New Zealand specimen possessed multiple ceso-
phageal glands and that there were no pouches on the first
diaphragm—two features known only in, and considered to
be almost diagnostic of, A. claparedii. At first, also, only
five pairs (the number occurring in A. claparedii) of
nephridia were seen in the Otago specimen, but finally a
much reduced pair was found in the segment anterior to the
one bearing the first fully developed nephridia. In the
other three specimens sent by Professor Benham there are
six pairs of fully developed nephridia, which is evidently the
normal condition. The lateral lobes of the prostomium of
these specimens were found to be more feebly developed than
those of A. claparedii. There were therefore two points
in which the southern specimens agreed with Levinsen’s
species, viz. the presence of multiple cesophageal glands and
the absence of diaphragmatic pouches; and two features in
which they differed, viz. the form of the lateral lobes of the
prostomium and the number of nephridia.
On sectioning the anterior end of one of the Otago speci-
mens a pair of large otocysts was found, each opening to the
exterior by a narrow tube. The presence of these well-
developed organs, in conjunction with the important differ-
ences above mentioned, finally settled that the New Zealand
specimens do not belong to the species A. claparedii, in
which the absence of otocysts is so characteristic and
remarkable a feature.
These specimens agree with A. marina in the number,
position, and character of their gills, in the number of their
nephridia, and in the general anatomy of their otocysts ; but
the southern specimens are clearly distinguished from
A. marina by their multiple cesophageal glands, by the
absence in the former of diaphragmatic pouches, and by
other less obvious features.
The only other known species to which the New Zea-
land specimens show any close similarity is A. assimilis,
Khiers (1897, pp. 103, 104), of which only the external
ANATOMY OF ARENICOLA ASSIMILIS. 739
characters are briefly described. Ehlers states that this
species closely resembles A. marina in general external
characters, but that in A. assimilis there are twenty
cheetigerous segments and asa rule there are thirteen pairs
of gills, the first being situated on the eighth chetigerous
annulus, but occasionally only twelve pairs of gills are
present. Hhlers also finds that, compared with A. marina,
the median lobe of the prostomium of A. assimilis is
proportionately smaller than the lateral ones, and the noto-
podial setee somewhat more feebly feathered.
The New Zealand specimens differ from Ehlers’ species in
the number of cheetigerous segments and in the position of
the first gill, but unfortunately Ehlers does not mention the
nephridia or nephridiopores, the cesophageal glands or the
otocysts,—important diagnostic characters concerning which
information was essential before the affinity of the New
Zealand specimens with A. assimilis could be either
accepted or rejected. My thanks are due to Dr. Michaelsen,
of the Hamburg Museum, who collected the specimens
examined by Ehlers, for kindly sending to me two complete
examples of A. assimilis from Uschuaia, in Tierra del Fuego,
and an incomplete specimen from Punta Arenas, in the Straits
of Magellan.
As Ehlers has given only a brief account even of the
external characters of his species,’ I propose to describe the
principal features of the three specimens given to me by Dr.
Michaelsen before proceeding to consider the New Zealand
specimens in detail.
! Since writing the above I have received, through the kindness of Pro-
fessor Ehlers (1901), a copy of his recently published monograph, ‘ Die Poly-
cheeten des magellanischen und chilenischen Strandes,’ in which he describes
(pp. 177, 178) the structure of the otocysts of A. assimilis, and also states
that the alimentary canal, the vascular system, and the nephridia of this
species agree, so far as he can ascertain, with those of A. marina. ‘his
agreement is, however, not quite so close as Mhilers’ statement would lead
one to expect, since, for example, the cesophageal glands in the latter species
are a single pair, while in A. assimilis there are several pairs. These points
are further discussed below,
740 J. H. ASHWORTH.
II. Arenicola assimilis, Ehlers.
The two complete specimens found at low-tide mark at
Uschuaia are 105mm. and 120 mm. long respectively. The
specimen from Punta Arenas would have been about 120 mm.
long if complete. Their colour in spirit is a fairly uniform
yellowish brown, but the tail of one specimen is of a some-
what darker colour. The body is slightly swollen in the
anterior region. The animal closely agrees in form with
specimens of A. marina of the same size.
External Characters.—The prostomium (see fig. 20) is
moderately developed; the two lateral lobes are in the form of
a V, the arms of which embrace the median lobe.’ ‘The
prostomium resembles that of A. marina and A. cristata,
except that the median lobe is proportionately smaller in
Ehlers’ species. ‘lhe nuchal organ is similar in its structure
and relations to that of A. marina. The metastomial
grooves indicating the track of the cesophageal connectives
are well marked.
There are twenty cheetigerous segments, the last thirteen
of which are branchiferous; the first gill is thus on the
eighth chetigerous annulus. In this character A.
assimilis differs from all other caudate Arenicolidz, in
which the first gill (except in those abnormal cases in which
the first true gill is missing) is on the seventh cheetigerous
annulus, Of the three specimens in my possession only one
has thirteen pairs of fully developed gills; in another the first
right gillis very minute; while in the third specimen the first
left gill is small, and the last gill on each side is considerably
smaller than the preceding one. Ehlers (1897, p. 104) records
specimens with only twelve pairs of gills.
The gills of the specimens from Uschuaia are dense bushy
structures resembling the dendritic type of gill found in
’ There is a rough sketch (‘‘d’aprés Ehlers”) of the prostomium of A.
assimilis ina memoir by Fauvel (1899, p. 178), which, however, does not give
an accurate impression of its form, as the two lateral lobes are represented as
separate, whereas they are actually united posteriorly to form the V-shaped
structure described above.
ANATOMY OF ARENICOLA ASSIMILIS. 741
littoral specimens of A. marina. There are in most of them
about eight main stems, each 2°5 mm. to 3 mm. long, bearing
five or six dichotomously subdivided branches on each side.
The gills of the specimen from Punta Arenas are larger and
of a somewhat more regular form. ‘The eight or nine stout
main stems are about 4 mm. long, and are regularly arranged
in radiating fashion; each bears six or seven pairs of
branches which divide dichotomously. Although at first
sight this gill seems to approach to the pinnate type, the
lateral branches are neither so numerous nor so regularly
arranged as in pinnate gills, and the gill may be regarded
as merely a well-developed example of the dendritic type.
The skin is subdivided into annuli. Between the prosto-
mium and the first chretigerous annulus there are five rings
(see fig. 20). The first four of these represent the region
found in other species of Arenicola which has been shown
to be composed of the peristomium (here represented by the
first two of these rings) fused with the first body-segment of
the post-larva, the sete in which disappear very early (see
figs. 19, 20). The fifth ring is the first annulus of the first
chetigerous segment, this segment being composed of three
annuli, viz. a chetigerous one and the annulus preceding
and following it. ‘The second and third chetigerous seg-
ments also consist of three annuli, the middle one bearing the
sete. The fourth and succeeding segments up to the end of
the branchial region are composed of five annuli, the fourth
of which is chetigerous. The region between any two
cheetigerous annuli behind the third is therefore subdivided
into four rings.
The epidermis of the tail is raised into numerous papille.
The segmentation of this region is only feebly marked, but it
is indicated, especially in the anterior portion of the tail,
by the presence of somewhat larger annuli placed at regular
intervals, upon which the epidermal papille are distinctly
larger than those on the intervening annuli. Each of these
larger rings is followed by a slight constriction, denoting the
presence internally of a septum, best seen in those parts of
742 J. H. ASHWORTH.
the tail which were stretched at the moment of death. The
space between two of the larger annuli is subdivided at the
anterior end of the tail into two or three rings, but further
back into from four to ten. These smaller annuli also bear
epidermal papilla, but in the anterior tail region they are
distinctly smaller than the papille found on the larger annuli.
Proceeding backwards along the tail, the difference in the
size of the annul and of the papille they bear may be
clearly recognised until the middle of the tail has been passed ;
then the papille become almost equal in size, and near the
anus if is impossible to distinguish any difference between
those of the various annuli. ‘There are about twenty-eight
segments in the tail of each of the complete specimens.
Setz.—The capillary sete (figs. 1, 1a) of the notopodium
are very similar to those of A. marina. They attain a
length of 4:5 mm., and on their distal fourth bear small
pointed processes, which, as Khlers (1897, p. 104) remarked,
are not so well developed as those of A. marina. The
processes are usually present on both sides of the seta; they -
are moderately obvious on one side, but on the other they are
very minute, and are borne on the edge of a thin border or
lamina. This lamina, which seldom exceeds 6 u in width,
extends along the seta for a distance of about one third its
length. In some of the setee the lamina is not denticulate at
its margin, and in others is only very faintly so; but it is
crossed by fine oblique lines, the intervals between which
correspond roughly to the size of the teeth on the dentigerous
lamin. From an examination of the sete of A. assimilis
and A. marina, it seems probable that the lamina at first
possesses an entire margin, but later this tends to break up
from the edge inwards, thus giving rise to the minute teeth
which are usually seen on full-grown sete. This explanation
would account for the fact that in some setz the margin of
the lamina is entire, while in others it bears either exceed-
ingly minute denticulations or the more obvious teeth shown
in fig. La. These three conditions are occasionally seen at
different points along the border of a single seta.
ANATOMY OF ARKENICOLA ASSIMILIS. 745
Setz similar to those above described are present in A.
marina, and in some examples the lamina is very well
marked, e. g. in a specimen of the Laminarian variety the thin
border extends for nearly a millimetre along the seta, and
attains a width of 20 uw. Similar sete are present in A.
eristata, but in A. claparedii! the lamina is not so well
developed, being short and narrow. In A. ecaudata and A.
grubii the lamina is also very narrow, seldom exceeding
about 3 in width.
The neuropodia of A. assimilis are easily seen, even in
the first segment. ‘They are especially well developed in
the branchial region, where each resembles a pair of closely
applied tumid lips, between which is the row of crotchets.
These (fig. 9) are often curved, and are 0°6 mm. to 0°7 mm. in
length, being considerably longer than cheetz from specimens
of A. marina of the same size. (The longest crotchets found
in a specimen of A. marina 125 mm. long are only 0°47 mm.
in length.) The rostrum is short and blunt, even in unworn
cheetee. There is a small subrostral enlargement, and about
six to nine teeth are present just behind the rostrum.
Musculature.—The musculature calls for little comment ;
it is similar to that of A. marina, except that the oblique
muscles are present along the whole animal from the first
diaphragm to the end of the tail. They are exceedingly thin
bands, somewhat broader in the posterior part of the gill
region, but even here seldom exceeding 0°5 mm. in width, and
as a rule they are only 0:2 mm to 0°3 mm. wide. There is a
dorsal mesentery in the first and second cheetigerous seg-
ments supporting the dorsal blood-vessel. The three dia-
phragms are, as usual, situated at the anterior ends of the first,
third, and fourth cheetigerous segments. ‘There are no
pouches on the first diaphragm. This condition was con-
sidered to be so marked a feature of A. claparedii that it
was given as one of the diagnostic characters of this species
(Gamble and Ashworth, 1900, pp. 533, 541), since all other
Arenicolide whose anatomy is fully investigated possess
' See Gamble and Ashworth, 1900, pl. xxiii, fig. 23,
744 J. H. ASHWORTH.
diaphragmatic pouches, and in some species they attain a
large size, e.g. in A. cristata they may reach a length of
12mm. Many of the blood-vesseis which cross the ccelom
obliquely to the nephridia and gills are provided with a very
obvious connective-tissue strand or band, which gradually
increases in size in the posterior segments of the gill region,
forming in the last two or three segments of this part of the
body an almost complete septum supporting the afferent and
efferent branchial vessels. There are well-developed caudal
septa.
Alimentary Cana].—The most striking feature of the
internal anatomy of A. assimilis is the presence of multiple
cesophageal pouches. ‘These are placed on the sides of the
cesophagus, just behind the third diaphragm. There are in
each of two specimens six, and in another eight, pouches on
each side. The anterior pair is long—-12 mm. in one speci-
men, 17 mm. in another,—and each of them is usually swollen
at or near its anterior free end, having a club-shaped appear-
ance. Their abundant blood-supply is evidenced by the
network seen in their walls. In the contracted condition
these anterior pouches are digitiform structures with a some-
what moniliform appearance. ‘The smaller posterior glands
are from 1 mm. to 4 mm. in length, and are pear-shaped or
oval sacs with rather thicker walls. As in other Areni-
colide, the cavity of each pouch is partially subdivided by
numerous septa produced by infolding of the wall; each
septum, therefore, is composed of two lamellw of glandular
epithelium, between which is a cavity filled with blood. The
partitions are very obvious in the smaller pouches, and in
the larger pouches when in a contracted condition; but when
these are fully distended the septa become mere ridges on
the inner wall of the pouch. In the presence of multiple
cesophageal pouches A. assimilis conforms to another of
the features hitherto considered to be peculiar to A.
claparedii, as in all other species of Arenicola in which
the pouches are known there is only a single pair.! In other
' Ehlers (1901, p. 177) states that the gut of A.assimilis agrees with
ANATOMY OF ARENICOLA ASSIMILIS. TA5
respects the alimentary canal resembles that of A. marina.
The ventral groove, which is well seen in the intestine, may
be traced forwards into the stomach to about the level of the
eighth or seventh seta.
Vascular System.—The vascular system closely agrees
with that of A. marina (see Gamble and Ashworth, 1898,
pl. u), except in the fifth and seventh chetigerous seg-
ments. In each of these there is only one pair of vessels,
afferent branches of the ventral vessel, passing to the
nephridia. The first pair of efferent branchial vessels is
situated in the eighth segment; this and the four succeeding
pans open into the subintestinal vessels, while the last eight
gills, 1. e. those of the thirteenth to twentieth segments,
return blood to the dorsal vessel. The body-wall is well
supphed with blood-vessels, especially in the anterior region ;
in sections of the peristomium and first cheetigerous seg-
ment there are numerous vessels lying either in the con-
nective tissue or in small ccelomic canals (see below) just
beneath the epidermis (fig. 22); in sections of some of the
posterior segments the vessels are not so abundant.
The heart is of moderate size, and has the usual relations.
There is a cardiac body formed by ingrowths, chiefly of the
posterior wall of the heart, and this is well developed in one
of the specimens 120 mm. long.
Celom.—The ccelom is spacious, as in A, marina. A
remarkable feature noticed at once in sections (fig. 22) of the
anterior end of A. assimilis is the large number of coelomic
spaces in the body-wall and between the muscles. In this
portion of the animal there are exceedingly numerous coelomic
canals lying in the subepidermal tissue of the body-wall,
penetrating into the muscle-bands, especially of the buccal
musculature, insinuating themselves between the brain-lobes
and between the brain and the prostomial epithelium, and
often accompanying the blood-vessels which supply the body-
that, of A. marina, but the presence of multiple cesophageal glands in the
former while there is only a single pair in the latter species is a point of
difference of considerable systematic importance.
746 J. H. ASHWORTH.
wall. In each of these canals the thin lining of ccelomic
epithelium may be easily recognised, and ccelomic corpuscles
may also be found in many of them. Similar canals are
present in A. marina (Gamble and Ashworth, 1898, p. 28),
in A. grubii, and to a less extent in the other species ;
but the development of these outgrowths of the ccelom
reaches its maximum in A. assimilis. They probably act
as nutritive, and possibly also as excretory and respiratory
channels. ‘here are very few coelomic canals in the pos-
terior part of the animal. The coelomic fluid and corpuscles
resemble, as far as can be ascertained in preserved specimens,
those of A. marina.
There are six pairs of nephridia, the ex-
Nephridia.
ternal openings of which are slightly posterior to the dorsal
ends of the fourth to the ninth neuropodia. The funnels of
the first pair of nephridia lie on the anterior face of the
third diaphragm. This condition is found again only in
A. marina; the first pair of nephridia of other species
corresponds in position to the second pair of A. assimilis
and A, marina. The dorsal lip of each nephrostome
(fig. 17) bears about twelve to fourteen spatulate or triangular
ciliated processes attached by their narrower ends. These
are subdivided distally, some of the larger ones into five or
six. The edge of the ventral lip of the nephrostome is
thrown into folds or frills, so that although it agrees in
general shape with the ventral lip of the nephrostome
of the marina section of the genus, the ventral nephro-
stomial lip of A. assimilis is quite distinguishable by this
peculiar character. his frilling is probably not due to con-
traction on killing, as it is not seen in specimens of
A. marina, A. claparedii, and A. cristata which have
been killed in a similar manner,
The nephrostome of the first nephridium lies on the
anterior face of the third diaphragm, and is directed an-
teriorly. It is smaller than any of the other nephrostomes,
its dorsal lip bears only eight to eleven processes, and the
frilling of the edge of the ventral lip is not so well marked.
ANATOMY OF ARENICOLA ASSIMILIS. 747
The first nephridium is usually distinctly smaller than any
of the others, a condition frequently noticed in A. marina.
Although this nephridium possesses a gonidial vessel, no
gonad is developed upon it.
Gonads.—The gonads are, as in other species, associated
with the nephridia, and are present on all except the first
pair. Hach gonad is a club-shaped mass of cells about 1°5
mm. long (fig. 17), formed by proliferation of the cells
covering the gonidial vessel (Gamble and Ashworth, 1900,
p. 921) immediately behind the nephrostome. The forma-
tion of ova and spermatozoa follows the same course as in
A.marina. ‘The ova present in the ccelomic fluid of the
specimen from Punta Arenas are apparently mature,! and
have a distinct but thin vitelline membrane (3 , thick).
They are not spherical, but somewhat discoidal. The face of
the disc is usually oval, and measures 0°19 to 0°20 mm. by 0°15
to 0°16 mm. in diameter. The thickness of the egg is about
0-075 mm. Measurements of a considerable number of well-
preserved unshrunk eggs from the ccelomic fluid show that
the three axes above named are fairly constant in propor-
tion. It will be convenient to correct here a statement in
the memoir by Dr. Gamble and myself (1900, p. 527) in
which- the ova of A. marina, A. claparedii, and A.
cristata are described as spherical. This isa mistake, as the
ova of all these species are flattened in one plane, like those of
A. assimilis described above. In A. marina? the face of
the egg is either circular (usually) and about 0°14 to 0°15 mm.
in diameter, or it is oval, with diameters of 0°16 and 0:12 to
0-14 mm., and the third axis of the egg is from 0:08 to 0:09
mm. in length. ‘The ova of A. claparedii are usually only
slightly oval, the two diameters of the face of the egg being
about 0°16 mm. and 0-14 to 0°16 mm. respectively, and the
1 This specimen Was taken in September, 1892.
2 The following description and measurements may replace those given on
p. 527 in the memoir cited above; they have been drawn either from living or
well-preserved ova, most of which have come into my hands since the com-
pletion of that memoir.
748 J. H. ASHWORTH.
thickness of the egg 0°07 mm. In A. cristata the three
axes of ova removed from a ripe female in Naples measure
0°155mm., 0°145 mm., and 0°07 mm. respectively.! The ova of
A. grubiiand A. ecaudata are not compressed in this way,
or only very slightly so. They are usually ovoid, and ripe ova
of the former species are 0°17 mm. long and 0°15 mm. broad
and thick. The largest ova of A. ecaudata which I have seen
have slightly smaller dimensions, but they are probably not
quite mature. The ova of A. grubii and A. ecaudata are
distinguished by their stout vitelline membrane, which is 5
to 6 4 thick; while in'A. claparedii, A. cristata, and A.
assimilis itis 2 to 3 m,and in A. marina only slightly over
1 «x in thickness.
Brain.—The brain of A. assimilis conforms to the
general plan seen in the marina section of the genus. It
consists of a pair of anterior lobes placed well forward in
the prostomium, a pair of posterior lobes which lie below the
nuchal organ, and an intermediate region which connects the
anterior and posterior lobes. The anterior lobes are short
but very broad ; in fact, this is by far the broadest part of
the brain; behind these lobes the brain gradually tapers.
The brain may be roughly compared in shape to two slightly
flattened pears lying side by side with their narrower faces
adjacent and fused along the middle third of their length.
The broad forwardly directed ends of the pears represent
the anterior cerebral lobes, while the tapering ends represent
the posterior lobes, which are continuous with two nerve-
tracts lying below the epithelium of the nuchal organ. The
anterior brain-lobes are separated in front by a ccelomic space.
Hach gives off anteriorly and dorsally a series of nerves
to the epithelium of the prostomium. ‘The anterior part
of these lobes consists almost entirely of small cells situated
in clusters and separated from one another by fibrous tracts
and by neuroglial tissue. Further back the delicate neuro-
pile which forms the core of these anterior lobes is well seen,
' See also Child (1900), p. 592, for further observations on the ova of A.
cristata.
ANATOMY OF ARENICOLA ASSIMILIS. 749
surrounded by clusters of nerve-cells and neuroglial tissue.
Bundles of nerve-fibrils may be traced from the bases of the
prostomial epithelial cells into the neuropile (see fig. 23).
Larger unipolar ganglion-cells are found immediately out-
side the neuropile, and particularly on the side nearest the
middle line. The cesophageal connectives arise from the
anterior lobes at the point where the neuropile reaches its
greatest development (fig. 22). The eyes are found on the
dorsal side of this part of the brain. There are four or five
on each of the anterior lobes. A little further back large
pyriform ganglion-cells become more numerous, and
especially on the inner side of the two anterior lobes just
before they unite in the middle line and for some distance
after their union (see fig. 23). Passing backwards along
this region, it is seen that the ganglion-cells become more
restricted to the dorsal and lateral faces of the brain, the
middle and ventral parts being composed largely of neuro-
pile, in which also neuroglial cells and fibrilla may be
recognised. The ganglion-cells of this region are more
intimately associated with the median part of the prostomium.
The posterior brain-lobes are small and tapering, and
eradually merge into two nerve-tracts which lie on the inner
side of the nuchal organ just below the sensory epithelium.
The brain of the specimen 120 mm. long is 0°65 mm. in
length, 1 mm. broad across the anterior lobes, and about 0-4
mm. deep in this region. It is most nearly like the brain of
A. marina, but is wider anteriorly. (The brain of a speci-
men of A. marina 120 mm. long is about 0°7 mm. wide.)
Gisophageal Connectives.—The csophageal connec-
tives arise from the lateral region of the broadest part of the
anterior cerebral lobes, i.e. just in front of their point of
union. Each is a stout fibrous cord with numerous cells on
its outer face, which, in the first part of its course, lies about
a millimetre below the epidermis, and is slung up in a
muscle sheet, which is attached to the subepidermal muscu-
lature by numerous muscle-strands (fig. 22). It is only in
the ventro-lateral region that the connectives approach the
voL. 46, PART 4.—NEW SERIES. Cee
750 J. H. ASHWORTH.
body-wall and finally come to lie upon the layer of circular
muscles. The connectives, the course of which is indicated
externally by the metastomial grooves, unite in the hinder
portion of the third annulus (cf. fig. 19). Hach gives off
numerous branches to the epidermis of the region through
which it passes ; in fact, the nerve-supply to the skin and
following segment is enormous—nerves pass into the raised
areas upon the skin and repeatedly branch, their terminations
lying in close contact with the bases of the epidermal cells
(fig.22). In addition to one or two nerves derived from each
connective, the skin of the region immediately below the
prostomium receives two moderately stout nerves which
arise from the ventral portion of each anterior cerebral lobe
close to the point of origin of the connective. These nerves
run on to the roof and sides of the mouth, and their branches
may be traced a considerable distance along the pharynx.
They are apparently more numerous on the dorsal than on
the ventral region of the pharynx. In many sections the
nerves may be seen sending branches along the axes of the
buccal papille (fig. 22). The connective gives off a very
short but stout nerve to the otocyst. Ganglion-cells are
present in moderate number around the point of origin of
this nerve. The nerve comes into contact with the otocyst
at the point where the tube leads off to the exterior, and is
intimately related to both structures. It provides the otocyst
with a sheath of nervous elements, which lies just below the
sensory epithelium, and also sends a small nervous sheath
along the tube.
Nerve-cord.—The nerve-cord is situated within the
layer of circular muscles. The right and left fibrous portions
are separated by a median vertical sheet of neuroglia. ‘The
ganglion-cells are distributed along the whole length of the
cord, and are not aggregated into ganglia. ‘They are
numerous, unipolar, pyriform, usually quite small cells with
deeply staining nuclei, aud most of them are situated in the
ventro-lateral regions of the cord; but some few larger ones
measuring from 15 to 30 «in length, and having vesicular
ANATOMY OF ARENICOLA ASSIMILIS. 751
nuclei, are found rather nearer the middle line in many of
the sections examined. The process of each cell is directed
dorsally into the lateral portion of the fibrous mass. The
spinal nerves arise in the same manner and position as in
other Arenicolide (Gamble and Ashworth, 1900, pp. 482,
483). Giant-fibres to the number of two or three are seen
in sections of the branchial region and tail. The giant-cells
are regularly arranged, being situated close to the posterior
border of each segment. In eight of the nine segments
examined there is only one giant-cell per segment, but in the
other segment two cells are present near together. The
giant-cells are placed in the extreme lateral regions of the
cord, and in the first piece of cord which was sectioned they
are situated alternately on the right and left sides, i.e. in
the seventeenth to twentieth chetigerous segments, and in
the first tail segment of this specimen they are situated
respectively R, L, R, L, R (see fig. 12). In sections of the
same specimen taken further forwards (tenth to thirteenth
segments) this curiously regular arrangement was not found,
the cells present in these segments being situated respectively
L, L, RR (two cells are present in this somite), R. The
average size of the cellsis 0:065 mm. long and 0:05 mm. broad
and deep. Each cell is pyriform, surrounded by a fibrillated
sheath, and sends out usually ouly one process, which passes
at once into the fibrous portion of the cord towards the
lateral giant-fibre. ‘lhe protoplasm of one cell, however, is
drawn out dorsally into five processes, one of which is much
thicker than the others, and may be easily traced into the
mid-dorsal region of the fibrous part of the cord. The
slender processes are traceable only a very short distance,
being lost either between the small ganglion-cells or imme-
diately on entering the fibrous part of the cord. There is in
most of the giant-cells a more deeply staining area in the
protoplasm close to the nucleus, due to the presence of
chromophilous granules. This probably corresponds to the
similar but better marked centrosphere seen in A. grubii
(Gamble and Ashworth, 1900, pp. 487, 488, and fig. 76).
752 J. H. ASHWORTH.
Sense Organs.—The eyes, of which there are four or five
on each side of the prostomium, are similar to those of other
Arenicolide (Gamble and Ashworth, 1900, pp. 506, 507).
They are situated either among the small nerve-cells on the
dorsal surface of the anterior cerebral lobes just in front of
their point of union, or in the epidermis immediately dorsal to
this region. Hach eye is composed of a cup-shaped mass
(6 to 12 « in diameter) of reddish-brown pigment spherules
grasping the base of a spherical or ovoid lens.
The otocysts! are remarkable for their large size. They
are oval sacs whose three internal diameters are 0 37, 0°36, and
0°25 mm. respectively. Their size may be better appreciated
after comparison with that of the otocysts of other species
(see figs. 13, 16). The largest otocysts seen while examining
four large specimens of A. marina for this purpose were
found in one about 180 mm. long, where they measure 0:07,
0-16, and 0:17 mm. along each of their three internal dia-
meters ; wlile in a specimen 250 min. long the corresponding
measurements are 0°15, 0:12, and 0°15 mm. (fig. 16). The
nearly spherical otocysts of full-grown specimens of A.
cristata (800 mm. long), A. grubii (180 mm.), and A.
ecaudata (180 mm.) have a mean internal diameter of
0-17 to 0:18 mm., 0°16 to 0°17 mm., and 012 to 0°13 mm.
respectively. From these figures it will be seen that the
otocysts of A. assimilis are much larger than those of
any other species. Hach opens to the exterior by a narrow
curved tube. The external opening is very minute, and at the
bottom of a groove situated immediately in front and to the
outer side of the lateral portion of the nuchal organ (fig. 22).
1 Ehlers (1901, pp. 177, 178) has recently described these organs and their
numerous spherical otoliths, consisting of a concentrically layered, evidently
secreted, material. These vary in size from 0°003 to 0°03 mm. The small ones
are compared to those of A. claparedii (the author really means A. grubii).
The larger otoliths are often irregular, and in a few a central foreign body may
be observed. ‘The opening of the tube into the cyst is very small, and Ehlers
thinks this is correlated with the character of the otoliths. He also states
that the otoeyst is apparently larger and its external opening nearer the
brain than in A. marina,
ANATOMY OF ARENICOLA ASSIMILIS. 7538
The opening is close to the point of origin of the cesophageal
connective, i.e. at the dorsal end of the metastomial groove,
so that it is more dorsally situated than the corresponding
opening in A, marina. ‘The lumen of the tube is small,
and in two of the four examined is almost obliterated alone
part of its length by approximation of the walls. In three of
the tubes there are fine particles of foreign matter at one or
more points. The otocyst and tube are lined with a cuticle
about 3 in thickness. The epithelial wall of the otocyst is
comparatively thin (80 to 40 ,). The sense cells are not
easily distinguishable, at first sight, from the supporting cells,
but in one series of sections they may be distinguished by
the presence of neuro-fibrille: in the former. Hach sense cell
is seen to be traversed by asingle fine fibril, which terminates
immediately below the cuticle. These cells and fibrille are
especially abundant in the wall of the otocyst near the
entrance of the tube, and they are also present in the
adjacent part of the tube. Below the epithelium is the
nervous sheath, among the fibres of which occur scattered
fusiform or stellate cells. The nerve-supply to the otocyst is
derived from the cesophageal connectives (see above, p. 750).
The otocyst contains the coagulated remains of the fluid with
which it was filled in life. Among this coagulum are
numerous minute spherical deeply staining granules, which
are probably secreted by some of the cells in the wall of the
otocyst (fig. 13). There are about forty or fifty otoliths in
each otocyst; they are usually spherical, but a few are oval,
and some are irregular, but have a rounded outline. They
nearly all show concentric markings indicating the method
of their formation by deposition of layer upon layer of a
secretion produced by cells in the wall of the otocyst. ‘he
largest otoliths are 35 w to 45 « in diameter. In the centre
of a few of them there is a minute refringent body, evidently
of foreign origin, forming the nucleus around which the
secreted matter has been deposited. Besides the contents
already named, there are in the otocyst several deeply
staining bodies varying in size from the minute granules
7a4 J. H. ASHWORTH.
present in the coagulum to spherical, oval, or elongate masses
10 w in diameter, which are either free or adhering to the
surface of one of the otoliths. Their appearance suggests
that they are composed of a substance similar to that of
which the otoliths are formed, although the latter are usually
much more lightly stained (fig. 15).
The nuchal organ of A. assimilis resembles that of A.
marina in its main features. The epithelium of the organ is
composed of exceedingly slender columnar cells; some of
these—the sense cells—are 70 p to 80 pw long, and only about
2 » wide, and have deeply staining nuclei. The intervening
supporting cells are a little stouter, and their nuclei stain less
deeply. Many of these cells are ciliated, and some are
elandular. Beneath the epithelium there is a layer of nerve
elements in connection with the posterior brain-lobes. From
this layer neuro-fibrillae may be traced into and through the
entire length of many of the sensory cells.
Similar fibrils may be seen in some of the epidermal cells
of the general body surface, and of the papille of the
proboscis.
III. Specimens of Arenicoia from New Zealand.
Three of these were collected in Otago Harbour, and are
respectively 136, 126, and 90 mm. long. Another specimen
from the Macquarie Islands is 217 mm. long. The Otago
specimens are of a light brown colour, the two larger ones
being darker in the anterior gill region, and the Macquarie
specimen is dark brown throughout its length.
External Characters.—The prostomium (fig. 20), the
nuchal organ, and the metastomial grooves agree in form and
relations with those of A. assimilis. ‘There are nineteen
cheetigerous segments, of which the last thirteen usually bear
gills. The first gill is thus situated on the seventh segment,
as in A. marina, A.claparedii, and A. cristata. The
gills of the two larger Otago specimens are all fully developed,
but in the smallest specimen the last right gill is smaller
ANATOMY OF ARENICOLA ASSIMILIS. 75D
than any of the others, and its fellow on the left side is
represented by a single filament about half a millimetre in
length and bifid at the tip. In the Macquarie specimen
there are only twelve pairs of gills, the first being well
developed and situated on the eighth chetigerous annulus.
The true first gill! is totally absent, a condition frequently
met with in A. marina. The gills of the Otago specimens
are of the pinnate type, and are beautifully regular in
arrangement. Hach consists of fourteen to sixteen main
stems, usually 3°5—4 mm. long (but in several cases reaching
6 mm.), which radiate from the base of the notopodium, and
are connected near their bases by a web-like membrane.
Each stem bears eleven to eighteen pairs of pinne, which are
either opposite or almost alternate in arrangement and
usually divide dichotomously. These gills are remarkable
for the enormous size of the afferent and efferent blood-
vessels, best seen in the main stems and in the larger
pinne. They closely resemble the gills of the Laminarian
variety of A. marina figured by Gamble and Ashworth
(1898, pl. i, fig. 2), except that the webbing at the base is
more pronounced in the southern specimens. The gills of
the Macquarie specimen are of a different type. They have
only seven or eight main trunks 3°5—5:°5 mm. long, each
bearing six or eight pinne on each side, and these are less
regularly arranged than in the Otago specimens. There is
no connecting membrane at the bases of the main trunks.
These gills resemble in form, but are larger than, those of the
Uschuaia specimens of A. assimilis (see p. 740).
The annulation of the skin is exactly like that of A.
assimilis (see p. 741).
Setz.—The notopodial sets (figs. 2, 24) taper more
abruptly at the tip than those of A. assimilis. Those of
the Otago specimen are 4 mm. long. The usual pointed
barbs or processes are present on one side of the shaft of the
seta, while on the other is a well-marked lamina, which
in most sete is 12 uw, and in some is 15 « broad. The
The pair of efferent vessels of this segment is also completely suppressed,
756 J. H. ASHWORTH.
margin of the lamina is in many cases entire, but im some is
very finely dentate. The sete of the Macquarie specimen
are about 5 mm. long, and the lamina is much narrower,
being only about 6 w in width.
The neuropodia are well developed ; the crotchets show an
interesting feature. On examining the post-rostral region
(fig. 7), there are seen to be about five to seven teeth—that is,
five, six, or seven teeth are in focus at the same time as the
rostrum, and lie approximately in the same plane. On focus-
sing slightly above or below this level, there comes into view
a number of teeth situated on the sides of the rostrum, so
that the latter projects from the centre of a series of teeth
arranged around its base (fig. 8). The small subrostral pro-
cess marks the position of the base of the lowest tooth of the
series. Only those cheetee which have not been much worn
show these lateral teeth.! The rostrum is slightly longer and
more pointed, and the enlargement near the middle of the
shaft better marked than in Ehlers’ species. Many of the
crotchets are strongly curved. ‘They vary in length in the
Otago specimens from 0°66 mm. to 0°8 mm., and in the Mac-
quarie specimen they reach a length of 0°86 mm. The last-
named cheetz are stouter, the rostrum more rounded at the
tip, and more nearly in line with the shaft, and the teeth are
more feebly marked ; these characters are due to the greater
age of the specimen from which the cheeta were taken.
Musculature.—The musculature is very similar to that
of A. assimilis, except that the muscles in the region of the
first diaphragm are more slender in the Otago specimens.
Oblique muscles are present alone the body from the first
diaphragm to the end of the tail. There are no pouches on
the first diaphragm.
Alimentary Canal.—The alimentary canal agrees most
minutely with that of A.assimilis. Multiple cesophageal
1 They are very well seen in the crotchets of post-larval stages (see fig. 10).
A similar series of teeth is present around the base of the rostrum of the
cheete of other Arenicolidee, but they are not so easily distinguished as in the
specimens above described,
ANATOMY OF ARENICOIA ASSIMILIS. LO
pouches, to the number of seven on each side, are present in
the three specimens examined. The anterior pair is long
and filiform or club-shaped, measuring in one case 15 mm.,
and in the other two specimens 24mm. in length. The other
pouches are pyriform or ovoid, and 5 to 5 mm. long.
Vascular System.—The vascular system agrees closely
with that of A. marina, and only differs from that of A.
assimilis in the position of the first efferent branchial
vessel. The first six efferent branchial vessels open into the
subintéstinal vessels, and the last seven into the dorsal
vessel. The heart of the specimen 126 mm. long contains a
moderately developed heart-body.
Nephridia.—There are six pairs of nephridia, opening, as
in A.marinaand A. assimilis, on the fourth to the ninth
segments. In the three specimens examined the first pair
is smaller than any of the others, and one of the nephridia is
considerably reduced, no funnel being visible. The funnels
of the first nephridia le on the anterior face of the first dia-
phragm, ‘heir dorsal lips bear about six broad, but usually
undivided, ciliated processes, and their ventral lips, though
small, are thrown into several of the peculiar folds or frills
as described above (p. 746) in the nephrostomes of A.
assimilis. The funnels of the other nephridia are larger ;
their dorsal lips bear about sixteen spatulate processes, most
of which are subdivided terminally into five or six, and their
ventral lips are thrown into some twenty or more folds. The
vesicles of the nephridia had been recently greatly distended
but are now almost empty.
Gonads.—The gonads are small and occur in the usual
position. None are present on the first pair of nephridia.
It is probable that the breeding season of these specimens
was practically over at the time of their capture (September,
1899). It is evident that the nephridial vesicles had been
recently subjected to great distension, and this was probably
due to the accumulation therein of genital products. Similar
' Except that in the Otago specimens there is in connection with the third
nephridium only one blood-vessel, viz. an afferent branch of the ventral vessel.
758 J. H. ASHWORTH,
distension of the vesicles occurs during the breeding season
in A. marina, A. grubii, and A. ecaudata (Gamble and
Ashworth, 1898, pl. ii, fig. 15, and 1900, pl. xxvi, fig. 47). On
staining and clearing the nephridia the vesicles are found to
still contain either a few large ova or masses of spermatids.
‘he ova are of the same somewhat flattened shape as those of
A. assimilis. ‘Their three axes measure 0°195 to 0°20 m.m,
0°16 to 0°175 mm., and 0:075 mm. respectively. (For the
measurements of the ova of other species see p. 747.)
Central Nervous System.—The brain resembles that
of the Uschuaia specimens, except that the anterior lobes are
much broader. In the specimen 126 mm. long the brain is
about 0°7 mm. long, and is broadest across the anterior lobes
at the point of origin of the cesophageal connectives. The
breadth of the brain at this point is 15 mm. and its depth
04mm. After the fusion of the two anterior lobes the brain
rapidly narrows, so that its middle region is only about
0-7 mm. broad. The structure of the anterior lobes is
exactly as described for A. assimilis on pp. 748, 749. Near
their point of union larger ganglion-cells occur near the middle
line, gradually increasing in number posteriorly and being found
over the whole dorsal face of the neuropile of the mid-brain.
In the posterior part of this region there are a few groups of
pyriform, fusiform, or pyramidal ganglon-cells, the stout
processes (usually only one to each cell) of which are united
into a number of bundles. These processes pass downwards
into the ventral portion of the neuropile, where they
branch freely (see fig. 23). Similar ganglion-cells extend
some distance into the posterior brain lobes. In other
respects the brain of these specimens conforms to the
description given on pp. 748, 749.
The cesophageal connectives arise, as usual, from the
posterior part of the anterior cerebral lobes. They lie
immediately below the epidermis of the metastomial groove,
and give off numerous nerves to the skin and buccal muscula-
ture. There is a slight swelling on the connective at the
origin of the nerve to the otocyst.
ANATOMY OF ARENICOLA ASSIMILIS. 709
The situation and structure of the nerve-cord agree with the
description given on p. 750. Sections taken in the mid-
branchial region show one, two, or sometimes three giant-
libres. Serial sections of three segments (fig. 11) of this part
show that in the first and last seements there are two giant-
cells, and in the middle one only one cell. When two cells
are present they lie, asin A. grubii, one behind the other,
the anterior one being only a little distance posterior to the
parapodium. The cells are laterally situated, pyriform in
shape, and their single process is directed into the adjacent
fibrous portion of the cord.
Sense Organs.—The nuchal organ and the reddish-brown
eyes have the usual structure and position,
The otocysts are somewhat smaller and le more laterally
than in A. assimilis. They are almost spherical sacs
(fig. 15) about 0°21 mm. in diameter, which communicate with
the exterior by a tube, whose external aperture occupies a
similar position to the corresponding opening in A. marina.
It is situated near the metastomial groove, but further from
the brain than in A. assimilis. The otoliths are of purely
external origin. They consist of numerous irregular bodies
(quartz-grains, fragments of spicules, etc.), without any of
the chitinoid covering which is usually associated with the
otoliths of A. marina, and which forms the major portion
of each otolith of A. assimilis. There are in each otocyst
from twenty to fifty moderately large bodies, the largest
being 55 w long and 27 w broad, and also a quantity of finer
débris of similar origin and character. ‘he lumen of the
tube is slit-lke, and about halfway down one of the tubes
there are at two points large foreign bodies. In the structure
of its wall the otocyst agrees with that of A. assimilis
(p. 753).
760 J. H. ASHWORTH.
IV. Systematic Position of Arenicola assimilis and of
the Specimens from New Zealand.
Arenicola assimilis is clearly distinguished from all
other species by the following characters :—(a) externally, by
its twenty chetigerous segments and the presence of the
first gill on the eighth segment; and (b) internally, by the
possession of six pairs of nephridia opening on the fourth to
the ninth segments, by the presence of numerous cesophageal
elands and of large otocysts opening to the exterior, and by
the absence of the pouches on the first diaphragm.
This species obviously falls within the caudate section of
the genus Arenicola. It has practically no points in com-
mon with A. eristata except those of generic value ; the two
species differ in every one of the characters named above,
Khlers’ species has some points of resemblance to A,
claparedii; in fact, the two most characteristic features of
the latter species are found in A. assimilis, viz. the
multiple cesophageal glands and the absence of diaphrag-
matic pouches. But these two species are clearly distin-
guished by the differences in the number of segments, the
position of the first gill, the number of nephridia, and the
presence in A. assimilis of large otocysts, such organs being
absent in A. claparedii.
The structures hitherto beheved to be diagnostic of
A. marina are also found in Khlers’ species, viz. six pairs
of nephridia opening on segments 4 to 9, and a pair of
open otocysts. These two species may be easily differentiated
by an inspection of the number of segments, the position of
the first gill, the cesophageal glands, and the first diaphragm
(to ascertain the presence or absence of pouches),
So that, although related in some degree to A. marina
and A. claparedii, A. assimilis is qumte distinct from
either, and may be easily determined by reference to the six
characters given above.
The determination of the systematic position of the New
ANATOMY OF ARENICOLA ASSIMILIS. 76!
Zealand specimens is a matter of considerable difficulty. In
the number of chetigerous segments and position of first gill
they resemble A. marina and A. claparedii, but the pro-
stomium is more nearly like that of the former. Internally
there are four characters, two of which are in agreement with
those of A. marina and in contrast to those of A. claparedii,
and two are vice versa
(1) the number and position of the
nephridia and the presence of open otocysts, and (2) the
presence of multiple cesophageal glands and the absence of
pouches on the first diaphragm. The absence of otocysts in
A. claparedii is so remarkable and characteristic a feature
that their presence in the Otago specimens, taken in con-
junction with the important differences in the number of
nephridia and the form of the prostomium, is sufficient to
exclude the southern specimens from Levinsen’s species.
While their relationship with A. marina rests on a stronger
basis, the internal differences are too important to be passed
over, and one must look elsewhere for a nearer ally.
Throughout the description of the anatomy of the New
Zealand specimeus it is striking how frequently a perfect
agreement occurs between them and A. assimilis. Their
prostomia are practically identical, and they further agree
in almost every internal character—the number and position
of their nephridia, their cesophageal glands, the absence of
pouches on the first diaphragm, the form and structure of the
brain, the large size of their open otocysts and of their ova.
The only differences are externally in the number of seg-
ments and the position of the first gill, and internally in the
vessels of the seventh segment and in the nature of the
otoliths. It is a question whether these differences are of
sufficient importance to justify the separation of the New
Zealand specimens as a distinct species.
The form of the otoliths is certainly very different. In
A. assimilis they are rounded, and consist almost entirely
of a substance secreted by the cells in the wall of the
otocyst, while in the New Zealand specimens they are
irregular foreign bodies (figs. 749, 751). Hhlers has re-
762 J. H. ASHWORTH.
marked (1901, p. 178) on the small size of the opening
from the tube into the otocyst in A. assimilis, and con-
sidered that this was connected with the form of the
otoliths. I had previously arrived at the conclusion that
their shape was due to the closure of the lumen of the tube,
and had examined a number of otocysts of A. marina to
obtain further evidence on this question. ‘The anterior ends
of nine specimens of the latter species have been sectioned,
und show considerable differences in the character of their
otoliths. Six of the specimens are comparatively young (from
about 17 to 65 mm. in length), and their otoliths are irregular
foreign bodies such as quartz-grains, portions of spicules,
frustules of diatoms, etc., which are almost naked, i. e. they
have either no secreted covering, or else it is a mere film, the
presence of which is indicated by its staining with haema-
toxylin. Of the remaining three older specimens, one, which
is about 170 mm. long, has irregular otoliths like those
described above, but in the other two, which are about 130
and 250 mm. long respectively, the otoliths have quite a
different appearance. They were at first irregular, but the
original particles have been covered by layer upon layer of
secreted substance, and the resultant otoliths have rounded
outlines (see fig. 16). The tubes of these two pairs of
otocysts are found to be practically closed along almost their
whole length, either by apposition of the walls or by the
blocking of the lumen by a granular substance secreted by
the gland-cells in the wall of the tube. In each case the
walls of the tube are so closely apposed that the lumen along
the greater part of its length is reduced to a slit not more
than 3 or 4 across, and even this space is occupied by a
thin band of the secreted substance mentioned above, thus
effectually closing the passage. The variation in the nature
of the otoliths is probably dependent on the condition of the
tube. At any rate, it is interesting to note that in the large
specimen (170 mm, long) with irregular otoliths mentioned
above, the lumen of the tube, as seen in section, is a fair-sized
slit, and is not encroached upon to any extent by secretion
ANATOMY OF ARENICOLA ASSIMILIS. 763
such as blocks the tubes in the specimens with rounded
otoliths. It is also worthy of note that in the other species
of Arenicola (cristata, grubii, ecaudata) which have
rounded or spherical otholiths, formed largely of secreted
inatter, the otocyst is a closed vesicle. It seems probable,
therefore, that the presence or absence of an open passage
connecting the otocyst to the exterior has considerable
influence upon the character of the otoliths, which varies
even in different specimens of the same species. The fact
that the otoliths of A. assimilis are rounded, while those of
the New Zealand specimens are irregular, is not of funda-
mental importance; it probably indicates that in the former
the tube leading from the otocyst to the exterior very soon
became blocked, and the otoliths are therefore largely com-
posed of material deposited around the small particles which
had gained access to the otocyst before the closure of its
tube. The otocysts of A. assimilis and of the New Zealand
specimens agree in the most important character, namely,
that each possesses a tube leading to the exterior; and the
modification which takes place in the former, causing a differ-
ence in the nature and shape of the otoliths, may be regarded
as of secondary importance, since a similar, though not so
marked a difference, may be observed within the limits of a
single species (A. marina). Tor further remarks on this
subject see p. 771.
While the number of chetigerous segments in the ecaudate
Arenicolidee varies greatly (from about twenty-four to forty in
A. grubii, and thirty-five to fifty-six in A. ecaudata), it is
peculiarly constant in three of the caudate species, there being
invariably nineteen in A. marina and A. claparedii, and
seventeen in A. cristata. In most American specimens of the
last-named species there is, however, an extension into the
tail of structures which are usually associated only with
parapodia, Small gills and cirriform processes occur upon
the first two or three tail segments of one specimen examined
(Gamble and Ashworth, 1900, p. 442, figs. 31, 32), and similar
processes are commonly present on American specimens, but
764 J. H. ASHWORTH.
have never been recorded in any Neapolitan specimen of this
species.
With this example in mind it is not difficult to believe that,
in a species probably widely ranging over the enormous
coast-line of the South Atlantic and Pacific Oceans, some
specimens may have become modified in the direction above
indicated, so that finally a condition was reached in which
some members of the species possess nineteen and others
twenty segments. If we suppose that an additional para-
podium and gill have been produced, the only alteration
necessary to bring such a form into line with A. assimilis
would be the loss of a gill at the anterior end of the series.
The reduction and absence of the first gill are so frequently
observed in A. marina (and to a less extent in almost all
other species) tlat the reduction and eventual loss of the first
eill of the hypothetical form are quite conceivable.
In my opinion the possession of an extra cheetigerous seg-
ment, though striking, is scarcely a sufficiently important
character to form by itself a test of specific value, and to be
used as the sole means of distinguishing two otherwise
identical forms. It seems preferable to regard the New
Zealand specimens as forming a variety of the species A.
assimilis, to which the name affinis may be given indicat-
ing its close connection with and resemblance to the type.
V. Post-larval Stages of Arenicola from the Falk-
land Islands.
After finding multiple cesophageal pouches in adult speci-
mens of Arenicola assimilis, it occurred to me that I
might be in error in the determination of the species of
certain post-larval Arenicolide from the Falkland Islands,
aud a re-examination of them became necessary. The
specimens were preserved in, and handed to me in, formalin,
and | examined them in that fluid two years ago. On finding
multiple cesophageal glands I had little hesitation in refer-
ring them to the species A. claparedii, because at that
ANATOMY OF ARENICOLA ASSIMILIS. 765
time the occurrence of several pairs of cesophageal ceca was
known only in this species, and, indeed, was considered to be
one of its most characteristic features. At the same time I
looked for the otocysts, but did not succeed in finding them."
They would have been moderately easy to see in post-larval
stages of A. marina of the same size, and finding no similar
structures in my post-larvee I therefore concluded (wrongly,
as it now appears) that otocysts were absent. With these
two features in mind, but relying especially on the very
obvious presence of several cesophageal pouches, I identified
the specimens as post-larval stages of A. claparedii, and as
such they were recorded by Miss Pratt (1901, p. 12). The
fact that these specimens had been obtained in the region in
which A.assimilis is found had not escaped my notice,
but as Ehlers’ account (1897, pp. 105, 104) stated that his
species closely resembled A. marina, it was naturally
concluded that the presence of multiple cesophageal glands
might still be regarded as a diagnostic character of A.
claparedii.
These post-larvee have now been carefully re-examined,
both entire and in sections, with the result that my previous
determination is found to be wrong; they are the youug
stages of A. assimilis, var. affinis.
The three specimens were found on the surface of the sea
near the Falkland Islands, by Mr. R. Vallentin, of New Quay,
and were handed over to me by Miss Pratt, of Owens
College, Manchester, who was working over Mr. Vallentin’s
collection of Polycheetes.
The specimens are 7°6, 8°7, and 11:1 mm. long respectively.
The largest specimen is provided with a transparent gelatin-
ous envelope about 1 mm. in diameter, which covers the
animal, except for a distance of a little over a millimetre at
each end. In general aspect these post-larvee resemble
those of A. marina,
! The nephridiopores were also examined, but on account of their minute size
it was impossible to make certain of their presence or absence on the critical seg-
ment (the fourth), and therefore their number could not be definitely ascertained.
vou. 46, PART 4,—NEW SERIES. DDD
766 I, H. ASHWORTH.
There are sparsely scattered greenish-brown pigment cells
in the epidermis.
The conical prostomium bears from two to four small
brownish-red eyes on each side (fig. 18). It is followed by a
region divided into two by a faint groove (figs. 18, 19).
The anterior portion of this region is the true peristomial
segment, and in the largest specimen is itself encircled by a
groove, which subdivides it into two annuli. ‘The posterior
part of the region above named corresponds to the segment
bearing the minute vestigial seta in the’ post-larve of
A. marina (Benham, 1893, p. 49). There is no trace of sete
in this segment in the post-larvee now under consideration.
In adults (see fig. 20) the region between the prostomium
and first chetigerous segment is divided into four rings
(see p. 741), in the third of which the cesophageal connec-
tives unite. By comparison with these post-larvee, it is seen
that the first two rings of the adult belong to the peristo-
mium and the other two to the first true body-somite, which,
in Arenicola, has lost its setae and has become fused with
the peristomium.
There are nineteen cheetigerous segments, in each of which
crotchets and sete may be clearly distinguished. ‘There are
two kinds of sete present in the notopodia. The more
numerous and longer ones are very similar to those of the
adult (fig. 3). They are about 0°3 mm. long, and bear a
lamina along about half their length. The shorter sete, about
0°25 mm. long, are obviously laminate for a short distance on
both sides (figs. 4, 5). They are almost lanceolate in shape,
and drawn out into long, slender tips. Only one of these
sete is usually present in each notopodium, in which there
are two to four sete altogether. There is a tendency, more
marked in the lanceolate sete, for the lamina to break up,
from the edge inwards, into fine, pointed teeth (figs. 4, 6).
The crotchets are 0:07 to 0°08 mm. long, andare distinguished
by the presence of a thickening, forming an encircling ridge
upon the shaft of the cheeta (fig. 10). This ridge lies just
below the level of the epidermis. As described on p. 756,
ANATOMY OF ARENICOLA ASSIMILIS. 767
the teeth are not confined to the region immediately behind
the rostrum, as on careful focussing they may be found also on
the sides of the rostrum. There is really, therefore, a circular
series of teeth from the centre of which the rostrum projects,
and the subrostral process is the lowest of this series.
Fie. 104 shows the appearance of the crotchet when the
rostrum is in sharp focus; in fig. 108 the other teeth seen on
focussing slightly upwards are added.
Each of the posterior tail segments is divided by shght
constrictions so as to present a tri-annulate appearance. The
anal seement is somewhat swollen, and the lips of the anus
are crenate.
There are no gills present in any of the specimens. ‘The
blood is light red in colour (in formalin).
After staining and clearing the specimens the alimentary
canal could be well seen (fig. 18). The muscular pharynx
leads into the thick-walled cesophagus, which bears on the
dorsal surface of its posterior portion the cesophageal glands,
of which there are from six to eight visible on each side ; the
anterior one is the largest. Just behind this point the
cesophagus is slightly constricted, and the two hearts lie on
its lateral walls. The stomach is a wider tube, and upon its
walls may be clearly seen the vessels of the gastric plexus
bounding the chlorogogenous areas. ‘The intestine, like that
of the adult, is thrown into concertina-like folds.
Sections show that the anterior part of the cesophagus is
ciliated, and that the stomach is lined by columnar cells,
many of which contain a vacuole near the end which adjoins
the digestive cavity.
In sections of the anterior ends of the two smaller speci-
mens the otocysts are not easily found ; they are much less
clearly differentiated at this stage than those of corresponding
post-larval stages of A. marina and A. ecaudata. They
are seen to be two small pits in the epidermis, the lips of
which are approximated so as to form a very short tube
(fiz. 21). Hach otocyst is somewhat triangular in section;
its apex is directed laterally and leads to the external open-
768 J. H. ASHWORTH.
ing. There are in each otocyst from four to six foreign
bodies (otoliths), all of which are apparently quartz-grains
except two; these are obviously fragments of spicules. The
otocysts of the specimen 11:1 mm. long are faintly visible in
a stained preparation of the whole animal cleared in thick
cedar-wood oil. They are about 404 by 25, in internal
diameter (fig. 18).
The nuchal organ is easily recognisable ; its cells are richly
ciliated (figs. 18, 21).
Neither giant-cells nor giant-fibres can be identified in the
nerve-cord at this stage.
Six pairs of nephridia may be traced in sections. The first
nephridium is small, and its anterior end runs forward and
pierces the third diaphragm. On the sixth nephridium the
gonidial vessel has a covering of cells which have large
spherical nuclei. This is the gonad, and it may also be dis-
tinguished, though not so clearly, on the fourth and fifth
nephridia.
The above-described post-larval stages are evidently not
young specimens of A. claparedii,as is shown by the presence
of otocysts and six pairs of nephridia. They are the young
stages of the variety of A. assimilis.
Ehlers (1897, p. 104) has recorded from Uschuaia a similar
aill-less specimen about 6°5 mm. long, which bears nineteen
cheetigerous segments. This post-larval stage was found
amone the “roots” of seaweeds (Tangwiirzeln), and had
probably recently settled down to its littoral habitat.
VI. Adult Specimens of Arenicola from the Falkland
Islands.
When the foregoing account was ready for press I received,
through the kindness of Mr. R. Vallentin, of New Quay,
Cornwall, five adult specimens of Arenicola from the Falk-
land Islands, and have thus been able to confirm some of the
observations described in the former part of this paper.
ANATOMY OF ARENICOLA ASSIMILIS. 769
The specimens were dug from the sand in Whale Sound,
Stanley Harbour, during the early months of this year (1902).
‘They are respectively 187, 185, 135, 128, and 121 mm. long.
Each has nineteen chetigerous segments, the seventh of
which bears the first and invariably small pair of gills. The
other external characters, e. g. the prostomium, annulation,
etc., agree exactly with those of the Otago specimens, while
internally the agreement is scarcely less perfect. In the two
specimens dissected there are six pairs of nephridia opening
on the fourth to the ninth segments. The first nephridium
is small, and its nephrostome is on the anterior face of the
third diaphragm. ‘The edge of the ventral lip of the larger
nephrostomes is thrown into numerous folds or frills, as
figured (see fig. 17). The vascular system agrees exactly with
that of the Otago specimens.
There are multiple cesophageal glands to the number of
twelve or thirteen on each side, the anterior ones digitiform
or club-shaped, the others pyriform or ovoid. There are no
pouches on the first diaphragm.
The only feature of an unusual character in the body of the
animal is the presence of a partial septum one segment
behind the third diaphragm. ‘This structure is homologous
to the septa met with in the posterior branchial region of
this and other Arenicolide. It is amembrane supporting both
the afferent and efferent vessels to the second pair of
nephridia, and is nearly 3 mm. across in its widest part. It
is not so extensive as the third diaphragm (which in the
same specimen is over 6 mm. across), as it does not reach
either the dorsal or the ventral body-wall. It may be
regarded as merely an exaggeration of the supporting
strands which are usually present in other species alongside
either one or both of the vessels to the nephridia (see, for
example, A. grubii, Gamble and Ashworth, 1900, pl. xxvi,
fies. 53, 54).
The two specimens examined are females which have
probably spawned, as only a very few ova are present in the
1 In one specimen the true first gill is absent on the right side.
770 J. H. ASHWORTH.
body-cavity. ‘These are large, and measure across their flat
faces 0°2 x 0°17 mm. (see pp. 747 and 758).
The anterior end of one of the specimens was cut into sections.
A pair of large otocysts is present (fig. 14). ‘hey are much
larger than those of the Otago specimens and a little larger
than those of the worms from Uschuaia (cf. figs. 13, 14, 15).
Their three diameters are respectively about 0°36, 0°38, and
0:28 mm. (compare the measurements on pp. 752 and 759).
The otoliths are all spherical or nearly so, and are com-
posed of a yellowish or brownish secreted substance. There
are in each otocyst two otoliths (fig. 14) considerably larger
than the rest. They are about 0°055 mm. in diameter, and in
the centre of each is a small irregular foreign body, probably
a quartz-grain. The smaller otoliths are usually from 0°02
to 0°03 mm. in diameter, and only rarely is a foreign particle
visible in them, though doubtless each has a very minute
central nucleus of this description. The two large otoliths
described above are probably the first otoliths of the post-
larval stage, which always remain distinguished by their
ereater size from those which are formed later. A similar
condition exists in A. ecaudata, in the post-larval stage of
which there is for some time only one otolith, which always
remains conspicuous, owing to its larger size (Gamble and
Ashworth, 1900, p. 504 and fig. 64).
The otocyst opens to the exterior by a tube, the external
opening of which corresponds in position to that of the Otago
specimens and of A. marina. ‘The lumen of the tube is of
moderate size along the greater part of its length, but is
reduced near its entrance to the otocyst in one case to a very
narrow passage, and in the other is practically obliterated.
The wall of the tube is remarkable for the presence of
large gland-cells, which are practically confined to the dorsal
wall. ‘They are almost ovoid in shape, and their cell-
contents are in the form of a reticulum. In the ventral
wall of the tube there are numerous elongate fusiform sense
cells.
The remaining structures shown in sections of the anterior
ANATOMY OF ARENICOLA ASSIMILIS. val
end are so exactly similar to those of the Otago specimens
that no further description of them is necessary.
The specimens above described are interesting from their
bearing on the discussion regarding the taxonomic value of
the shape of otoliths (see p. 761). The only difference
between the Falklands specimen and those from New Zealand
is that in the former the otoliths are spherical and composed
almost entirely of a secreted substance, while in the latter
they consist of irregular foreign bodies, such as sand-grains
and fragments of spicules. There can be no doubt that the
two sets of specimens belong to the same species, or rather
to the same variety, so that (as was also proved for A.
marina, see p. 762) the shape of the otoliths varies in
different specimens of the same species or variety. The
closure of the tube of the otocyst along part of its length and
the presence of the numerous large gland-cells in its wall are
probably the principal factors in determining the shape and
nature of the otoliths of the Falklands specimens. Having
proved the presence of spherical otoliths in some examples of
A. assimilis, var. affinis, it will be noticed that one of the
differences (discussed on pp. 761—763) between this new
variety and the type of the species disappears ; so that now the
only features by which they may be distinguished are (1) the
presence of twenty chetigerous segments in the type of the
species, whereas the new variety possesses only nineteen, and
(2) the slightly different position of the external opening of
the otocyst. As the latter is too fine a character for ready
application in systematic work, it may be said that the deter-
mination rests upon the number of chetigerous segments.
Another striking feature about the otocysts is the great
difference in their size in specimens of the variety from the
two localities. Whereas in the Otago specimens their
average diameter is 0°21 mm. (in a specimen 136 mm. long),
in one (128 mm. long) from the Falklands their average
diameter is 0°34 mm. (cf. figs. 14, 15), so that the internal
volume of the latter is about four times that of the former.
Adult specimens of the new variety are now recorded from
772 J. H. ASHWORTH.
the same locality as the post-larval stages described on pp. 764
—768. There can be no doubt that the latter are stages in
development of the former. Judging from Ehlers’ record
(1897, p. 104) of the capture near Uschuaia of a aill-less
specimen 6°5 mm. long with nineteen chetigerous segments, it
seems probable that the variety occurs at this place along
with typical specimens of the species.
VII. Distribution of Arenicola assimilis.
Ehlers (1901, p. 178) records the occurrence of A.
assimilis in collections from the Straits of Magellan (Punta
Arenas and Susanna Cove), the Beagle Channel (Uschuaia
and Lapataia Nueva), South Georgia, Chile (Schmarda),
Kerguelen (Grube), and California.
Schmarda’s (1861, pp. 51, 52) A. piscatorum from Chile
and Grube’s (1878, pp. 511, 554) A. piscatorum, Cuv., var.,
from Kerguelen, are both included by Ehlers under the
species A.assimilis. Although Schmarda gives a brief
description of some points in the anatomy of his specimens
he unfortunately does not mention any characters which
enable their identity to be definitely settled. With respect
to Grube’s specimen from Kerguelen the only information
given is that most of the branchiferous segments are divided
into only four annul, and owing to this feature Grube dis-
tinguished his specimens as a variety of A. piscatorum,
‘There is no evidence to show that any of these specimens
belong to the species A. assimilis,
Khlers (1897, p. 104) states that in the Gottingen collec-
tion there is a species of Arenicola! from California in
which there are twelve pairs of gills which agree in position
with those of A. assimilis, and these specimens are dis-
1 | thank Professor Ehlers for sending to me by letter the further informa-
tion that this is a duplicate from Professor Agassiz’s collection, which was
sent to Godttingen to be worked over, ‘The rest of the specimens were
returned to Professor Agassiz, and are doubtless those referred to on the next
page.
ANATOMY OF ARENICOLA ASSIMILIS. 773
tinguishable from A. marina only by this character. No
mention is made of other features which would have been
much more valuable as diagnostic characters, but the
difference in the number of gills is accepted as a sufficient
ground for separating the specimens from A. marina, not-
withstanding the well-known liability to reduction (from
thirteen to twelve pairs) in the number of branchie in this
species. As will be seen from the discussion below, it is very
probable that Ehlers’ specimen does not belong to either of
these species, and that this is an example of the confusion
due to placing an implicit reliance on the value of external
features in discriminating species of Arenicola. On such
a variable and insufficient character as the number of gills
Ehlers bases his diagnosis of the Californian specimen, and
refers it to the species A.assimilis. ‘This is the only
evidence in support of his record of this species from Cali-
fornia.
I have recently re-examined specimens of Arenicola
from a collection made by Professor Agassiz, near Crescent
City, California, sent to Dr. Gamble and myself from the
Harvard Museum, and identified by us (1900, p. 423) as
A. claparedii. These specimens are the more interesting
because they are accompanied by a label? indicating that they
have passed through the hands of Professor Ehlers, and that he
considered them to belong to a new species nearly related to
A. marina (= piscatorum). Itisalmost certain that these
are the same specimens which Ehlers has recorded as A. ass1-
milis. ‘There are five specimens, in three of which there
are twelve pairs of gills, the first situated on the eighth
chetigerous segment. In each of the other two specimens
there are twelve gills on the left side (the first being on the
eighth segment), accompanied in one case by thirteen gills
on the right, the first being very small and borne on the
seventh segment, while on the right side of the other
‘The writing upon the label, which is now faint, is as follows :—
*Arenicola, n. sp. nahe piscator. 7 vor Segm. 12(13) Kiementrag.
Californien (H. Ehlers).”
774 J. H. ASHWORTH.
specimen there are only eleven gills, the first of which is on
the ninth segment. It may therefore be said that it is usual
to find the first gill in these specimens on the eighth segment
as in A. assimilis. Dissections of two of the specimens
show that there are five pairs of nephridia, multiple ceso-
ageal glands, and no pouches on the first diaphragm; and
sections of the anterior end prove conclusively that there are
no otocysts. All these points are so characteristic of
A. claparedii that there can be no doubt that the speci-
mens belong to this species.
I am indebted to Dr. H. P. Johnson for two specimens of
Arenicola from Puget Sound, Washington. In one of
these! there are thirteen pairs of gills, but in the other the
seventh segment bears a gill only on the right side, the first
left gill being on the following segment. Dissections of the
specimens and sections of the anterior end of one of them
fully confirm the determination of their species made by Dr.
Johnson (1901, p. 421) ; they are undoubted A. claparedii.
It is therefore highly probable that Ehlers is in error in
recording A. assimglis from California. In the first place,
his determination of the species of the Californian specimens
rests solely upon a character which is very variable and
almost useless for distinguishing species ; secondly, a re-
examination of what are probably the very same specimens
proves them to be A. claparedii, and this species has been
recorded from another point on the west coast of the
United States.
A revision of Ehlers’ record of the distribution of A. assi-
milis therefore becomes necessary, and may be given as
follows :—Adult typical specimens of A. assimilis have been
recorded from several places in the extreme south of the
' It is remarkable that of the seven specimens examined from the west
coast of the United States this is the only one which possesses the full
number of gills, On the contrary, it is unusual to find any departure from the
normal number in Neapolitan specimens of A. claparedii; out of thirty-nine
examined only two show a reduction in the number of gills; in each case
there are thirteen on the left side, but only twelve on the right.
ANATOMY OF ARENICOLA ASSIMILIS. tO
American continent and from South Georgia. Others
forming a new variety but agreeing with the type, except in
the number of chetigerous segments, are now recorded from
Otago Harbour, the Macquarie Islands, and the Falkland
Islands. Post-larval stages of the variety have been obtained
off Stanley Harbour (Hast Falkland) and near Uschuaia.
VIII. Specific Characters of the Caudate Areni-
colide.
Appended isa revised summary of the characters of the
caudate Arenicolide, which clearly shows by what features,
both external and internal, A. assimilis may be readily
recognised and distinguished from other species with which
it is hiable to be confused. It cannot be too strongly urged
that attention should be directed by systematists chiefly to
internal characters in the discrimination of the species of
Arenicola. No determination of A. marina, A. clapa-
redii, or A. assimilis can be considered complete or
entirely trustworthy which relies solely on external characters.
It is impossible to distinguish these three species with
certainty unless reference be made to the nephridia,
cesophageal glands, and otocysts, the two former being of
especial use in this connection.
The characters! of the caudate Arenicolide may be briefly
stated thus:
A distinct tail present; the parapodia and gills do not
extend to the posterior end of the animal. The body is
often swollen anteriorly. Gills, pinnate or derivable from
the pinnate type, eleven to thirteen pairs, the first (which
may be small or even absent) on the seventh or eighth
chetigerous segment. Prostomium consisting of a median
and two lateral lobes. Nephrostomes with dorsal lip well
provided with flattened, spatulate, ciliated, vascular processes ;
" The following is a revision of a part of the summary published by Dr.
Gamble and myself (1900, p. 540), to which reference may be made for the
characters of the genus and of the ecaudate species.
776 J. H. ASHWORTH.
ventral lip ciliated, entire (i. e. not deeply notched as in the
ecaudate Arenicolidz). Gonads small, ova discoidal.
(a) A. marina, Linn.—Nineteen chetigerous segments.
Thirteen pairs of gills; the first, which is on the seventh
segment, may be reduced (or suppressed). Otocysts opening
to the exterior. Otoliths, numerous foreign bodies (quartz-
grains, etc.), which may, however, be covered with a layer
of secreted chitinoid substance, giving them a rounded out-
line. Six pairs of nephridia opening on segments 4 to 9.
One pair of cesophageal pouches, cylindrical, club-shaped,
or conical. Diaphragmatic pouches (on the first diaphragm)
small, globular, or flask-shaped.
Found on both sides of the North Atlantic.
(b) A. assimilis, Ehlers.—Twenty chetigerous segments.
Thirteen pairs of gills, the first of which is situated on the
eighth segment (the first gill is liable to be reduced or
suppressed). Otocysts large, opening to the exterior. Otoliths
numerous ; spherical or rounded chitinoid bodies. Six pairs
of nephridia opening on segments 4 to 9. Several pairs
of cesophageal pouches ; the anterior pair long, club-shaped,
or filiform; the others much smaller and pear-shaped. No
pouches on the first diaphragm.
Recorded from the extreme south of the American
continent.
(c) A. assimilis, var. affinis, Ashworth.—Nineteen
chetigerous segments. Thirteen pairs of gills, the first
(liable to reduction or suppression) on the seventh segment.
Otocysts large, opening to the exterior. Otoliths numerous,
aud composed either of foreign bodies (quartz-grains, etc.)
or of spherical chitinoid bodies. Other characters as in the
type of the species (see above).
Recorded from Otago Harbour, New Zealand, the Mac-
quarie Islands, the Falkland Islands.
(d) A. claparedii, Levinsen—Nineteen chetigerous seg-
ments. ‘Thirteen pairs of gills, the first on the seventh
segment (this pair of gills is liable to reduction or suppres-
sion, especially in specimens from the west coast of North
ANATOMY OF ARENICOLA ASSIMILIS. What
America). Lateral lobes of prostomium well developed. No
otocysts. Five pairs of nephridia opening on segments 5
to 9. Two or more pairs of oesophageal pouches, the
anterior pair long and slender or club-shaped, the others
shorter, usually pyriform. No pouches on the first diaphragm.
Recorded from the Mediterranean and from the west coast
of the United States.
(e) A. cristata, Stimpson.—Seventeen chetigerous seg-
ments. Eleven pairs of gills, the first on the seventh
segment. Otocysts, closed spherical sacs each containing a
single large, spherical, chitinoid otolith. Six pairs of
nephridia opening on segments 5 to 10. One pair of
cesophageal pouches cylindrical or club-shaped. Diaphrag-
matic pouches (on the first diaphragm) large and finger-
shaped.
Found in the Mediterranean, in the West Indies, and on the
eastern shores of North America south of latitude 40° N.
IX. Summary of Results.
1. The anatomy of Arenicola assimilis is fully de-
scribed for the first time. Although Ehlers states that the
species differs from A. marina only in the number of
chetigerous segments (nineteen in the latter, twenty in the
former), in the position of the first gill and in the relative
size of the middle lobe of the prostomium, further examina-
tion shows that there are other important points of difference,
e. g. A. assimilis possesses multiple cesophageal glands
and has no pouches on the first diaphragm.
2. Specimens of Arenicola are described from Otago
Harbour (New Zealand) and from the Macquarie Islands
which differ from the type in the number of chetigerous
segments (nineteen) and situation of the first gill. There is
also a difference in the shape of the otoliths ; in the type they
are spherical or rounded, while in the New Zealand specimens
they are irregular. These specimens belong to a new variety
(var. affinis) of the species.
778 J. H. ASHWORTA.
3. In the discussion of the systematic position of the
Otago specimens it is concluded that the form of the
otoliths is not sufficiently reliable to form a character by
which species may be discriminated. In A. marina the
otoliths are usually irregular, but two out of nine specimens
examined possess rounded otoliths. In these cases the
otoliths were at first irregular foreign bodies, but they
have been covered with layer upon layer of secreted sub-
stance, and now have a rounded outline. In each of these
two cases the tube which placed the otocyst in communica-
tion with the exterior has become either wholly or partially
blocked, either by apposition of its walls or by the secretion
into the lumen of a glandular substance which forms an
effectual plug. In the seven specimens of A. marina with
irregular otoliths the tubes are not closed in this way. It is
concluded that the presence of spherical or rounded otoliths
is associated with the closure of the tube, and it is pointed
out in support of this conclusion that the other species
(cristata, grubii, and ecaudata) in which spherical
otoliths are found have closed otocysts (pp. 761—763).
4, The brain is well developed. The ganglion-cells of its
middle region are large (especially in the Otago specimens)
and send processes into the neuropile, where they branch
freely.
5. Giant-fibres and segmentally arranged giant-cells are
present in the nerve-cord. They have the same structure as
in A. erubii.
6. The otocysts of A. assimilis are distinguished by their
size. ‘They are considerably larger than those of any other
species. Neuro-fibrille may be traced from the nervous
sheath of the otocyst into and along the whole length of the
sense cells of the otocystic epithelium. These cells and
fibrils are especially abundant near the point of entrance of
the tube to the otocyst.
7. There is a large nerve-supply to the skin and proboscis.
Neuro-fibrilla may be seen in some of the cells of the general
body-surface and of the papilla of the proboscis,
ANATOMY OF ARENICOLA ASSIMILIS. 779
8. Post-larval specimens of A. assimilis, var. affinis, are
described from the Falkland Islands. They possess an
acheetous segment between the peristomium and_ first
chetigerous segment (as in similar stages of A. marina and
A. ecaudata). By comparison with the adult the hmits in
the latter of the peristomium and acheetous body-segment
may be determined (figs. 19, 20).
9. Adult specimens of A. assimilis, var. affinis are also
described from Stanley Harbour, East Falkland. They are
remarkable for the large size of their otocysts, the internal
volume of which is about four times that of the otocysts of
the Otago specimens (figs. 14, 15). It is evident that a con-
siderable variation in the size of these organs may occur in
specimens of the same species or variety from different
localities. The otoliths, several of which contain an irregular
foreign body, are spherical, and in one specimen two of them
are much larger than any of the others. ‘They are the first
two otoliths of the post-larval stage which have continually
received fresh depositions of secreted substance, and always
remain distinguished from those formed later by their larger
size. These specimens from the Falklands differ from the
Otago specimens in the nature of their otoliths. Here is
additional evidence that the character of the otoliths con-
tained in otocysts provided with a tube leading to the
exterior is not a feature upon which much value should be
placed in systematic work. Blocking of the tube (as occurs
in the Falklands specimen) converts the otocyst into a closed
sac, in which spherical otoliths are formed, while in other
specimens (e.g. those from Otago Harbour) in which the
tube remains open the otoliths are irregular foreign bodies,
such as sand-grains, which are able to gain access to the
otocyst.
10. Ehlers records A. assimilis from the Straits of
Magellan, the Beagle Channel, South Georgia, Chile
(Schmarda), Kerguelen (Grube), and California. It is shown
that there is no evidence in support of the last three
records. Schmarda’s and Grube’s specimens are insufficiently
780 J. H. ASHWORTH.
described, and no character is mentioned by which their
species may be determined. Ehlers’ diagnosis of the
Californian worms rests solely upon a character which is
very variable and almost useless for distinguishing species.
A re-examination of what are probably the same specimens
proves them to be A. claparedii, and this species has also
been recorded from another point on the west coast of the
United States (pp. 772—774). Incidentally it may be men-
tioned that specimens of A. claparedii from the west coast
of North America almost invariably bear only twelve gills
either on one or both sides, while Neapolitan specimens have
usually the full number (thirteen pairs).
A. assimilis may be regarded as the characteristically
southern species of the genus. Adult typical specimens are
recorded from several points in the extreme south of America
and from South Georgia. A new variety (var. affinis),
differing from the type only in the number of cheetigerous
segments, is now recorded from Otago Harbour (New Zea-
land), the Macquarie Islands, and the Falkland Islands.
Post-larval stages of the variety have been taken off the
Falklands and near Uschuaia, in the Beagle Channel.
X. LITERATURE.
1861. Scnmanrpa, L. K.—‘ Neue Wirbellose Thiere,’ Band i, p. 51, Leipzig,
1861.
1878. Grune, K.—“ Anneliden-Ausbeute S.M.S. Gazelle,” ‘ Monatsber, d. K.
Akad. d. Wissensch. zu Berlin, aus dem Jahre 1877,’ pp. 511, 554,
Berlin, 1878.
1893. Bentnam, W. B.—‘ Journal Marine Biol, Association,’ New Series,
vol. ili, p. 48, 1898.
1897. Enters, E.—* Polycheten,’ ‘Hamburger Magalhaenische Sammel-
reise,’ pp. 108, 104, Hamburg, 1897.
1898. Game, F. W., anv Asnwortn, J. H.—‘ Quart. Journ. Mier. Sci.,’
vol. xli, p. 1, 1898.
1899. Fauver, P.—* Mémoires de la Société nationale des Sciences naturelles
et mathématiques de Cherbourg,’ tome xxxi, p. 178, Cherbourg,
1899.
ANATOMY OF ARENICOLA ASSIMILIS. 781
1900. Cutnp, C. M.—‘ Archiv f. Entwickelungsmechanik,’ Band ix, p. 587,
1900.
1900. Gameie, F. W., anp AsHwortH, J. H.—‘ Quart. Journ. Mier. Sci.,’
xiii, p. 419, 1900.
1901. Prarr, E. Mi—‘ Memoirs and Proceedings of the Manchester Lit. and
Phil. Society,’ vol. xlv, p. 12, 1901.
1901. Jounson, H. P.—‘ Proceedings of the Boston Society of Natural His-
tory,’ vol. xxix, p. 421, 1901.
1901. Enters, £.—‘ Die Polychaten des magellanischen und chilenischen
Strandes,’ pp. 177, 178, Beriin, 1901.
EXPLANATION OF PLATES 36 & 37,
Hlustrating Dr. J. H. Ashworth’s memoir on ‘The Anatomy
of Arenicola assimilis, Ehlers, and of a New Variety
of the Species, with some Observations on the Post-
larval Stages.
List oF REFERENCE LETTERS.
A. B.S. Achztous segment of body between peristomium and _ first
chetigerous segment. Axt. Cer. Z. Anterior lobe of brain. B/. Bladder of
nephridium. B.V. Blood-vessel. C. Cuticle. Ch. Seg.’ First cheetigerous
segment. Cal. Ceelom. Conx. Tiss. Connective tissue. pid. Epidermis.
£.T. Entrance to tube leading from otocyst to exterior. Ht. Op. Of. External
opening of otocyst. Gang.C. Ganglion-cell. Gast. Lat. Lateral gastric
vessel. G@/.C. Gland cell. Goxz. Gonad. Goz. V. Gonidial vessel. Hé.
Heart. Met. Gr. Metastomial groove. Middle Comm. Middle commissure
of brain. M/. Cire. Circular muscles. MM. Long. Longitudinal muscles. AZo.
Mouth. J/. Sh. Buce. Muscular sheath of buccal mass. NV. Band of nerve-
fibres from prostomial epithelium to brain, WV. 4f. Afferent vessel of
nephridium. MW. Buee. Nerve to buccal mass and papillae of ‘ proboscis.’
N.C. Nerve-cord. WN. Epid. Nerve to epidermis. Nm. Ch, Neuropodial
chet. Not. S$. Notopodial sete. Nphm.D. Dorsal lip of nephrostome.
Nphn. V. Ventral lip of nephrostome. pile. Neuropile. Nue. Gr. Nuchal
groove. Oc. Kye. i. (Hsophagus. WM. Conn. (Msophageal connective.
.G/, (Esophageal gland. O¢. Otocyst. O¢h. Otolith. O¢. 7. Tube of
otocyst. Pap. Papilla of “proboscis.” Per. Peristomium, PA. Pharynx.
Post. Cer. L. Posterior lobe of brain. Prost. Prostomium. Prost. Epith.
Epithelium of prostomium. Prost. Lat. Lateral lobe of prostomium.
vou. 46, pARkT 4,—NEW SERIES. EEE
782 J. H. ASHWORTH.
Prost. Mid. Middle lobe of prostomium. Sp. Fragment of spicule. S¢.
Stomach. V.V. Ventral vessel.
PLATE 36.
Fre. 1.—Distal half of a seta from the fifth notopodium of a specimen of
Arenicola assimilis from Uschuaia. x 100.
Fic. 1 a.—A portion of the seta more highly magnified. Note the lamina
on the left bearing fine teeth. x 750.
Fie. 2.—Distal portion (two fifths) of a seta from the sixth notopodium of
a specimen of A.assimilis, var. affinis, from Otago Harbour. xX 100.
Fig. 2a.—A portion of the same seta more highly magnified. Nete the
broad lamina crossed by fine oblique lines. x 500.
Fics. 3, 4, 5.—Notopodial sete from a post-larval specimen (7°6 mm. long)
of A. assimilis, var. affinis. Most of the sete are of the kind shown in
Fig. 3, but in each notopodium there is one seta of the type seen in Fig. 5.
Sete of the kind shown in Fig. 4 are much less common than the preceding ;
only two specimens were seen in ten notopodia. The lamina is breaking up
on one side near its tip into fine teeth. x 320.
l'1c. 6.—Tip of a seta of the same kind as shown in Fig. 5. Note the fine
teeth on the margin of the lamina on one side. x 600.
Fic. 7.—A crotchet from the fifteenth neuropodium of an Otago specimen
(var. affinis). x 100.
Fic. 8.—The head of a crotchet from another Otago specimen, to show the
teeth situated on the sides of the rostrum. x 280.
Fic. 9.—A crotchet from the fifteenth neuropodium of A. assimilis from
Uschuaia. xX 100.
Fic, 10.—Two erotchets from the sixteenth neuropodium of a post-larval
specimen (7°6 mm. long) of A. assimilis, var. affinis, 4 shows the appear-
ance of the cheta when the rostrum and post-rostral teeth are in focus; in B
the teeth on the sides of the rostrum are also shown, ‘The subrostral process
is now seen to be one of the series of teeth, ‘The dotted line indicates the
level of the epidermis, x 600.
Fie. 11.—Diagram of a portion of the nerve-cord of A. assimilis, var.
affinis, from Otago Harbour, to show the distribution of the giant nerve-
cells. The transverse lines indicate the position of the neuropodia, the
numbers of which they bear. The nerve-cord is magnified about 10 times
and the cells 40 times.
Fre. 12.—Diagram of a portion of the nerve-cord of A. assimilis from
Uschuaia, to show the distribution of the giant nerve-cells. The trans-
verse lines indicate the position of the neuropodia, ‘The last giant-cell shown
ANATOMY OF ARENICOLA ASSIMILIS. 783
situated (probably) in the first tail segment. The nerve-cord is magnified
about 10 times and the cells 40 times.
Fic. 138.—Shows the size of the otocyst of a specimen (120 mm. long) of
A. assimilis from Uschuaia. The oval outline is a camera drawing of the
cuticle which lines the otocyst. The tube which leads from the exterior
enters the otocyst near the point marked #7. 7. The otoliths are rounded, and
many of them show concentric markings, indicating their deposition in layers.
In the centre of several of them small foreign bodies may be distinguished.
Attached to some of the otoliths are other small rounded bodies of a similar
nature, but which stain more deeply. The minute deeply staining spherules
(indicated by the dots) are also composed of a similar substance. x 210.
(Cf. Figs. 14, 15.)
Fie. 14.—Camera drawing of the cuticle lining the otocyst of a specimen
(128 mm. long) of A. assimilis, var. affinis, from the Falkland Islands. The
otocyst is somewhat larger than the one shown in Fig. 13. The otoliths are
nearly spherical. (They are not all present in one section; some are added
from another section.) The two larger ones are probably the first otoliths of
the post-larva, which are easily distinguished by their size from those which
are formed later. In the centre of each of the large otoliths an irregular
foreign body may be seen. x 210. (Cf. Figs. 13, 15.)
Fie. 15.—Camera drawing of the cuticle lining the otocyst of a specimen
(136 mm. long) of A. assimilis, var. affinis, from Otago Harbour. The
otocyst is much smaller than either of the two preceding. The otoliths are
irregular bodies, chiefly quartz-grains, but two small fragments of spicules
(Sp.) are seen lying close together. x 210. (Cf. Figs. 18, 14.)
Fic. 16.—Camera drawing of the cuticle lining the otocyst of A. marina
(about 10 inches long). The tube connecting this otocyst to the exterior is
almost blocked, and in consequence the otoliths, which were originally small
irregular foreign particles, are now assuming a rounded outline, due to the
deposition upon them of layer upon layer of secreted substance; see, for
example, the otolith containing the spicule fragment (Sp.). Note the small
size of the otocyst compared to those shown in Figs. 18, 14, and 15. x 210.
Fie. 17.—Fifth nephridium of A. assimilis (specimen from Punta
Arenas). The dorsal lip of the nephrostome (Nphm. D.) bears the usual
ciliated vascular processes, while the edge of the ventral lip (Nplm. V.) is
thrown into numerous folds or frills. Note the gonad, a somewhat club-
shaped mass of cells around the gonidial vessel (seen by transparency through
the gonad). x 10.
Fic. 18.—Left aspect of the anterior portion of a post-larval specimen of
A. assimilis, var. affinis, from the Falkland Islands. ‘The total length of
this specimen is 11‘l1 mm. Note the four cup-shaped eyes on the prostomium.
Between the prostomium and the first chetigerous segment is a region
784 J. H. ASHWORTH.
imperfectly divided into two by a groove ; the anterior part—the peristomium
(Per.)—is again partially subdivided into two annuli, in the anterior of which
the otocyst (O/.) may be faintly seen ; the posterior part (4. B. S.) is the first
body-segment, which, however, does not bear any traces of sete. The suc-
ceeding segments hear notopodial and neuropodial sete. The former usually
consist of two or three capilliform bristles and one lanceolate seta (see also
Figs. 3—6). The alimentary canal is seen by transparency through the body-
wall. The buccal mass leads into the cesophagus, which dilates in the fourth
chetigerous segment, and in the sixth bears the glands (@. G/.), seven of
which may be recognised. Immediately behind this the cesophagus is con-
tracted, and leads into the stomach, on whose surface the almost rectangular
chlorogogenous areas are already differentiated. The nephridiopores are not
shown, as they are too minute to be detected with certainty. x 50.
Fic. 19.—The anterior end of the same specimen. Ventral aspect. The
slit-like ccelomic cavity between the two anterior brain-lobes is seen by trans-
parency in the prostomium. The metastomial field—the area included
between the oesophageal connectives—is slightly raised above the level of the
ceneral epidermis. The specimen shows the peristomium (Per.), the achatous
hody-segment (4..B.8.), and the first chetigerous segment (Ch. Seg.'). See
description of previous figure. Compare these parts with those of the adult
shown in Fig. 20. x 50.
Fic. 20.—The anterior end of an. adult specimen of A. assimilis, var.
affinis, from Otago Harbour. Dorsal aspect. Note the prostomium with
its V-shaped lateral lobes embracing the median one. ‘The nuchal groove
(Nuc. Gr.) and the origin of the metastomial groove (Met. Gr.) are shown.
The first chetigerous segment (Ch. Seg.’) consists of three annuli. The region
between this and the prostomium is divisible into two almost equal parts, an
anterior part—the peristomium (Per.)—in which the two annuli are not very
regular, and a posterior part, consisting of two annuli, which form the achi-
tous body-segment (4. B.S.). x8. (Cf. Fig. 19.) The form of the pro-
stomium and the annulation of the skin of type specimens of A, assimilis
from Uschuaia are exactly the same as shown in this figure.
Fre. 21.—Transverse section of a post-larval specimen of A. assimilis
var, affinis, 8°7 mm. long. The section passes somewhat obliquely through
the posterior brain-lobes, the nuchal organ, and the otocyst of the right side
The two posterior cerebral lobes (Post. Cer. Z.) are seen in the prostomium
separated by a portion of the ccelomic cavity. They are closely applied to the
nuchal organ, the ciliated epithelium of which is well seen on the left, but is
cut obliquely on the right. The right otocyst is seen as an invagination of
the epidermis, the mouth of the pit being narrowed to form the tube of the
otocyst. Already a few foreign bodies (otoliths) have gained admittance.
Note the large gland-cells scattered in the epithelium, tie sections of the
ANATOMY OF ARENICOLA ASSIMILIS. 785
cesophageal connectives, the pharynx, the circular aud longitudinal muscles.
The nuclei of the muscle-fibres have been omitted. x 210.
PLATE 37.
Fic. 22.—Transverse section of the anterior end of a specimen of A.
assimilis, from Uschuaia. The section passes through the anterior part of
the brain at the point of origin of the cesophageal connectives. The two
anterior cerebral lobes are seen in the prostomium in close relation to its
epithelium ; bands of nerve-fibres (V.) may be seen passing from the latter
into the brain. The cells in the brain are represented by the dots shown in
the figure. The spaces seen in the brain and in the subepidermal tissue are
canaliform prolongations of the coelomic cavity. On the left the section has
passed somewhat obliquely through the skin, and shows the numerous
branches given off from the cesophageal connective, and ending in the basal
part of the epidermis. The dots in these nerves represent the nerve-cells
which are present. Immediately to the left of the prostomium is the external
opening of the otocyst (Hv. Op. Ot.), the tube being cut through along a con-
siderable part of its length; a transverse section of the inner part of the tube
is seen just ventral to this. In the lower part of the figure the buccal mass
is seen cut across; the elevations of the epithelium (Pap.) shown are the
papille of the “proboscis.” The nerves (WV. Buce.) which supply these are
shown. Note the strong musculature of the buccal mass. x.30.
Fic. 23.—Transverse section of the middle portion of the brain of a speci-
men of A. assimilis var. affinis, from Otago Harbour. Only the left half
of the section is drawn; the median plane is indicated by the two vertical
lines. In the upper part of the section the intimate relation of the prostomial
epithelium and the brain may be observed; the ganglion-cells extend up to,
and lie among the bases of, the epithelial cells. The dots in the brain repre-
sent the nuclei of small nerve-cells, which are usually arranged in groups or
clusters. The large cells in the middle of the figure show the form and posi-
tion of the larger ganglion-cells of the brain. Their processes extend down-
wards into the neuropile, where they branch. Note the numerous fibres
passing from the left half of the brain across the middle line to the right, form-
ing the middle cerebral commissure. The isolated cell on the right side is
drawn from another section. x 210.
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