The Works of Francis Balfour 1-20: Difference between revisions

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1 I employ here the same letters to indicate the stages as in my "Monograph on  
1 I employ here the same letters to indicate the stages as in my "Monograph on  
Elasmobranch Fishes."  
Elasmobranch Fishes."
 
 
 
==XXI. On the evolution of the Placenta, and on the possibility of employing the characters of the Placenta in the classification of the Mammalia==
 
 
 
FROM Owen's observations on the Marsupials it is clear that
the yolk-sack in this group plays an important (if not the most
important) part, in absorbing the maternal nutriment destined
for the foetus. The fact that in Marsupials both the yolk-sack
and the allantois are concerned in rendering the chorion vascular,
makes it a priori probable that this was also the case in the
primitive types of the Placentalia ; and this deduction is supported by the fact that in the Rodentia, Insectivora, and Cheiroptera this peculiarity of the foetal membranes is actually found.
In the primitive Placentalia it is also probable that from the
discoidal allantoic region of the chorion simple foetal villi, like
those of the Pig, projected into uterine crypts ; but it is not
certain how far the umbilical region of the chorion, which was
no doubt vascular, may also have been villous. From such a
primitive type of fcetal membranes divergencies in various
directions have given rise to the types of foetal membranes found
at the present day.
 
In a general way it may be laid down that variations in any
direction which tended to increase the absorbing capacities of
the chorion would be advantageous. There are two obvious
ways in which this might be done, viz. (i) by increasing the
complexity of the foetal villi and maternal crypts over a limited
area, (2) by increasing the area of the part of the chorion covered
by the placental villi. Various combinations of the two processes would also, of course, be advantageous.
 
1 From the Proceedings of the Zoological Society of London, t88i.
 
 
 
THE EVOLUTION OF THE PLACENTA. 735
 
The most fundamental change which has taken place in all
the existing Placentalia is the exclusion of the umbilical vesicle
from any important function in the nutrition of the foetus.
 
The arrangement of the foetal parts in the Rodentia, Insectivora, and Cheiroptera may be directly derived from the
primitive form by supposing the villi of the discoidal placental
area to have become more complex, so as to form a deciduate
discoidal placenta, while the yolk-sack still plays a part, though
physiologically an unimportant part, in rendering the chorion
vascular.
 
In the Carnivora, again, we have to start from the discoidal
placenta, as evinced by the fact that in the growth of the placenta the allantoic region of the placenta is at first discoidal,
and only becomes zonary at a later stage. A zonary deciduate
placenta indicates an increase both in area and in complexity.
The relative diminution of the breadth of the placental zone in
late foetal life in the zonary placenta of the Carnivora is probably
due to its being on the whole advantageous to secure the nutrition of the foetus by insuring a more intimate relation between
the foetal and maternal parts, than by increasing their area of
contact. The reason of this is not obvious, but, as shewn below,
there are other cases where it is clear that a diminution in the
area of the placenta has taken place, accompanied by an increase
in the complexity of its villi.
 
The second type of differentiation from the primitive form of
placenta is illustrated by the Lemuridae, the Suidae, and Manis.
In all these cases the area of the placental villi appears to have
increased so as to cover nearly the whole subzonal membrane,
without the villi increasing to any great extent in complexity.
From the diffused placenta covering the whole surface of the
chorion, differentiations appear to have taken place in various
directions. The placenta of Man and Apes, from its mode of
ontogeny, is clearly derived from a diffused placenta (very
probably similar to that of Lemurs) by a concentration of the
foetal villi, which are originally spread over the whole chorion, to
a disk-shaped area, and by an increase in their arborescence.
Thus the discoidal placenta of Man has no connexion with, and
ought not to be placed in, the same class as those of the Rodentia, Cheiroptera, and Insectivora.
 
 
 
736 THE EVOLUTION OF THE PLACENTA.
 
The polycotyledonary forms of placenta are due to similar
.concentrations of the fcetal villi of an originally diffused placenta.
 
In the Edentata we have a group with very varying types of
placenta. Very probably these may all be differentiations within
the group itself from a diffused placenta such as that found in
Manis. The zonary placenta of Orycteropus is capable of being
easily derived from that of Manis by the disappearance of the
fcetal villi at the two poles of the ovum. The small size of the
umbilical vesicle in Orycteropus indicates that its discoidal placenta is not, like that of the Carnivora, directly derived from a
type with both allantoic and umbilical vascularization of the
chorion. The discoidal and dome-shaped placentae of the
Armadillos, Myrmecophaga, and the Sloths may easily have been
.formed from a diffused placenta, just as the discoidal placenta of
the Simiidse and Hominidae appears to have been formed from a
diffused placenta like that of the Lemuridae.
 
The presence of zonary placentae in Hyrax and ElepJias does
not necessarily afford any proof of affinity of these types with
the Carnivora. A zonary placenta may be quite as easily derived from a diffused placenta as from a discoidal placenta ; and
the presence of two villous patches at the poles of the chorion in
: Elephas very probably indicates that its placenta has been evolved
from a diffused placenta.
 
Although it would not be wise to attempt to found a classification upon the placental characters alone, it may be worth
while to make a few suggestions as to the affinities of the orders
of Mammalia indicated by the structure of the placenta. We
clearly, of course, have to start with forms which could not be
grouped with any of the existing orders, but which might be
called the Protoplacentalia. They probably had the primitive
type of placenta described above : the nearest living representatives of the group are the Rodentia, Insectivora/and Cheiroptera. Before, however, these three groups had become dis.tinctly differentiated, there must have branched off from the
.primitive stock the ancestors of the Lemuridae, the Ungulata,
and the Edentata.
 
It is obvious on general anatomical grounds that the Monkeys
and Man are to be derived from a primitive Lemurian type ; and
 
 
 
THE EVOLUTION OF THE PLACENTA. 737
 
with this conclusion the form of the placenta completely tallies.
The primitive Edentata and Ungulata had no doubt a diffused
placenta which was probably not very different from that of the
primitive Lemurs ; but how far these groups arose quite independently from the primitive stock, or whether they may have
had a nearer common ancestor, cannot be decided from the
structure of the placenta. The Carnivora were certainly an
offshoot from the primitive placental type which was quite independent of the three groups just mentioned ; but the character
of the placenta of the Carnivora does not indicate at what stage
in the evolution of the placental Mammalia a primitive type of
Carnivora was first differentiated.
 
No important light is thrown by the placenta on the affinities
of the Proboscidea, the Cetacea, or the Sirenia ; but the character
of the placenta in the latter group favours the view of their being
related to the Ungulata.
 
 
 
XXII. ON THE STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS 1 . By F. M. BALFOUR and W. N. PARKER.
 
(With Plates 3442.)
TABLE OF CONTENTS.
 
PAGE
 
INTRODUCTION 739
 
GENERAL DEVELOPMENT 74
 
BRAIN
 
Adult brain 759
 
Development of the brain . . _ 7^4
 
Comparison of the larval and adult brain of Lepidosteiis, together with
some observations on the systematic value of the characters of the
Ganoid brain 767
 
SENSE ORGANS
 
Olfactory organ 77 '
 
Anatomy of the eye H>.
 
Development of the eye 771
 
SUCTORIAL Disc 774
 
MUSCULAR SYSTEM 775
 
SKELETON
 
Vertebral column and ribs of the adult 77^
 
Development of the vertebral column and ribs 778
 
Comparison of the vertebral column of Lepidosteus with that of other
 
forms 79 3
 
The ribs of Fishes 793
 
The skeleton of the ventral lobe of the tail fin, and its bearing on the
 
nature of the tail fin of the various types of Pisces . . . 80 1
 
EXCRETORY AND GENERATIVE ORGANS
 
Anatomy of the excretory and generative organs of the female . 810
 
Anatomy of the excretory and generative organs of the male . 813
 
Development of the excretory and generative organs . . . . 815
 
Theoretical considerations 822
 
1 From the Philosophical Transactions of the Royal Society, 1882.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 739
 
 
 
THE ALIMENTARY CANAL AND ITS APPENDAGES PAGE
 
Topographical anatomy of the alimentary canal 828
 
Development of the alimentary canal and its appendages . . . 831
 
THE GILL ON THE HYOID ARCH 835
 
THE SYSTEMATIC POSITION OF LEPIDOSTEUS . . . . . . 836
 
LIST OF MEMOIRS ON THE ANATOMY AND DEVELOPMENT OF LEPIDOSTEUS 840
 
LIST OF REFERENCE LETTERS . . . . 841
 
EXPLANATION OF PLATES 842
 
 
 
INTRODUCTION.
 
THE following paper is the outcome of the very valuable gift
of a series of embryos and larvae of Lepidostens by Professor Alex.
Agassiz, to whom we take this opportunity of expressing our
most sincere thanks. The skull of these embryos and larvae has
been studied by Professor Parker, and forms the subject of a
memoir already presented to the Royal Society.
 
Considering that Lepidosteus is one of the most interesting of
existing Ganoids, and that it is very closely related to species of
Ganoids which flourished during the Triassic period, we naturally
felt keenly anxious to make the most of the opportunity of
working at its development offered to us by Professor Agassiz'
gift. Professor Agassiz, moreover, most kindly furnished us with
four examples of the adult Fish, which have enabled us to make
this paper a study of the adult anatomy as well as of the development.
 
The first part of our paper is devoted to the segmentation,
formation of the germinal layers, and general development of the
embryo and larva. The next part consists of a series of sections
on the organs, in which both their structure in the adult and
their development are dealt with. This part is not, however, in
any sense a monograph, and where already known, the anatomy
is described with the greatest possible brevity. In this part of
the paper considerable space is devoted to a comparison of the
organs of Lepidosteus with those of other Fishes, and to a statement of the conclusions which follow from such comparison.
 
The last part of the paper deals with the systematic position
of Lepidosteus and of the Ganoids generally.
 
 
 
74 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
 
 
GENERAL DEVELOPMENT.
 
The spawning of Lepidosteus takes place in the neighbourhood of New York about May 2Oth. Agassiz (No. i) 1 gives an
account of the process from Mr S. W. Carman's notes, which we
venture to quote in full.
 
" Black Lake is well stocked with Bill-fish. When they
appear, they are said to come in countless numbers. This is
only for a few days in the spring, in the spawning season, between
the 1 5th of May and the 8th of June. During the balance of the
season they are seldom seen. They remain in the deeper parts
.of the lake, away from the shore, and, probably, are more or less
nocturnal in habits. Out of season, an occasional one is caught
on a hook baited with a minnow. Commencing with the 2Oth
of April, until the I4th of May we were unable to find the Fish,
or to find persons who had seen them during this time. Then a
fisherman reported having seen one rise to the surface. Later,
others were seen. On the afternoon of the i8th, a few were
found on the points, depositing the spawn. The temperature at
the time was 68 to 69 on the shoals, while out in the lake the
mercury stood at 62 to 63. The points on which the eggs were
laid. were of naked granite, which had been broken by the frost
and heat into angular blocks of 3 to 8 inches in diameter. The
blocks were tumbled upon each other like loose heaps of brickbats, and upon and between them the eggs were dropped. The
points are the extremities of small capes that make out into the
lake. The eggs were laid in water varying in depth from 2 to
14 inches. At the time of approaching the shoals, the Fish
might be seen to rise quite often to the surface to take air. This
they did by thrusting the bill out of the water as far as the
corners of the mouth, which was then opened widely and closed
with a snap. After taking the air, they seemed more able to
remain at the surface. Out in the lake they are very timid, but
once buried upon the shoals they become quite reckless as to
what is going on about them. A few moments after being driven
 
1 The numbers refer to the list of memoirs of the anatomy and development given
at the end of this memoir.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS; 741
 
off, one or more of the males would return as if scouting. If
frightened, he would retire for some time ; then another scout
would appear. If all promised well, the females, with the attendant males, would come back. Each female was accompanied
by from one to four males. Most often, a male rested against
each side, with their bills reaching up toward the back of her
head. Closely crowded together, the little party would pass
back and forth over the rocky bed they had selected, sometimes
passing the same spot half-a-dozen times without dropping an
egg, then suddenly would indulge in an orgasm ; and, lashing
and plashing the water in all directions with their convulsive
movements, would scatter at the same instant the eggs and the
sperm. This ended, another season of moving slowly back and
forth was observed, to be in turn followed by another of excitement. The eggs were excessively sticky. To whatever they
happened to touch, they stuck, and so tenaciously that it was
next to impossible to release them without tearing away a
portion of their envelopes. It is doubtful whether the eggs
would hatch if removed. As far as could be seen at the time,
upon or under the rocks to which the eggs were fastened there
was an utter absence of anything that might serve as food for
the young Fishes.
 
" Other Fishes, Bull-heads, &c., are said to follow the Bill-fish
to eat the spawn. It may be so. It was not verified. Certainly
the points under observations were unmolested. During the
afternoon of the i8th of May a few eggs were scattered on
several of the beds. On the igth there were more. With the
spear and the snare, several dozens of both sexes of the Fish
were taken. Taking one out did not seem greatly to startle the
others. They returned very soon. The males are much smaller
than the average size of the females ; and, judging from those
taken, would seem to have as adults greater uniformity in size.
The largest taken was a female, of 4 feet ii inch in length.
Others of 2 feet 6 inches contained ripe ova. With the igth of
May all disappeared, and for a time the weather being meanwhile cold and stormy there were no signs of their continued
existence to be met with. Nearly two weeks later, on the 3ist
of May, as stated by Mr Henry J. Perry, they again came up,
not in small detachments on scattered points as before, but in
 
 
 
742 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
multitudes, on every shoal at all according with their ideas of
spawning beds. They remained but two days. During the
summer it happens now and then that one is seen to come up for
his mouthful of air ; beyond this there will be nothing to suggest
the ravenous masses hidden by the darkness of the waters."
 
Egg membranes, The ova of Lepidosteus are spherical bodies
of about 3 millims. in diameter. They have a double investment
consisting of (i) an outer covering formed of elongated, highly
refractive bodies, somewhat pyriform at their outer ends (Plate
34, fig. i/,/*.), which are probably metamorphosed follicular
cells 1 , and (2) of an inner membrane, divided into two zones,
viz. : an outer and thicker zone, which is radially striated, and
constitutes the zona radiata (s. r.}, and an inner and narrow
homogeneous zone (2. r'.\
 
Segmentation. We have observed several stages in the segmentation, which shew that it is complete, but that it approaches
the meroblastic type more nearly than in the case of any other
known holoblastic ovum.
 
Our earliest stage shewed a vertical furrow at the upper or
animal pole, extending through about one-fifth of the circumference (Plate 34, fig. I), and in a slightly later stage we found a
second similar furrow at right angles to the first (Plate 34, fig. 2).
We have not been fortunate enough to observe the next phases
of the segmentation, but on the second day after impregnation
(Plate 34, fig. 3), the animal pole is completely divided into small
segments, which form a disc, homologous to the blastoderm of
meroblastic ova ; while the vegetative pole, which subsequently
forms a large yolk-sack, is divided by a few vertical furrows, four
of which nearly meet at the pole opposite the blastoderm (Plate
34, fig. 4). The majority of the vertical furrows extend only a
short way from the edge of the small spheres, and are partially
intercepted by imperfect equatorial furrows.
 
 
 
1 We have examined the structure of the ovarian ova in order to throw light on
the nature of these peculiar pyriform bodies. Unfortunately, the ovaries of our adult
examples of Lepidosteus were so badly preserved, that we could not ascertain anything on this subject. The ripe ova in the ovary have an investment of pyriform
bodies similar to those of the just laid ova. With reference to the structure of the
ovarian ova we may state that the germinal vesicles are provided with numerous
nucleoli arranged in close proximity with the membrane of the vesicle.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 743
 
Development of the embryo. We have not been able to work
out the stages immediately following the segmentation, owing to
want of material ; and in the next stage satisfactorily observed,
on the third day after impregnation, the body of the embryo is
distinctly differentiated. The lower pole of the ovum is then
formed of a mass in which no traces of the previous segments or
segmentation furrows could any longer be detected.
 
Some of the dates of the specimens sent to us appear to have
been transposed ; so that our statements as to ages must only be
taken as approximately correct.
 
Third day after impregnation. In this stage the embryo is
about 3*5 millims. in length, and has a somewhat dumb-bell shaped
outline (Plate 34, fig. 5). It consists of (i) an outer area (p. z]
with some resemblance to the area pellucida of the Avian
embryo, forming the parietal part of the body ; and (2) a central
portion consisting of the vertebral and medullary plates and the
axial portions of the embryo. In hardened specimens the
peripheral part forms a shallow depression surrounding the
central part of the embryo.
 
The central part constitutes a somewhat prominent ridge, the
axial part of it being the medullary plate. Along the anterior
half of this part a dark line could be observed in all our specimens, which we at first imagined to be caused by a shallow groove.
We have, however, failed to find in our sections a groove in this
situation except in a single instance (Plate 35, fig. 20, x), and are
inclined to attribute the appearance above-mentioned to the
presence of somewhat irregular ridges of the outer layer of the
epiblast, which have probably been artificially produced in the
process of hardening.
 
The anterior end of the central part is slightly dilated to form
the brain (.) ; and there is present a pair of lateral swellings
near the anterior end of the brain which we believe to be the
commencing optic vesicles. We could not trace any other clear
indications of the differentiation of the brain into distinct lobes.
 
At the hinder end of the central part of the embryo a very
distinct dilatation may also be observed, which is probably homologous with the tail swelling of Teleostei. Its structure is more
particularly dealt with in the description of our sections of this
stage.
 
 
 
744 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
After the removal of the egg-membranes described above
we find that there remains a delicate membrane closely attached
to the epiblast. This membrane can be isolated in distinct
portions, and appears to be too definite to be regarded as an
artificial product.
 
We have been able to prepare several more or less complete
series of sections of embryos of this stage (Plate 35, figs. 18 22\
These sections present as a whole a most striking resemblance
to those of Teleostean embryos at a corresponding stage of
development.
 
Three germinal layers are already fully established. The
epiblast (ep.} is formed of the same parts as in Teleostei, viz. :
of an outer epidermic and an inner nervous or mucous stratum.
In the parietal region of the embryo these strata are each
formed of a single row of cells only. The cells of both strata
are somewhat flattened, but those of the epidermic stratum are
decidedly the more flattened of the two.
 
Along the axial line there is placed, as we have stated
above, the medullary plate. The epidermic stratum passes over
this plate without undergoing any change of character, and
the plate is entirely constituted of the nervous stratum of the
epidermis.
 
The medullary plate has, roughly speaking, the form of a
solid keel, projecting inwards towards the yolk. There is no
trace, at this stage at any rate, of a medullary groove ; and as
we shall afterwards shew, the central canal of the cerebro-spinal
cord is formed in the middle of the solid keel. The shape of
this keel varies according to the region of the body. In the
head (Plate 35, fig. 18, m.c.}, it is very prominent, and forming^
as it does, the major part of the axial tissue of the body, impresses
its own shape on the other parts of the head and gives rise to
a marked ridge on the surface of the head directed towards the
yolk. In the trunk (Plate 35, fig's. 19, 20) the keel is much less
prominent, but still projects sufficiently to give a convex form
to the surface of the body turned towards the yolk.
 
In the head, and also near the hind end of the trunk, the
nervous layer of the epiblast continuous with the keel on each
side is considerably thicker than the lateral parts of the layer.
The thickening of the nervous layer in the head gives rise to
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 745
 
what has been called by Gotte l " the special sense plate," owing
to its being subsequently concerned in the formation of parts of
the organs of special sense. We cannot agree with Gotte in
regarding it as part of the brain.
 
In the keel itself two parts may be distinguished, viz.: a
superficial part, best marked in the region of the brain, formed
of more or less irregularly arranged polygonal cells, and a deeper
part of horizontally placed flatter cells. The upper part is
mainly concerned in the formation of the cranial nerves, and of
the dorsal roots of the spinal nerves.
 
The mesoblast (ms.) in the trunk consists of a pair of independent plates which are continued forwards into the head,
and in the prechordal region of the latter, unite below the
medullary keel.
 
The mesoblastic plates of the trunk are imperfectly divided
into vertebral and lateral regions. Neither longitudinal sections
nor surface views shew at this stage any trace of a division of
the mesoblast into somites. The mesoblast cells are polygonal,
and no indication is as yet present of a division into splanchnic
and somatic layers.
 
The notochord (nc.) is well established, so that its origin
could not be made out. It is, however, much more sharply
separated from the mesoblastic plates than from the hypoblast,
though the ventral and inner corners of the mesoblastic plates
which run in underneath it on either side, are often imperfectly
separated from it. It is formed of polygonal cells, of which
between 40 and 50 may as a rule be seen in a single section.
No sheath is present around it. It has the usual extension in
front.
 
The hypoblast (/y.) has the form of a membrane, composed of
a single row of oval cells, bounding the embryo on the side
adjoining the yolk.
 
In the region of the caudal swelling the relations of the
germinal layers undergo some changes. This region may, from
the analogy of other Vertebrates be assumed to constitute the
lip of the blastopore. We find accordingly that the layers become more or less fused. In the anterior part of the tail
 
1 " Ueb. d. Entwick. d. Central Nerven Systems d. Teleoslier," Arc/iiv fur inikr.
Anat. Vol. xv. 1878.
 
B. . 48
 
 
 
746 STRUCTURE AND DEVELOPMENT OF I.EPIDOSTEUS.
 
swelling, the boundary between the notochord and hypoblast
becomes indistinct. A short way behind this point (Plate 35,
fig. 21), the notochord unites with the medullary keel, and a
neurenteric cord, homologous with the neurenteric canal of other
Ichthyopsida, is thus established. In the same region the boundary between the lateral plates of mesoblast and the notochord,
and further back (Plate 35, fig. 22), that between the mesoblast
and the medullary keel, becomes obliterated.
 
Fifth day after impregnation. Between the stage .last described and the next stage of which we have specimens, a considerable progress has been made. The embryo (Plate 34, figs.
6 and 7) has grown markedly in length and embraces more than
half the circumference of the ovum. Its general appearance is,
however, much the same as in the earlier stage, but in the
cephalic region the medullary plate is divided by constrictions
into three distinct lobes, constituting the regions of the forebrain, the mid-brain, and the hind-brain. The fore-brain (Plate
34, fig. 6,f.b.} is considerably the largest of the three lobes, and
a pair of lateral projections forming the optic vesicles are
decidedly more conspicuous than in the previous stage. The
mid-brain (m.b.} is the smallest of the three lobes, while the
hind-brain (h.b) is decidedly longer, and passes insensibly into
the spinal cord behind.
 
The medullary keel, though retaining to a great extent the
shape it had in the last stage, is no longer completely solid.
Throughout the whole region of the brain and in the anterior
part of the trunk (Plate 35, figs. 23, 24, 25) a slit-like lumen has
become formed. We are inclined to hold that this is due to the
appearance of a space between the cells, and not, as supposed by
Oellacher for Teleostei, to an actual absorption of cells, though
we must admit that our sections are hardly sufficiently well preserved to be conclusive in settling this point. Various stages in
its growth may be observed in different regions of the cerebrospinal cord. When first formed, it is a very imperfectly defined
cavity, and a few cells may be seen passing right across from
one side of it to the other. It gradually becomes more definite,
and its wall then acquires a regular outline.
 
The optic vesicles are now to be seen in section (Plate 35,
fig. 23, op.} as flattish outgrowths of the wall of the fore-brain,
 
 
 
STRUCTURE AND DEVELOPMENT OF I.KHDOSTKUS. 747
 
 
 
into which the lumen of the third ventricle is prolonged for a
short distance.
 
The brain has become to some extent separate from the
superjacent epiblast, but the exact mode in which this is effected
is not clear to us. In some sections it appears that the separation
takes place in such a way that the nervous keel is only covered
above by the epidermic layer of the epiblast, and that the
nervous layer, subsequently interposed between the two, grows
in from the two sides. Such a section is represented in Plate 35,
fig. 24. Other sections again favour the view that in the isolation
of the nervous keel, a superficial layer of it remains attached to
the nervous layer of the epidermis at the two sides, and so,
from the first, forms a continuous layer between the nervous
keel and the epidermic layer of the epiblast (Plate 35, fig. 25).
In the absence of a better series of sections we do not feel able
to determine this point. The posterior part of the nervous keel
retains the characters of the previous stage.
 
At the sides of the hind-brain very distinct commencements
of the auditory vesicles are apparent. They form shallow pits
(Plate 35, fig. 24, au.} of the thickened part of the nervous
layer adjoining the brain in this region. Each pit is covered
over by the epidermic layer above, which has no share in its
formation.
 
In many parts of the lateral regions of the body the nervous
layer of the epidermis is more than one cell deep.
 
The mesoblastic plates are now divided in the anterior part
of the trunk into a somatic and a splanchnic layer (Plate 35, fig.
25, so., sp.), though no distinct cavity is as yet present between
these two layers. Their vertebral extremities are somewhat
wedge-shaped in section, the base of the wedge being placed
at the sides of the medullary keel. The wedge-shaped portions
are formed of a superficial layer of 'palisade-like cells and an
inner kernel of polygonal cells. The superficial layer on the
dorsal side is continuous with the somatic mesoblast, while the
remainder pertains to the splanchnic layer.
 
The diameter of the notochord has diminished, and the cells
have assumed a flattened form, the protoplasm being confined to
an axial region. In consequence of this, the peripheral layer
appears clear in transverse sections. A delicate cuticular sheath
 
48-2
 
 
 
748 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
is formed around it. This sheath is probably the commencement of the permanent sheath of later stages, but at this
stage it cannot be distinguished in structure from a delicate
cuticle which surrounds the greater part of the medullary
cord.
 
The hypoblast has undergone no changes of importance.
 
The layers at the posterior end of the embryo retain the
characters of the last stage.
 
Sixth day after impregnation. At this stage (Plate 34, fig. 8)
the embryo is considerably more advanced than at the last stage.
The trunk has decidedly increased in length, and the head forms
a relatively smaller portion of the whole. The regions of the
brain are more distinct. The optic vesicles (op.} have grown
outwards so as to nearly reach the edges of the area which forms
the parietal part of the body. The fore-brain projects slightly
in front, and the mid-brain is seen as a distinct rounded prominence. Behind the latter is placed the hind-brain, which passes
insensibly into the spinal cord. On either side of the mid- and
hind-brain a small region is slightly marked off from the rest of
the parietal part, and on this are seen two more or less transversely directed streaks, which, by comparison with the Sturgeon 1 .
we are inclined to regard as the two first visceral clefts (br.c.}.
We have, however, failed to make them out in sections, and
owing to the insufficiency of our material, we have not even
studied them in surface views as completely as we could have
wished.
 
The body is now laterally compressed, and more decidedly
raised from the yolk than in the previous stages. In the lateral
regions of the trunk the two segmental or archinephric ducts
(sg.} are visible in surface views : the front end of each is placed
at the level of the hinder border of the head, and is marked by
a flexure inwards towards the middle line. The remainder of
each duct is straight, and extends backwards for about half the
length of the embryo. The tail has much the same appearance
as in the last stage.
 
The vertebral regions of the mesoblastic plates are now segmented for the greater part of the length of the trunk, and the
 
1 Salensky, " Recherches s. le Developpement du Sterlet." Archives de Biol.
Vol. n. 1881, pi. xvii. fig. 27.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 749
 
somites of which they are composed (Plate 36, fig. 30, pr.) are
very conspicuous in surface views.
 
Our sections of this stage are not so complete as could be
desired : they shew, however, several points of interest.
 
The central canal of the nervous system is large, with welldefined walls, and in hardened specimens is filled with a coagulum. It extends nearly to the region of the tail.
 
The optic vesicles, which are so conspicuous in surface views,
appear in section (Plate 35, fig. 26, op.} as knob-like outgrowths
of the fore-brain, and very closely resemble the figures given by
Oellacher of these vesicles in Teleostei 1 .
 
From the analogy of the previous stage, we are inclined to
think that they have a lumen continuous with that of the forebrain. In our only section through them, however, they are
solid, but this is probably due to the section merely passing
through them to one side.
 
The auditory pits (Plate 35, fig. 27, au.} are now well marked,
and have the form of somewhat elongated grooves, the walls of
which are formed of a single layer of columnar cells belonging
to the nervous layer of the epidermis, and extending inwards so
as nearly to touch the brain.
 
In an earlier stage it was pointed out that the dorsal part of
the medullary keel was different in its structure from the remainder, and that it was destined to give rise to the nerves.
The process of differentiation is now to a great extent completed, and may best be seen in the auditory region (Plate 35, fig.
27, VIII.). In this region there was present during the last stage
a great rhomboidal mass of cells at the dorsal region of the brain
(Plate 35, fig. 24, VIII.). In the present stage, this, which is the
rudiment of the seventh and auditory nerves, is seen growing
down on each side from the roof of the hind-brain, between the
brain and the auditory involution, and abutting against the wall
of the latter.
 
Rudiments of the spinal nerves are also seen at intervals
as projections from the dorsal angles of the spinal cord (Plate
36, fig. 29, sp.1t.}. They extend only for a short distance
outwards, gradually tapering off to a point, and situated
 
1 "Beitrage zur Entwick. d. Knochenfische," Zeit.f. wiss. Zool. Vol. xxm. 1873,
taf. m. fig. ix. 2.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
 
 
between the epiblast and the dorsal angles of the mesoblastic
somites.
 
The process of formation of the cranial nerves and dorsal
roots of the spinal nerves is, it will be seen, essentially the same
as that already known in the case of Elasmobranchii, Aves, &c.
The nerVes afise as outgrowths of a special crest of cells, the
neural crest of Marshall, which is placed along the dorsal angle
of the cord. The peculiar position of the dorsal roots of the
spinal nerves is also very similar to what has been met with in
the early stages of these structures by Marshall in Birds 1 , and
by one of us in Elasmobranchs 2 .
 
In the parietal region a cavity has now appeared in part of
the trunk betweeri the splanchnic and somatic layers of the
mesoblast (Plate 36, fig. 29, b.c^), the somatic layer (so.) consisting of a single row of columnar cells on the dorsal side, while
the remainder of each somite is formed of the splanchnic layer
(j/'.). In many of the sections the somatic layer is separated by
a considerable interval from the epiblast.
 
We have been able to some extent to follow the development of the segmental duct. The imperfect preservation of our
specimens has, as in other instances, rendered the study of the
point somewhat difficult, but we believe that the figure representing the development of the duct some way behind its front end
(Plate 36, fig. 29) is an accurate representation of 'what may be
seen in a good many of our sections.
 
It appears from these sections that the duct (Plate 36, fig. 29,
.$.) is developed as a hollow ridge-like outgrowth of the somatic
layer of mesoblast, directed towards the epiblast, in which it
causes a slight bulging. The cavity of the ridge freely communicates with the body-cavity. The anterior part of this ridge
appears to be formed first. Very soon, in fact, in an older
embryo belonging to this stage, the greater part of the groove
becomes segmented off as a duct lying between the epiblast and
somatic mesoblast (Plate 36, fig. 28, sg.}, while the front end still
remains, as we believe, in communication with the body-cavity
by an anterior pore.
 
1 Journal of Anat and Physiol. Vol. xi. p. 491, plates xx. and xxi.
 
2 " Elasmobranch Fishes," p. 156, plates 10 and 13. [This edition, p. 378,
pi. ii, 14-]
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 75 1
 
This mode of development corresponds in every particular
with that observed in Teleostei by Rosenberg and Oellacher.
 
The structure of the notochord (nc.) at this stage is very
similar to that observed by one of us in Elasmobranchii 1 . The
cord is formed of transversely arranged flattened cells, the outer
parts of which are vacuolated, while the inner parts are granular,
and contain the nuclei. This structure gives rise to the appearance in transverse sections of an axial darker area and a peripheral lighter portion.
 
The hypoblast retains for the most part its earlier constitution,
but underneath the notochord, in the trunk, it is somewhat thickened, and the cells at the two sides spread in to some extent
under the thickened portion (Plate 36, fig. 29, s.nc.}. This thickening, as is shewn in transverse sections at the stage when the
segmental duct becomes separated from the somatic mesoblast
(Plate 36, fig. 28, s.nc.), is the commencement of the subnotochordal rod.
 
The tail end' of the embryo still retains its earlier characters.
Seventh day after impregnation. Our series of specimens of
this stage is very imperfect, and we are only able to call attention
to the development of a certain number of organs.
 
Our sections clearly establish the fact that the optic vesicles
are now hollow processes of the fore-brain. Their outer ends
are dilated, and are in contact with the external skin. The
formation of the optic cup has not, however, commenced. The
nervous layer of the skin adjoining the outer wall of the optic
cup is very slightly thickened, constituting the earliest rudiment
of the lens.
 
In one of our embryos of this day the developing auditory
vesicle still has the form of a pit, but in the other it is a closed
vesicle, already constricted off from the nervous layer of the
epidermis.
 
With reference to the development of the excretory duct we
cannot add much to what we have already stated in describing
the last stage.
 
The duct is considerably dilated anteriorly (Plate 36, fig. 31,
.$#.); but our sections throw no light on the nature of the abdominal pore. The posterior part of the duct has still the form
 
1 " Elasmobranch Fishes," p. 136, plate 11, fig. 10. [This edition, p. 354, pi. 12.]
 
 
 
752 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
of a hollow ridge united with somatic mesoblast (Plate 36, fig.
32, sg.).
 
During this stage, the embryo becomes to a small extent
folded off from the yolk-sack both in front and behind, and in the
course of this process the anterior and posterior extremities of
the alimentary tract become definitely established.
 
We have not got as clear a view of the process of formation
of these two sections of the alimentary tract as we could desire,
but our observations appear to shew that the process is in many
respects similar to that which takes place in the formation of
the anterior part of the alimentary tract in Elasmobranchii 1 .
One of us has shewn that in Elasmobranchs the ventral wall of
the throat is formed not by a process of folding in of the hypoblastic sheet as in Birds, but by a growth of the ventral face of
the hypoblastic sheet on each side of and at some little distance
from the middle line. Each growth is directed inwards, and
the two eventually meet and unite, thus forming a complete
ventral wall for the gut. Exactly the same process would seem
to take place in Lepidosteus, and after the lumen of the gut is in
this way established, a process of mesoblast on each side also
makes its appearance, forming a mesoblastic investment on the
ventral side of the alimentary tract. Some time after the alimentary tract has been thus formed, the epiblast becomes folded
in, in exactly the same manner as in the Chick, the embryo
becoming thereby partially constricted off from the yolk (Plate
36, figs. 33, 34).
 
The form of the lumen of the alimentary tract differs somewhat in front and behind. In front, the hypoblastic sheet
remains perfectly flat during the formation of the throat, and thus
the lumen of the latter has merely the form of a slit. The lumen
of the posterior end of the alimentary tract is, however, narrower
and deeper (Plate 36, figs. 33, 34, a/.). Both in front and behind,
the lateral parts of the hypoblastic sheet become separated from
the true alimentary tract as soon as the lumen of the latter is
established.
 
It is quite possible that at the extreme posterior end of the
embryo a modification of the above process may take place, for
 
1 F. M. Balfour, "Monograph on the Development of Elasmobranch Fishes,"
p. 87, plate 9, fig. 2. [This edition, p. 303, pi. 10.]
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 753
 
 
 
in this region the hypoblast appears to us to have the form of
a solid cord.
 
We could detect no true neurenteric canal, although a more
or less complete fusion of the germinal layers at the tail end of
the embryo may still be traced.
 
During this stage the protoplasm of the notochordal cells,
which in the last stage formed a kind of axial rod in the centre
of the notochord, begins to spread outwards toward the sheath
of the notochord.
 
Eighth day after impregnation. The external form of the embryo (Plate 34, fig. 9) shews a great advance upon the stage last
figured. Both head and body are much more compressed laterally and raised from the yolk, and the head end is folded off for
some distance. The optic vesicles are much less prominent
externally. A commencing opercular fold is distinctly seen.
Our figure of this stage is not, however, so satisfactory as we
could wish.
 
A thickening of the nervous layer of the external epiblast
which will form the lens (Plate 36, fig. 35, /.) is more marked
than in the last stage, and presses against the slightly concave
exterior wall of the optic vesicle (op.). The latter has now
a large cavity, and its stalk is considerably narrowed.
 
The auditory vesicles (Plate 36, fig. 36, au.) are closed, appearing as hollow sacks one on each side of the brain, and are no
longer attached to the epiblast.
 
The anterior opening of the segmental duct can be plainly
seen close behind the head. The lumen of the duct is considerably larger.
 
The two vertebral portions of the mesoblast are now separated by a considerable space from the epiblast on one side and
from the notochord on the other, and the cells composing them
have become considerably elongated from side to side (Plate 36,
fig. 37, MS.).
 
In some sections the aorta can be seen (Plate 36, fig. 37, ##.)
lying close under the sub- notochordal rod, between it and the
hypoblast, and on either side of it a slightly larger cardinal vein
(cd. v.}.
 
The protoplasm of the notochord has now again retreated
towards the centre, shewing a clear space all round. This is
 
 
 
754 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
 
 
most marked in the region of the trunk (Plate 36, fig. 37). The
sub-notochordal rod (s. nc.) lies close under it.
 
A completely closed fore-gut, lined by thickened hypoblast,
extends about as far back as the auditory sacks (Plate 36, figs. 35
and 36, /.'). In the trunk the hypoblast, which will form the
walls of the alimentary tract, is separated from the notochord
by a considerable interval.
 
Ninth day after impregnation : External characters. Very
considerable changes have taken place in the external characters
of the embryo. It is about 8 millims. in length, and has assumed
a completely piscine form. The tail especially has grown in
length, and is greatly flattened from side to 'side : it is wholly
detached from the yolk, and bends round towards the head,
usually with its left side in contact with the yolk. It is provided with well-developed dorsal and ventfal fin-folds, which
meet each other round the end of the tail, the tail fin so formed
being, nearly symmetrical. The head is not nearly so much
folded off from the yolk as the tail. At its front end is placed
a disc with numerous papillae, of which we shall say more hereafter. This disc is somewhat bifid, and is marked in the centre
by a deep depression.
 
Dorsal to it, on the top of the head, are two widely separated
nasal pits. On the surface of the yolk, in front of the head, is to
be seen the heart, just as in Sturgeon embryos. Immediately
below the suctorial disc is a slit-like space, forming the mouth.
It is bounded below by the two mandibular arches, which meet'
ventrally in the median line. A shallow but well-marked depression on each side of the head indicates the posterior boundary
of the mandibular arch. Behind this is placed the very conspicuous hyoid arch with its rudimentary opercular flap ; and in
the depression, partly covered over by the latter, may be seen a
ridge, the external indication of the first branchial arch.
 
Eleventh day after impregnation : External characters. The
embryo (Plate 34, fig. 10) is now about 10 millims. in length, and
in several features exhibits an advance upon the embryo of the
previous stage.
 
The tail fin is now obviously not quite symmetrical, and
the dorsal fin-fold is continued for nearly the whole length of the
trunk. The suctorial disc (Plate 34, fig. 1 1, s.d.} is much more
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 755
 
 
 
prominent, and the papillae (about 30 in number) covering it are
more conspicuous from the surface. It is not obviously composed of two symmetrical halves. The opercular flap is larger,
and the branchial arches behind it (two of which may be made
out without dissection) are more prominent.
 
The anterior pair of limbs is now visible in the form of two
longitudinal folds projecting in a vertical direction from the
surface of the yolk-sack at the sides of the body.
 
The stages subsequent to hatching have been investigated
with reference to the external features and to the habits by
Agassiz, and we shall enrich our own account by copious quotations from his memoir.
 
He states that the first batch were hatched on the eighth 1
day after being laid. " The young Fish possessed a gigantic
yolk-bag, and the posterior part of the body presented nothing
specially different from the general appearance of a Teleostean
embryo, with the exception of the great size of the chorda. The
anterior part, however, was most remarkable ; and at first, on
seeing the head of this young Lepidosteus, with its huge mouthcavity extending nearly to the gill-opening, and surmounted by
a hoof-shaped depression edged with a row of protuberances
acting as suckers, I could not help comparing this remarkable
structure, so utterly unlike anything in Fishes or Ganoids, to the
Cyclostomes, with which it has a striking analogy. This organ
is also used by Lepidostetts as a sucker, and the moment the
young Fish is hatched he attaches himself to the sides of the
disc, and there remains hanging immovable; so firmly attached,
indeed, that it requires considerable commotion in the water to
make him loose his hold. Aerating the water by pouring it from
a height did not always produce sufficient disturbance to loosen
the young Fishes. The eye, in this stage, is rather less advanced
than in corresponding stages in bony Fishes ; the brain is also
comparatively smaller, the otolith ellipsoidal, placed obliquely in
the rear above the gill-opening. . . . Usually the gill-cover is
pressed closely against the sides of the body, but in breathing an
opening is seen through which water is constantly passing, a
 
1 This statement of Agassi/, does not correspond with the dates on the specimens
sent to us a fact no doubt due to the hatching not taking place at the same time for
all the larva;.
 
 
 
756 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
strong current being made by the rapid movement of the pectorals,
against the base of which the extremity of the gill-cover is closely
pressed. The large yolk-bag is opaque, of a bluish-gray colour.
The body of the young Lepidostens is quite colourless and transparent. The embryonic fin is narrow, the dorsal part commencing
above the posterior end of the yolk-bag ; the tail is slightly
rounded, the anal opening nearer the extremity of the tail than
the bag. The intestine is narrow, and the embryonic fin extending from the vent to the yolk-bag is quite narrow. In a somewhat more advanced stage, hatched a few hours earlier, the
upper edge of the yolk-bag is covered with black pigment cells,
and minute black pigment cells appear on the surface of the
alimentary canal. There are no traces of embryonic fin-rays
either in this stage or the one preceding ; the structure of the
embryonic fin is as in bony Fishes previous to the appearance
of these embryonic fin-rays finely granular. Seen in profile,
the yolk-bag is ovoid ; as seen from above, it is flattened, rectangular in front, with rounded corners, tapering to a rounded
point towards the posterior extremity, with re-entering sides."
 
We have figured an embryo of 1 1 millims. in length, shortly
after hatching (Plate 34, fig. 12), the most important characters
of which are as follows : The yolk-sack, which has now become
much reduced, forms an appendage attached to the ventral
surface of the body, and has a very elongated form as compared
with its shape just before hatching. The mouth, as also noticed
by Agassiz, has a very open form. It is (Plate 34, fig. 13, m.}
more or less rhomboidal, and is bounded behind by the mandibular arch (?;/.) and laterally by the superior maxillary processes
(s. mx). In front of the mouth is placed the suctorial disc (s. </.), the
central papillae of which are arranged in groups. The opercular
fold (Ji. op.} is very large, covering the arches behind. A wellmarked groove is present between the mandibular and opercular
arches, but so far as we can make out it is not a remnant of the
hyomandibular cleft.
 
The pectoral fins (Plate 34, fig. \2,pc.f?} are very prominent
longitudinal ridges, which, owing to their being placed on the
surface of the yolk-sack, project in a nearly vertical direction : a
feature which is also found in many Teleostean embryos with
large yolk-sacks.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 757
 
 
 
No traces of the pelvic fins have yet become developed.
 
The positions of the permanent dorsal, anal, and caudal fins,
as pointed out by Agassiz, are now indicated by a deposit of
pigment in the embryonic fin.
 
In an embryo on the sixth day after hatching, of about 15
millims. in length, of which we have also given a figure (Plate
34, fig. 14), the following fresh features deserve special notice.
 
In the region of the head there is a considerable elongation
of the pre-oral part, forming a short snout, at the end of which
is placed the suctorial disc. At the sides of the snout are placed
the nasal pits, which have become somewhat elongated anteriorly.
 
The mouth has lost its open rhomboidal shape, and has
become greatly narrowed in an antero-posterior direction, so
that its opening is reduced to a slit. The mandibles and maxillary processes are nearly parallel, though both of them are
very much shorter than in the adult. The operculum is now a
very large flap, and has extended so far backwards as to cover
the insertion of the pectoral fin. The two opercular folds nearly
meet ventrally.
 
The yolk-sack is still more reduced in size, one important
consequence of which is that the pectoral fins (pc.f.) appear to
spring out more or less horizontally from the sides of the body,
and at the same time their primitive line of attachment to the
body becomes transformed from a longitudinal to a more or less
transverse one.
 
The first traces of the pelvic fins are now visible as slight
longitudinal projections near the hinder end of the yolk-sack
 
 
 
The pigmentation marking the regions of the permanent fins
has become more pronounced, and it is to be specially noted
that the ventral part of the caudal fin (the permanent caudal) is
considerably more prominent than the dorsal fin opposite to it.
 
The next changes, as Agassiz points out, " are mainly in the
lengthening of the snout ; the increase in length both of the
lower and upper jaw ; the concentration of the sucker of the
sucking disc ; and the adoption of the general colouring of
somewhat older Fish. The lobe of the pectoral has become
specially prominent, and the outline of the fins is now indicated
by a fine milky granulation. Seen from above, the gill-cover is
 
 
 
STRUCTURE AND DEVELOPMENT OF I.EPIDOSTEUS.
 
 
 
seen to leave a large circular opening leading to the gill-arches,
into which a current of water is constantly passing, by the lateral
expansion and contraction of the gill-cover; the outer extremity
of the gill-cover covers the base of the pectorals. In a somewhat older stage the snout has become more elongated, the
sucker more concentrated, and the disproportionate size of the
terminal sucking-disc is reduced ; the head, when seen from
above, becoming slightly elongated and pointed."
 
In a larva of about 18 days old and 21 millims. in length, of
which we have not given a figure, the snout has grown greatly
in length, carrying with it the nasal organs, the openings of
which now appear to be divided into two parts. The suctorial
disc is still a prominent structure at the end of the snout. The
lower jaw has elongated correspondingly with the upper, so that
the gape is very considerable, though still very much less than
in the adult.
 
The opercular flaps overlap ventrally, the left being superficial. They still cover the bases of the pectoral fins. The
latter are described by Agassiz as being " kept in constant rapid
motion, so that the fleshy edge is invisible, and the vibration
seems almost involuntary, producing a constant current round
the opening leading into the cavity of the gills."
The pelvic fins are somewhat more prominent
The yolk-sack, as pointed out by Agassiz, has now disappeared as an external appendage.
 
After the stage last described the young Fish rapidly approaches the adult form. To shew the changes effected we
have figured the head of a larva of about a month old and
23 millims. in length (Plate 34, fig. 15). The suctorial disc,
though much reduced, is still prominent at the end of the snout.
Eventually, as shewn by Agassiz, it forms the fleshy globular
termination of the upper jaw.
 
The most notable feature in which the larva now differs in
its external form from the adult is in the presence of an externally heterocercal tail, caused by the persistence of the primitive caudal fin as an elongated filament projecting beyond the
permanent caudal (Plate 41, fig. 68).
 
Delicate dermal fin-rays are now conspicuous in the peripheral parts of all the permanent fins. These rays closely
 
 
 
STRUCTURE AND DEVELOPMENT OF LEHDOSTEUS. 759
 
resemble the horny fin-rays in the fins of embryo Elasmobranchs in their development and structure. They appear
gradually to enlarge to form the permanent rays, and we have
followed out some of the stages of their growth, which is in
many respects interesting. Our observations are not, however,
complete enough to publish, and we can only say here that their
early development and structure proves their homology with
the horny fibres or rays in fins of Elasmobranchii. The skin is
still, however, entirely naked, and without a trace of its future
armour of enamelled scales.
 
The tail of a much older larva, 1 1 centims. in length, in
which the scales have begun to be formed, is shewn in Plate 34,
fig. 1 6.
 
We complete this section of our memoir by quoting the
following passages from Agassiz as to the habits of the young
fish at the stages last described :
 
" In the stages intervening between plate iii, fig. 19, and
plate iii, fig. 30, the young Lepidosteus frequently swim about,
and become readily separated from their point of attachment.
In the stage of plate iii, fig. 30, they remain often perfectly quiet
close to the surface of the water; but, when disturbed, move
very rapidly about through the water. . . . The young
already have also the peculiar habit of the adult of coming to
the surface to swallow air. When they go through the process
under water of discharging air again they open their jaws wide,
and spread their gill-covers, and swallow as if they were choking,
making violent efforts, until a minute bubble of air has become
liberated, when they remain quiet again. The resemblance to a
Sturgeon in the general appearance of this stage of the young
Lepidosteus is quite marked."
 
 
 
BRAIN.
I. A natomy.
 
The brain of Lepidosteus has been figured by Busch (whose
figure has been copied by Miklucho-Maclay, and apparently by
Huxley), by Owen and by Wilder (No. 15). The figure of the
latter author, representing a longitudinal section through the
 
 
 
/60 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
brain, is the most satisfactory, the other figures being in many
respects inaccurate ; but even Wilder's figure and description,
though taken from the fresh object, appear to us in some
respects inadequate. He offers, moreover, fresh interpretations
of certain parts of the brain which we shall discuss in the sequel.
 
We have examined two brains which, though extremely soft,
were, nevertheless, sufficiently well preserved to enable us to
study the external form. We have, moreover, made a complete
series of transverse sections through one of the brains, and our
sections, though utterly valueless from a histological point of view,
have thrown some light on the topographical anatomy of the
brain.
 
Plate 38, figs. 47 A, B, and C, represent three views of the
brain, viz. : from the side, from above, and from below. We will
follow in our description the usual division of the brain into forebrain, mid-brain, and hind-brain.
 
The fore-brain consists of an anterior portion forming the
cerebrum, and a posterior portion constituting the thalamencephalon.
 
The cerebrum at first sight appears to be composed of (a)
a pair of posterior and somewhat dorsal lobes, forming what have
usually been regarded as the true cerebral hemispheres, but
called by Wilder the prothalami, and (b) a pair of anterior and
ventral lobes, usually regarded as the olfactory lobes, from which
the olfactory nerves spring. Mainly from a comparison with
our embryonic brains described in the sequel, we are inclined to
think that the usual interpretations are not wholly correct, but
that the true olfactory lobes are to be sought for in small enlargements (Plate 38. figs. 47 A, B, and C, o/f.) at the front end of the
brain 1 from which the olfactory nerves spring. The cerebrum
proper would then consist of a pair of anterior and ventral lobes
(ce.}, and of a pair of posterior lobes (ce'.\ both pairs uniting to
form a basal portion behind.
 
The two pairs of lobes probably correspond with the two
parts of the cerebrum of the Frog, the anterior of which, like
that of Lepidosteus, was held to be the olfactory lobe, till Gotte's
researches shewed that this view was not tenable.
 
1 The homoiogies of the olfactory lobes throughout the group of Fishes require
further investigation.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 761
 
 
 
The anterior lobes of the cerebrum have a conical form, tapering anteriorly, and are completely separated from each other.
The posterior lobes, as is best shewn in side views, have 'a
semicircular form. Viewed from above they appear as rounded
prominences, and their dorsal surface is marked by two conspicuous furrows (Plate 38, fig. 47 B, ce'.}, which have been noticed
by Wilder, and are similar to those present in many Teleostei.
Their front ends overhang the base of the anterior cerebral
lobes. The basal portion of the cerebrum is an undivided lobe,
the anterior wall of which forms the lamina terminalis.
 
What we have above described as the posterior cerebral
lobes have been described by Wilder as constituting the everted
dorsal border of the basal portion of the cerebrum.
 
The portion of the cerebro-spinal canal within the cerebrum
presents certain primitive characters, which are in some respects
dissimilar to those of higher types, and have led Wilder to
hold the posterior cerebral lobes, together with what we have
called the basal portion of the cerebrum, to be structures
peculiar to Fishes, for which he has proposed the name " prothalami."
 
In the basal portion of the cerebrum there is an unpaired
slit-shaped ventricle, the outer walls of which are very thick.
It is provided with a floor formed of nervous matter, in part of
which, judging from Wilder's description, a well-marked commissure is placed. We have found in the larva a large commissure in this situation (Plate 37, figs. 44 and 45, a.c.) ; and
it may be regarded as the homologue of the anterior commissure
of higher types. This part of the ventricle is stated by Wilder
to be without a roof. This appears to us highly improbable. We
could not, however, determine the 'nature of the roof from our
badly preserved specimens, but if present, there is no doubt that
it is extremely thin, as indeed it is in the larva (Plate 37, fig.
46 B). In a dorsal direction the unpaired ventricle extends so
as to separate the two posterior cerebral lobes. Anteriorly the
ventricle is prolonged into two horns, which penetrate for a
short distance, as the lateral ventricles, into the base of the
anterior cerebral lobes. The front part of each anterior cerebral
lobe, as well as of the whole of the posterior lobes, appears solid
in our sections ; but Wilder describes the anterior horns of the
B. 49
 
 
 
762 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
ventricle as being prolonged for the whole length of the anterior
lobes.
 
In the embryos of all Vertebrates the cerebrum is not at
first divided into two lobes, so that the fact of the posterior part
of the cerebrum in Lepidosteus and probably other Ganoids
remaining permanently in the undivided condition does not
appear to us a sufficient ground for giving to the lobes of this
part of the cerebrum the special name of prothalami, as proposed by Wilder, or for regarding them as a section of the
brain peculiar to Fishes.
 
The thalamencephalon (///.) contains the usual parts, but is
is some respects peculiar. Its lateral walls, forming the optic
thalami, are thick, and are not sharply separated in front from
the basal part of the cerebrum ; between them is placed the
third ventricle. The thalami are of considerable extent, though
partially covered by the optic lobes and the posterior lobes of
the cerebrum. They are not, however, relatively so large as
in other Ganoid forms, more especially the Chondrostei and
Polypterus.
 
On the roof of the thalamencephalon is placed a large thinwalled vesicle (Plate 38, figs. 47 A and B, v.tk.), which undoubtedly
forms the most characteristic structure connected with this part
of the brain. Owing to the wretched state of preservation of
the specimens, we have found it impossible to determine the
exact relations of this body to the remainder of the thalamencephalon; but it appears to be attached to the roof of the
thalamencephalon by a narrow stalk only. It extends forwards
so as to overlap part of the cerebrum in front, and is closely
invested by a highly vascular layer of the pia mater.
 
No mention is made by Wilder of this body ; nor is it represented in his figures or in those of the other anatomists who
have given drawings of the brain of Lepidosteus. It might at
first be interpreted as a highly-developed pineal gland, but a
comparison with the brain of the larva (vide p. 764) shews that
this is not the case, but that the body in question is represented
in the larva by a special outgrowth of the roof of the thalamencephalon. The vesicle of the roof of the thalamencephalon is
therefore to be regarded as a peculiar development of the tela
choroidea of the third ventricle.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 763
 
How far this vesicle has a homologue in the brains of other
Ganoids is not certain, since negative evidence on this subject is
all but valueless. It is possible that a vesicular sack covering
over the third ventricle of the Sturgeon described by Stannius 1 ,
and stated by him to be wholly formed of the membranes of the
brain, is really the homologue of our vesicle.
 
Wiedersheim 2 has recently described in Protopterns a body
which is undoubtedly homologous with our vesicle, which he
describes in the following way :
 
" Dorsalwarts ist das Zwischenhirn durch ein tiefes, von
Hirnschlitz eingenommenes Thai von Vorderhirn abgesetzt ;
dasselbe ist jedoch durch eine hautige, mit der Pia mater zusammenhangende Kuppel oder Kapsel uberbruckt."
 
This " Kuppel " has precisely the same relations and a very
similar appearance to our vesicle. The true pineal gland is
placed behind it. It appears to us possible that the body found
by Huxley 3 in Ceratodus, which he holds to be the pineal gland,
is in reality this vesicle. It is moreover possible that what has
usually been regarded as the pineal gland in Petromyzon may
in reality be the homologue of the vesicle we have found in
Lepidosteus.
 
We have no observations on the pineal gland of the adult,
but must refer the reader for the structure and relations of this
body to the embryological section.
 
The infundibulum (Plate 38, fig. 47 A, in.) is very elongated.
Immediately in front of it is placed the optic chiasma (Plate 38,
figs. 47 A and C, op.c/i.) from which the optic fibres can be traced
passing along the sides of the optic thalami and to the optic
lobes, very much as in M tiller's figure of the brain of Polypterus,
 
On the sides of the infundibulum are placed two prominent bodies, the lobi inferiores (l.in), each of which contains a
cavity continuous with the prolongation of the third ventricle
 
1 " Ueb. d. Gehirn des Stors," Mailer's Arc/iiv, 1843, and Lehrbuch d. vergl. Anat.
d. Wirbelthiere. Cattie, Archives de Biologie, Vol. in. 1882, has recently described
in Acipenser sturio a vesicle on the roof of the thalamencephalon, whose cavity is
continuous with the third ventricle. This vesicle is clearly homologous with that in
Lepidosteus. (June 28, 1882.)
 
2 R. Wiedersheim, Morphol. Studien, 1880, p. 71.
 
3 "On Ceratodus Forstcri" &.C., Proc. Zool. Soc. 1876.
 
492
 
 
 
764 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
into the infundibulum. The apex of the infundibulum is enlarged,
and to it is attached a pituitary body (//.).
 
The mid-brain is of considerable size, and consists of a basal
portion connecting the optic thalami with the medulla, and a
pair of large optic lobes (op.l.}. The iter a tertio ad quartum
ventriculum, which forms the ventricle of this part of the brain,
is prolonged into each optic lobe, and the floor of each prolongation is taken up by a dome-shaped projection, the homologue
of the torus semicircularis of Teleostei.
 
The hind-brain consists of the usual parts, the medulla
oblongata and the cerebellum. The medulla presents no peculiar
features. The sides of the fourth ventricle are thickened and
everted, and marked with peculiar folds (Plate 38, figs. 47 A
and B, m.o.).
 
The cerebellum is much larger than in the majority of
Ganoids, and resembles in all essential features the cerebellum
of Teleostei. In side views it has a somewhat S-shaped form,
from the presence of a peculiar lateral sulcus (Plate 38, fig. 47 A,
cd.). As shewn by Wilder, its wall actually has in longitudinal
section this form of curvature, owing to its anterior part projecting forwards into the cavity of the iter 1 . This forward projection is not, however, so conspicuous as in most Teleostei.
The cerebellum contains a -large unpaired prolongation of the
fourth ventricle.
 
 
 
1 1 . Development.
 
The early development of the brain has already been described ; and, although we do not propose to give any detailed
account of the later stages of its growth, we have thought it
worth while calling attention to certain developmental features
which may probably be regarded as to some extent characteristic
of the Ganoids. With this view we have figured (Plate 37, figs.
44, 45) longitudinal sections of the brain at two stages, viz.:
of larvae of 15 and 26 millims., and transverse sections (Plate 37,
figs. 46 A G) of the brain of a larva at about the latter stage
(25 millims.).
 
1 In Wilder's figure the walls of the cerebellum are represented as much too thin.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 765
 
The original embryonic fore-brain is divided in both embryos
into a cerebrum (ce.) in front and a thalamencephalon (th.) behind.
In the younger embryo the cerebrum is a single lobe, as it is
in the brains of all Vertebrate embryos ; but in the older larva
it is anteriorly (Plate 37, fig. 46 A) completely divided into
two hemispheres. The roof of the undivided posterior part of
the cerebrum is extremely thin (Plate 37, fig. 46 B). Near the
posterior border of the base of the cerebrum there is a great
development of nervous fibres, which may probably be regarded
as in part equivalent to the anterior commissure (Plate 37, figs.
44, 45 a.c.).
 
Even in the oldest of the two brains the olfactory lobes are
very slightly developed, constituting, however, small lateral and
ventral prominences of the front end of the hemispheres. From
each of them there springs a long olfactory nerve, extending for
the whole length of the rostrum to the olfactory sack.
 
The thalamencephalon presents a very curious structure, and
is relatively a more important part of the brain than in the
embryo of any other form which we know of. Its roof, instead
of being, as usual, compressed antero-posteriorly 1 , so as to be
almost concealed between the cerebral hemispheres and the optic
Jpbes (mid-brain), projects on the surface for a length quite equal
to that of the cerebral hemispheres (Plate 37, figs. 44 and 45, th.}.
In the median line the roof of the thalamencephalon is thin
and folded ; at its posterior border is placed the opening of
the small pineal gland. This body is a papilliform process of
the nervous matter of the roof of this part of the brain, and
instead of being directed forwards, as in most Vertebrate types,
tends somewhat backwards, and rests on the mid-brain behind
(Plate 37, figs. 44, 45, and 46 C and D, /.). The roof of the
thalamencephalon immediately in front of the pineal gland forms
a sort of vesicle, the sides of which extend laterally as a pair
of lobes, shewn in transverse sections in Plate 37, figs. 46 C and
D, as th.L This vesicle becomes, we cannot doubt, the vesicle
on the roof of the thalamencephalon which we have described in
the adult brain. Immediately in front of the pineal gland the
roof of the thalamencephalon contains a transverse commissure
 
1 Vide F. M. Balfour, Comparative Embryology, Vol. II. figs. 248 and 250.
 
 
 
766 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
(Plate 37, fig. 46 C, z.}, which is the homologue of a similarly
situated commissure present in the Elasmobranch brain 1 , while
behind the pineal gland is placed the posterior commissure. The
sides of the thalamencephalon are greatly thickened, forming
the optic thalami (Plate 37, figs. 46 C and D, op.th^, which are
continuous in front with the thickened outer walls of the hemispheres. Below, the thalamencephalon is produced into a very
elongated infundibulum (Plate 37, figs. 44, 45, 46 E, in.}, the
apex of which is trilobed as in Elasmobranchii and Teleostei.
The sides of the infundibulum exhibit two lobes, the lobi inferiores (Plate 37, fig. 46 E, /./.), which are continued posteriorly
into the crura cerebri.
 
The pituitary body 2 (Plate 37, figs. 44, 45, 46 E,/A) is small,
not divided into lobes, and provided with a very minute lumen.
 
In front of the infundibulum is the optic chiasma (Plate 37,
fig. 46 D, op.ch.}, which is developed very early. It is, as stated
by Mtiller, a true chiasma.
 
The mid-brain (Plate 37, figs. 44 and 45, m. b.} is large, and
consists in both stages of (i) a thickened floor forming the crura
cerebri, the central canal of which constitutes the iter a tertio ad
quartum ventriculum ; and (2} the optic lobes (Plate 37, figs. 46
E, F, G, op. /.) above 5 each of which is provided with a cavity
continuous with the median iter. The optic lobes are separated
dorsally and in front by a well-marked median longitudinal
groove. Posteriorly they largely overlap the cerebellum. In the
anterior part of the optic lobes, at the point where the iter joins
the third ventricle, there may be seen slight projections of the
floor into the lumen of the optic lobes (Plate 37, fig. 46 E).
These masses probably become in the adult the more conspicuous
 
1 Vide F. M. Balfour, Comparative Embryology, Vol. II. pp. 355 6 [the original
edition], where it is suggested that this commissure is the homologue of the grey
commissure of higher types.
 
8 We have not been able to work out the early development of the pituitary body
ns satisfactorily as we could have wished. In Plate 37, fig. 40, there is shewn an
invagination of the oral epithelium to form it ; in Plate 37, figs. 41 and 42, it is represented in transverse section in two consecutive sections. Anteriorly it is still connected with the oral epithelium (fig. 41), while posteriorly it is free. It is possible
that an earlier stage of it is shewn in Plate 36, fig. 35. Were it not for the evidence
in other types of its being derived from the epiblast we should be inclined to regard it
as hypoblaslic in origin.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 767
 
prominences of the floor of the ventricles of the optic lobes,
which we regard as homologous with the tori semicirculares of
the brain of the Teleostei.
 
The hind-brain is formed of the usual divisions, viz. : cerebellum and medulla oblongata (Plate 37, figs. 44 and 45, cb., md.).
The former constitutes a bilobed projection of the roof of the
hind-brain. Only a small portion of it is during these stages left
uncovered by the optic lobes, but the major part extends forwards
for a considerable distance under the optic lobes, as shewn in
the transverse sections (Plate 37, figs. 46 F and G, cb.) ; and
its two lobes, each with a prolongation .of its cavity, are continued forwards beyond the opening of the iter into the fourth
ventricle.
 
It is probable that the anterior horns of the cerebellum are
equivalent to the prolongations of the cerebellum into the central
cavity of the optic lobes of Teleostei, which are continuous with
the so-called fornix of Gottsche.
 
 
 
III. Comparison of the larval and adult brain of Lepidosteus,
together with some observations on the systematic value of the
characters of the Ganoid brain.
 
The brain of the older of the two larvae, which we have
described, sufficiently resembles in most of its features that of
the adult to render material assistance in the interpretation of
certain of the parts of the latter. It will be remembered that in
the adult brain the parts usually held to be olfactory lobes were
described as the anterior cerebral lobes. The grounds for this
will be apparent by a comparison of the cerebrum of the larva
and adult. In the larva the cerebrum is formed of (i) an unpaired
basal portion with a thin roof, and (2) of a pair of anterior lobes,
with small olfactory bulbs at their free extremities.
 
The basal portion in the larva clearly corresponds in the
adult with the basal portion, together with the two posterior
cerebral lobes, which are merely special outgrowths of the dorsal
edge of the primitive basal portion. The pair of anterior lobes
have exactly the same relations in the larva as in the adult,
except that in the former the ventricles are prolonged for their
 
 
 
768 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
whole length instead of being confined to their proximal portions.
If, therefore, our identifications of the larval parts of the brain
are correct, there can hardly be a question as to our identifications
of the parts in the adult. As concerns these identifications, the
comparison of the brain of our two larvae appears conclusive in
favour of regarding the anterior lobes as parts of the cerebrum,
as distinguished from the olfactory lobes, in that they are clearly
derived from the undivided anterior portion of the cerebrum of
the younger larva.
 
The comparison of the larval brain with that of the adult
again appears to us to leave no doubt that the vesicle attached
to the roof of the thalamencephalon in the adult is the same
structure as the bilobed outgrowth of this roof in the larva ; and
since there is in addition a well-developed pineal gland in the
larva with the usual relations, there can be no ground for identifying the vesicle in the adult with the pineal gland.
 
Miiller, in his often quoted memoir (No. 1 3), states that the
brains of Ganoids are peculiar and distinct from those both of
Teleostei and Elasmobranchii ; but in addition to pointing out
that the optic nerves form a chiasma he does not particularly
mention the features, to which he alludes in general terms. More
recently Wilder (No. 15) has returned to this subject; and
though, as we have already had occasion to point out, we cannot
accept all his identifications of the parts of the Ganoid brain, yet
he has called attention to certain characteristic features of the
cerebrum which have an undoubted systematic value.
 
The distinctive characters of the Ganoid brain are, in our
opinion, (i) the great elongation of the region of the thalamencephalon ; and (2) the unpaired condition of the posterior part
of the cerebrum, and the presence of so thin a roof to the
ventricle of this part as to cause it to appear open above.
 
The immense length of the region of the thalamencephalon
is a feature in the Ganoid brain which must at once strike any
one who examines figures of the brains of Chondrostei, Polypterus,
or Amia. It is less striking in the adult Lepidosteus, though here
also we have shewn that the thalamencephalon is really very
greatly developed ; but in the larva of Lepidosteus this feature is
still better marked, so that the brain of the larva may be described
as being more characteristically Ganoid than that of the adult.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 769
 
The presence of a largely developed thalamencephalon at
once distinguishes a Ganoid brain from that of a Teleostean
Fish, in which the optic thalami are very much reduced ; but
Lepidosteus shews its Teleostean affinities by a commencing
reduction of this part of the brain.
 
The large size of the thalamencephalon is also characteristic of
the Ganoid brain in comparison with the brain of the Dipnoi ;
but is not however so very much more marked in the Ganoids
than it is in some Elasmobranchii.
 
On the whole, we may consider the retention of a large
thalamencephalon as a primitive character.
 
The second feature which we have given as characteristic
of the Ganoid brain is essentially that which has been insisted
upon by Wilder, though somewhat differently expressed by
him.
 
The simplest condition of the cerebrum is that found in the
larva of Lepidosteus, where there is an anterior pair of lobes, and
an undivided posterior portion with a simple prolongation of the
third ventricle, and a very thin roof. The dorsal edges of the
posterior portion, adjoining the thin roof, usually become somewhat everted (cf. Wilder), and in Lepidosteus these edges have in
the adult a very great development, and form (vide Plate 38, fig.
47 A C, ce.) two prominent lobes, which we have spoken of as
the posterior cerebral lobes.
 
These characters of the cerebrum are perhaps even more
distinctive than those of the thalamencephalon.
 
In Teleostei the cerebrum appears to be completely divided
into two hemispheres, which are, however, all but solid, the lateral
ventricles being only prolonged into their bases. In Dipnoi
again there is either (Protopterus, Wiedersheim 1 ) a completely
separated pair of oval hemispheres, not unlike those of the lower
Amphibia, or the oval hemispheres are not completely separated
from each other (Ceratodus, Huxley 2 , Lepidosiren, Hyrtl 3 ) ; in
either case the hemispheres are traversed for the whole length by
lateral ventricles which are either completely or nearly completely
separated from each other.
 
1 Alorphol. Studicn, in. Jena, 1880.
 
2 "On Ceratodus Forsteri," Proc. Zool. Soc. 1876.
 
3 Lepidosiren paradqxa. Prag. 1845.
 
 
 
770 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
In Elasmobranchii the cerebrum is an unpaired though
bilobed body, but traversed by two completely separated lateral
ventricles, and without a trace of the peculiar membranous roof
found in Ganoids.
 
Not less interesting than the distinguishing characters of the
Ganoid brain are those cerebral characters which indicate affinities
between Lepidostens and other groups. The most striking of
these are, as might have been anticipated, in the direction of the
Teleostei.
 
Although the foremost division of the brain is very dissimilar
in the two groups, yet the hind-brain in many Ganoids and the
mid-brain also in Lepidosteus approaches closely to the Teleostean
type. The most essential feature of the cerebellum in Teleostei
is its prolongation forwards into the ventricles of the optic
vesicles as the valvula cerebelli. We have already seen that
there is a homologous part of the cerebellum in Lepidosteus ;
Stannius also describes this part in the Sturgeon, but no such
part is represented in M tiller's figure of the brain of Polypterus,
or described by him in the text.
 
The cerebellum is in most Ganoids relatively smaller, and
this is even the case with Amia; but the cerebellum of Lepidosteus
is hardly less bulky than that of most Teleostei.
 
The presence of tori semicirculares on the floor of the midbrain of Lepidosteus again undoubtedly indicates its affinities with
the Teleostei, and such processes are stated by Stannius to be
absent in the Sturgeon, and have not, so far as we are aware,
been described in other Ganoids. Lastly we may point to the
presence of well-developed lobi inferiores in the brain of Lepidosteus as an undoubted Teleostean character.
 
On the whole, the brain of Lepidosteus, though preserving its
true Ganoid characters, approaches more closely to the brain
of the Teleostei than that of any other Ganoid, including even
A mia.
 
It is not easy to point elsewhere to such marked resemblances
of the Ganoid brain, as to the brain of the Teleostei.
 
The division of the cerebrum into anterior and posterior
lobes, which is found in Lepidosteus, probably reappears again,
as already indicated, in the higher Amphibia. The presence of
the peculiar vesicle attached to the roof of the thalamencephalon
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 7/1
 
has its parallel in the brain of Protopterus, and as pointing in
the same direction a general similarity in the appearance of the
brain of Polypterus to that of the Dipnoi may be mentioned.
 
There appears to us to be in no points a close resemblance
between the brain of Ganoids and that of Elasmobranchii.
 
 
 
SENSE ORGANS.
 
Olfactory organ.
 
Development. The nasal sacks first arise during the late embryonic period in the form of a pair of thickened patches of the
nervous layer of the epiblast on the dorsal surface of the front
end of the head (Plate 37, fig. 39, ol.). The patches very soon
become partially invaginated ; and a small cavity is developed
between them and the epidermic layer of the epiblast (Plate 37,
figs. 42 and 43, ol.}. Subsequently, the roof of this space, formed
by the epidermic layer of the epiblast, is either broken through
or absorbed ; and thus open pits, lined entirely by the nervous
layer of the epidermis, are formed.
 
We are not acquainted with any description of an exactly
similar mode of origin of the olfactory pits, though the process
is almost identical with that of the other sense organs.
 
We have not worked out in detail the mode of formation of
the double openings of the olfactory pits, but there can be but
little doubt that it is caused by the division of the single opening into two.
 
The olfactory nerve is formed very early (Plate 37, fig. 39, I),
and, as Marshall has found in Aves and Elasmobranchii, it
arises at a stage prior to the first differentiation of an olfactory
bulb as a special lobe of the brain.
 
The Eye.
 
Anatomy. We have not made a careful histological examination of the eye of Lepidosteus, which in our specimens was not
sufficiently well preserved for such a purpose ; but we have
 
 
 
772 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
found a vascular membrane enveloping the vitreous humour on
its retinal aspect, which, so far as we know, is unlike anything
which has so far been met with in the eye of any other adult
Vertebrate.
 
The membrane itself is placed immediately outside the hyaloid membrane, i.e. on the side of the hyaloid membrane bounding the vitreous humour. It is easily removed from the retina,
to which it is only adherent at the entrance of the optic nerve.
In both the eyes we examined it also adhered, at one point, to
the capsule of the lens, but we could not make out whether this
adhesion was natural, or artificially produced by the coagulation
of a thin layer of albuminous matter. In one instance, at any
rate, the adhesion appeared firmer than could easily be produced
artificially.
 
The arrangement of the vessels in the membrane is shewn
diagrammatically in Plate 38, fig. 49, while the characteristic
form of the capillary plexus is represented in Plate 38, fig. 50.
 
The arterial supply appears to be derived from a vessel perforating the retina close to the optic nerve, and obviously homologous with the artery of the processus falciformis and pecten
of Teleostei and Birds, and with the arteria centralis retinae of
Mammals. From this vessel branches diverge and pursue a
course towards the periphery. They give off numerous branches,
the blood from which enters a capillary plexus (Plate 38, figs.
49 and 50) and is collected again by veins, which pass outwards
and finally bend over and fall into (Plate 38, fig. 49) a circular
vein (cr. z>.) placed at the outer edge of the retina along the
insertion of the iris (ir). The terminal branches of some of the
main arteries appear also to fall directly into this vein.
 
The membrane supporting the vessels just described is composed of a transparent matrix, in which numerous cells are
embedded (Plate 38, fig. 50).
 
Development. In the account of the first stages of development of LepidosteuS) the mode of formation of the optic cup, the
lens, &c., have been described (vide Plates 35 and 36, figs. 23,
26, 35). With reference to the later stages in the development
of the eye, the only subject with which we propose to deal is the
growth of the mesoblastic processes which enter the cavity of
the vitreous humour through the choroid slit.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 773
 
Lepidosteus is very remarkable for the great number of mesoblast cells which thus enter the cavity of the vitreous humour,
and for the fact that these cells are at first unaccompanied by any
vascular structures (Plate 37, fig. 43, v.h). The mesoblast cells
are scattered through the vitreous humour, and there can be no
doubt that during early larval life, at a period however when
the larva is certainly able to see, every histologist would consider the vitreous humour to be a tissue formed of scattered
cells, with a large amount of intercellular substance ; and the
fact that it is so appears to us to demonstrate that Kessler's
view of the vitreous humour being a mere transudation is not
tenable.
 
In the larva five or six days after hatching, and about
15 millims. in length, the choroid slit is open for its whole
length. The edges of the slit near the lens are folded, so as to
form a ridge projecting into the cavity of the vitreous humour,
while nearer the insertion of the optic nerve they cease to exhibit any such structure. The mesoblast, though it projects
between the lips of the ridge near the lens, only extends through
the choroid slit into the cavity of the vitreous humour in -the
neighbourhood of the optic nerve. Here it forms a lamina with
a thickened edge, from which scattered cells in the cavity of the
vitreous humour seem to radiate.
 
At a slightly later stage than that just described, bloodvessels become developed within the cavity of the vitreous
humour, and form the vascular membrane already described in
the adult, placed close to the layer of nerve-fibres of the retina,
but separated from this layer by the hyaloid membrane (Plate
38, fig. 48, v.s/1.). The artery bringing the blood to the above
vascular membrane is bound up in the same sheath as the optic
nerve, and passes through the choroid slit very close to the optic
nerve. Its entrance into the cavity of the vitreous humour is
shewn in Plate 38, fig. 48 (vs.); its relation to the optic nerve in
Plate 37, fig. 46, C and D (vs.).
 
The above sheath has, so far as we know, its nearest analogue
in the eye of Alytes, where, however, it is only found in the
larva.
 
The reader who will take the trouble to refer to the account
of the imperfectly-developed processus falcifprmis of the Elas
 
 
774 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
mobranch eye in the treatise On Comparative Embryology, by
one of us 1 , will not fail to recognize that the folds of the retina
at the sides of the choroid slit, and the mesoblastic process
passing through this slit, are strikingly similar in Lepidosteus
and Elasmobranchii ; and that, if we are justified in holding
them to be an imperfectly-developed processus falciformis in the
one case, we are equally so in the other.
 
Johannes Miiller mentions the absence of a processus falciformis as one of the features distinguishing Ganoids and Teleostei. So far as the systematic separation of the two groups
is concerned, he is probably perfectly justified in this course ;
but it is interesting to notice that both in Ganoids and Elasmobranchii we have traces of a structure which undergoes a very
special development in the Teleostei, and that the processus
falciformis of Teleostei is therefore to be regarded, not as an
organ peculiar to them, but as the peculiar modification within
the group of a primitive Vertebrate organ.
 
SUCTORIAL Disc.
 
One of the most remarkable organs of the larval Lepidosteus
is the suctorial disc, placed at the front end of the head, to
which we have made numerous allusions in the first section of
this memoir.
 
The external features of the disc have been fully dealt with
by Agassiz, and he also explained its function by observations
on the habits of the larva. We have already quoted (p. 755)
a passage from Agassiz' memoir shewing how the young Fishes
use the disc to attach themselves firmly to any convenient
object. The discs appear in fact to be highly efficient organs of
attachment, in that the young Fish can remain suspended by
them to the sides of the jar, even after the water has been
lowered below the level at which they are attached.
 
The disc is formed two or three days before hatching, and
from Agassiz' statements, it appears to come into use immediately the young Fish is liberated from the egg membranes.
 
We have examined the histological structure of the disc at
various ages of its growth, and may refer the reader to Plate 34,
 
1 Vol. II. p. 414 [the original edition].
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 775
 
figs. 1 1 and 13, and Plate 37, figs. 40 and 44. The result of our
examination has been to shew that the disc is provided with
a series of papillae often exhibiting a bilateral arrangement.
The papillae are mainly constituted of highly modified cells of
the mucous layer of the epidermis. These cells have the form
of elongated columns, the nucleus being placed at the base, and
the main mass of the cells being filled with a protoplasmic reticulum. They may' probably be regarded as modified mucous
cells. In the mesoblast adjoining the suctorial disc there are
numerous sinus-like vascular channels.
 
It does not appear probable that the disc has a true sucking
action. It is unprovided with muscular elements, and there
appears to be no mechanism by which it could act as a sucking
organ. We must suppose, therefore, that its adhesive power
depends upon the capacity of the cells composing its papillae to
pour out a sticky secretion.
 
 
 
MUSCULAR SYSTEM.
 
There is a peculiarity in the muscular system of Lepidosteus,
which so far as we know has not been previously noticed. It is
that the lateral muscles of each side are not divided, either in
the region of the trunk or of the tail, into a dorso-lateral and
ventro-lateral division.
 
This peculiarity is equally characteristic of the older larvae
as of the adult, and is shewn in Plate 41, figs. 67, 72, and 73,
and Plate 42, figs. 74 76. In the Cyclostomata the lateral
muscles are not divided into dorsal and ventral sections ; but
except in this group such a division has been hitherto considered
as invariable amongst Fishes.
 
This character must, without doubt, be held to be the indication of a very primitive arrangement of the muscular system.
In the embryos of all Fishes with the usual type of the lateral
muscles, the undivided condition of the muscles precedes the
divided condition ; and in primitive forms such as the Cyclostomata and Amphioxus the embryonic condition is retained, as it
is in Lepidosteus.
 
 
 
776 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
SKELETON.
PART I. Vertebral column and ribs of the adult.
 
A typical vertebra from the trunk of Lepidosteus has the
following characters (Plate 42, figs. 80 and 81).
 
The centrum is slightly narrower in the middle than at its
two extremities. It articulates with adjacent vertebrae by a
convex face in front and a concave face behind, being thus,
according to Owen's nomenclature, opisthoccelous. It presents
on its under surface a well-marked longitudinal ridge, which in
many vertebrae is only united at its two extremities with the
main body of the vertebra.
 
From the lateral borders of the centrum there project, at a
point slightly nearer the front than the hind end, a pair of prominent haemal processes (h.a.} } to the ends of which are articulated the ribs. These processes have a nearly horizontal direction in the greater part of the trunk, though bent downwards in
the tail.
 
The neural arches (n.a.) have a somewhat complicated form.
They are mainly composed of two vertical plates, the breadth
of the basal parts of which is nearly as great as the length of
the vertebrae, so that comparatively narrow spaces are left between the neural arches of successive vertebrae for the passage
of the spinal nerves. Some little way from its dorsal extremity
each neural arch sends a horizontal process inwards, which meets
its fellow and so forms a roof for the spinal canal. These processes appear to be confined to the posterior parts of the vertebrae, so that at the front ends of the vertebrae, and in the
spaces between them, the neural canal is without an osseous
roof. Above the level of this osseous roof there is a narrow
passage, bounded laterally by the dorsal extremities of the
neural plates. This passage is mainly filled up by a series of
cartilaginous elements (Plate 42, figs. 80 and 81, t.c.) (probably
fibro-cartilage), which rest upon the roof of the neural canal.
Each element is situated intervertebrally, its anterior end being
wedged in between the two dorsal processes of the neural arch
of the vertebra in front, and its posterior end extending for some
 
 
 
STRUCTURE AND DEVELOPMENT OF LEFIDOSTEUS. 777
 
 
 
distance over the vertebra behind. The successive elements are
connected by fibrous tissue, and are continuous dorsally with
a fibrous band, known as the ligamentum longitudinale superius
(Plate 42, figs. 80 and 81, /./.), characteristic of Fishes generally,
and running continuously for the whole length of the vertebral
column. Each of the cartilaginous elements is, as will be afterwards shewn, developed as two independent pieces of cartilage,
and might be compared with the dorsal element which usually
forms the keystone of the neural arch in Elasmobranchs, were
not the latter vertebral instead of intervertebral in position.
More or less similar elements are described by Gotte in the
neural arches of many Teleostei, which also, however, appear to
be vertebral ly placed, and he has compared them and the corresponding elements in the Sturgeon with the Elasmobranch
cartilages forming the keystone of the neural arch. Gotte does
not, however, appear to have distinguished between the cartilaginous elements, and the osseous elements forming the roof of
the spinal canal, which are true membrane bones ; it is probable
that the two are not so clearly separated in other types as in
Lepidosteus.
 
The posterior ends of the neural plates of the neural arches
are continued into the dorsal processes directed obliquely upwards and backwards, which have been somewhat unfortunately
described by Stannius as rib-like projections of the neural arch.
The dorsal processes of the two sides do not meet, but between
them is placed a median free spinous element, also directed
obliquely upwards and backwards, which forms a kind of roof
for the groove in which the cartilaginous elements and the ligamentum longitudinale are placed.
 
The vertebrae are wholly formed of a very cellular osseous
tissue, in which a distinction between the bases of the neural
and haemal processes and the remainder of the vertebra is not
recognizable. The bodies of the vertebras are, moreover, directly
continuous with the neural and haemal arches.
 
The ribs in the region of the trunk are articulated to the
ends of the long haemal processes. They envelop the bodycavity, their proximal parts being placed immediately outside
the peritoneal membrane, along the bases of the intermuscular
septa. Their distal ends do not, however, remain close to the
B. 50
 
 
 
778 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
peritoneal membrane, but pass outwards along the intermuscular
septa till their free ends come into very close proximity with the
skin. This peculiarity, which holds good in the adult for all the
free ribs, is shewn in one of the anterior ribs of an advanced
larva in Plate 41, fig. 72 (rb.}. We are not aware that this has
been previously noticed, but it appears to us to be a point not
without interest in all questions which concern the homology of
rib-like structures occupying different positions in relation to the
muscles. Its bearings are fully dealt with in the section of this
paper devoted to the consideration of the homologies of the ribs
in Fishes.
 
As regards the behaviour of the ribs in the transitional region
between the trunk and the tail, we cannot do better than translate the description given by Gegenbaur of this region (No. 6,
p. 411): "Up to the 34th vertebra the ribs borne by the laterally and posteriorly directed processes present nothing remarkable, though they have gradually become shorter. The ribs of
the 35th vertebra exhibit a slight curvature outwards of their
free ends, a peculiarity still more marked in the 36th. The last
named pair of ribs converge somewhat in their descent backwards so that both ribs decidedly approach before bending outwards. The 37th vertebra is no longer provided with freely
terminating ribs, but on the contrary, the same pair of processes
which in front was provided with ribs, bears a short forked
process as the haemal arch. The two, up to this point separated
ribs, have here formed a haemal arch by the fusion of their lower
ends, which arch is movable just like the ribs, and, like them,
is attached to the vertebral column'' ' \ !
 
In the region of the tail-fin the haemal arches supporting the
caudal fin-rays are very much enlarged.
 
 
 
PART II. Development of the vertebral column and ribs.
 
The first development and early histological changes of the
notochord have already been given, and we may take up the
history of the vertebral column at a period when the notochord
forms a large circular rod, whose cells are already highly vacuolated, while the septa between the vacuoles form a delicate
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 779
 
wide-meshed reticulum. Surrounding the notochord is the
usual cuticular sheath, which is still thin.
 
The first indications of the future vertebral column are to be
found in the formation of a distinct mesoblastic investment of
the notochord. On the dorsal aspect of the notochord, the
mesoblast forms two ridges, one on each side, which are prolonged upwards so as to meet above the neural canal, for which
they form a kind of sheath. On the ventral side of the notochord there are also two ridges, which are, however, except on
the tail, much less prominent than the dorsal ridges.
 
The changes which next ensue are practically identical with
those which take place in Teleostei. Around the cuticular
sheath of the notochord there is formed an elastic membrane
the membrana elastica externa. At the same time the basal
parts of the dorsal, or as we may perhaps more conveniently call
them, the neural ridges of the notochord become enlarged at
each intermuscular septum, and the tissue of these enlargements
soon becomes converted into cartilage, thus forming a series of
independent paired neural processes riding on the membrana
elastica externa surrounding the notochord, and extending about
two-thirds of the way up the sides of the medullary cord. They
are shewn in transverse section in Plate 41, fig. 67 (n.a.), and in
a side view in fig. 68 (n.a.}.
 
Simultaneously with the neural arches, the haemal arches
also become established, and arise by the formation of similar
enlargements of the ventral or haemal ridges. In the trunk they
are very small, but in the region of the tail their condition is
very different. At the front end of the anal fin the paired
haemal arches suddenly enlarge and extend ventralwards (Plate
41, fig. 67, h.a.}.
 
Each succeeding pair of arches becomes larger than the one
in front, and the two elements of each arch first nearly meet
below the caudal vein (Plate 41, fig. 67) and finally actually do
so, forming in this way a completely closed haemal canal. At
the point where they first meet the permanent caudal fin commences, and here (Plate 41, fig. 68) we find that not only do the.
haemal arches meet and coalesce below the caudal vein, but they
are actually produced into long spines supporting the fin-rays of
the caudal fin, which thus differs from the other fins in being
 
502
 
 
 
780 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
supported by parts of the true vertebral column and not by
independently formed elements of the skeleton.
 
Each of the large caudal haemal arches, including the spine,
forms a continous whole, and arises at an earlier period of larval
life than any other part of the vertebral column. We noticed
the first indications of the neural arches in the larva of about a
week old, while they are converted into fully formed cartilage in
the larva of three weeks.
 
The neural and haemal arches, resting on th'e membrana
elastica externa, do not at this early stage in the least constrict
the notochord. They grow gradually more definite, till the larva
is five or six weeks old and about 26 millims. in length, but
otherwise for a long time undergo no important changes. During the same period, however, the true sheath of the notochord
greatly increases in thickness, and the membrana elastica externa becomes more definite. So far it would be impossible to
distinguish the development of the vertebral column of Lepidosteus from that of a Teleostean Fish.
 
Of the stages immediately following we have unfortunately
had no examples, but we have been fortunate enough to obtain
some young specimens of Lepidosteus^, which have enabled us to
work out with tolerable completeness the remainder of the developmental history of the vertebral column. In the next oldest
larva, of about 5 '5 centims., the changes which have taken place
are already sufficient to differentiate the vertebral column of
Lepidosteus from that of a Teleostean, and to shew how certain
of the characteristic features of the adult take their origin.
 
In the notochord the most important and striking change
consists in the appearance of a series of very well marked vertebral constrictions opposite the insertions of the neural and hcemal
arches. The first constrictions of the notochord are thus, as in
other Fishes, vertebral; and although, owing to the growth of
the intervertebral cartilage, the vertebral constrictions are subsequently replaced by intervertebral constrictions, yet at the same
time the primitive occurrence of vertebral constrictions demonstrates that the vertebral column of Lepidosteus is a modification
of a type of vertebral column with biconcave vertebrae.
 
1 These specimens were given to us by Professor W. K. Parker, who received
them from Professor Burl G. Wilder.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
 
 
The structure of the gelatinous body of the notochord has
undergone no important change. The sheath, however, exhibits
certain features which deserve careful description. In the first
place the attention of the observer is at once struck by the fact
that, in the vertebral regions, the sheath is much thicker ('014
millim.) than in the intervertebral ('005 millim.), and a careful
examination of the sheath in longitudinal sections shews that
the thickening is due to the special differentiation of a superficial
part (Plate 41, fig. 69, s/i.~) of the sheath in each vertebral region.
This part is somewhat granular as compared to the remainder,
especially in longitudinal sections. It forms a cylinder (the walj
of which is about *oi millim. thick) in each vertebral region,
immediately within the membrana elastica externa. Between
it and the gelatinous tissue of the notochord within there is a
very thin unmodified portion of the sheath, which is continuous
with the thinner intervertebral parts of the sheath. This part of
the sheath is faintly, but at the same time distinctly, concentrically striated a probable indication of concentric fibres. The
inner unmodified layer of the sheath has the appearance in
transverse sections through the vertebral regions of an inner
membrane, and may perhaps be Kolliker's "membrana elastica
interna."
 
We are not aware that any similar modification of the sheath
has been described in other forms.
 
The whole sheath is still invested by a very distinct membrana elastica externa (m.e/).
 
The changes which have taken place in the parts which form
the permanent vertebrae will be best understood from Plate 41,
figs. 69 71. From the transverse section (fig. 70) it will be
seen that there are still neural and haemal arches resting upon
the membrana elastica externa ; but longitudinal sections (fig. 69)
shew that laterally these arches join a cartilaginous tube, embracing the intervertebral regions of the notochord, and continuous
from one vertebra to the next.
 
It will be convenient to treat separately the neural arches,
the haemal arches with their appendages, and the intervertebral
cartilaginous rings.
 
The neural arches, except in the fact of embracing a relatively
smaller part of the neural tube than in the earlier stage, do not
 
 
 
782 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
at first sight appear to have undergone any changes. Viewed
from the side, however, in dissected specimens, they are seen to
be prolonged upwards so as to unite above with bars of cartilage
directed obliquely backwards. An explanation of this appearance is easily found in the sections. The cartilaginous neural
arches are invested by a delicate layer of homogeneous bone,
developed in the perichondrium, and this bone is prolonged
beyond the cartilage and joins a similar osseous investment of
the dorsal bars above mentioned. The whole of these parts
may, it appears to us, be certainly reckoned as parts of the
neural arches, so that at this stage each neural arch consists of:
(i) a pair of basal portions resting on the notochord consisting
of cartilage invested by bone, (2) of a pair of dorsal cartilaginous
bars invested in bone (n.a'.}, and (3) of osseous bars connecting
(i) and (2).
 
Though, in the absence of the immediately preceding stages,
it is not perfectly certain that the dorsal pieces of cartilage are
developed independently of the ventral, there appears to us every
probability that this is so ; and thus the cartilage of each neural
arch is developed discontinuously, while the permanent bony
neural arch, which commences as a deposit of bone partly in the
perichondrium and partly in the intervening membrane, forms a
continuous structure.
 
Analogous occurrences have been described by Gotte in
Teleostei.
 
The dorsal portion of each neural arch becomes what we
have called the dorsal process of the adult arch.
 
Between the dorsal processes of the two sides there is placed
a median rod of cartilage (Plate 41, fig. 70, i. s.), which in its
development is wholly independent of the true neural arches,
and which constitutes the median spinous element of the adult.
In tracing these backwards it becomes obvious that they are
homologous with the interspinous elements supporting the dorsal
fin, in that they are replaced by these interspinous elements in
the region of the dorsal fin, and that the interspinous bones
occupy the same position as the median spinous processes.
This homology was first pointed out by Gotte in the case of the
Teleostei.
 
Immediately beneath this rod is placed the longitudinal
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 783
 
ligament (Plate 41, fig. 70, /./.), but there is as yet no trace of a
junction between the neural arches of the two sides in the space
between the longitudinal ligament and the spinal cord.
 
The basal parts of the neural arches of the two sides are
united dorsally by a thin cartilaginous layer resting on the
sheath of the notochord, but they are not united ventrally with
the haemal arches.
 
The haemal processes in the trunk are much more prominent
than in the preceding stage, and their bases are united ventrally
by a tolerably thick layer of cartilage. In the trunk they are
continuous with the so-called ribs of the adult (Plate 41, fig. 70) ;
but in order to study the nature of these ribs it is necessary to
trace the modifications undergone by the haemal arches in passing from the tail to the trunk.
 
It will -be remembered that at an earlier stage the haemal
arches in the region of the tail-fin were fully formed, and that
through the anterior part of the caudal region the haemal processes were far advanced in development, and just in front of
the caudal fin had actually met below the caudal vein.
 
The mode of development of the haemal arches in the tail as
unjointed cartilaginous bars investing the caudal arteries and
veins is so similar to that of the caudal haemal arches of
Elasmobranchii, that it appears to us impossible to doubt their
identity in the two groups 1 .
 
The changes which have taken place by this stage with
reference to the haemal arches of the tail are not very considerable.
 
In the case of a few more vertebrae the haemal processes
 
1 Gegenbaur (No. 6) takes a different view on this subject, as is clear from the
following passage in this memoir (pp. 369 370) :" Each vertebra of Lepidosteus
thus consists of a section of the notochord, and of the cartilaginous tissue surrounding
its sheath, which gives origin to the upper arches for the whole length of the vertebral
column, and in the caudal region to that of the lower arches also. The latter do not
however complete the enclosure of a lower canal, but this is effected by special independent
elements, which are to be interpreted as homologues of the ribs." (The italics are
ours.) While we fully accept the homology between the ribs and the lower elements
of the kemal arches of the tail, the view expressed in the italicised section, to the
effect that the lower parts of the caudal arches are not true haemal arches but are
independently formed elements, is entirely opposed to our observations, and has we
believe only arisen from the fact that Gegenbaur had not the young larvae to work
with by which alone this question could be settled.
 
 
 
784 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
have united into an arch, and the spinous processes of the arches
in the region of the caudal fin have grown considerably in
length. A more important change is perhaps the commencement of a segmentation of the distal parts of the haemal arches
from the proximal. This process has not, however, as yet resulted in a complete separation of the two, such as we find in
the adult.
 
If the haemal processes are traced forwards (Plate 42, figs.
75 and 76) from the anterior segment where they meet ventrally,
it will be found that each haemal process consists of a basal
portion, adjoining the notochord, and a peripheral portion.
These two parts are completely continuous, but the line of a
future separation is indicated by the structure of the cartilage,
though not shewn in our figures. As the true body-cavity of
the trunk replaces the obliterated body-cavity of 'the caudal
region, no break of continuity will be found in the structure of
the haemal processes (Plates 41 and 42, figs. 73 and 74), but
while the basal portions grow somewhat larger, the peripheral
portions gradually elongate and take the form of delicate rods
of cartilage extending ventralwards, on each side of the bodycavity, immediately outside the peritoneal membrane, and along
the lines of insertion of the intermuscular septa. These rods
obviously become the ribs of the adult.
 
As one travels forwards the ribs become continually longer
and more important, and though they are at this stage united'
with the haemal processes in every part of the trunk, yet they
are much more completely separated from these processes in
front than behind (Plate 41, fig. 72).
 
In front (Plate 41, fig. 72), each rib (rb.} t after continuing its
ventral course for some distance, immediately outside the peritoneal membrane, turns outwards, and passes along one of the
intermuscular septa till it reaches the epidermis. This feature
in the position of the ribs is, as has been already pointed out in
the anatomical part of this section, characteristic of all the ribs
of the adult.
 
It is unfortunate that we have had no specimens shewing the
ribs at an earlier stage of development ; but it appears hardly
open to doubt that iJie ribs are originally continuous with tlie
hcenial processes, and that the indications of a separation between
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 785
 
those two parts at this stage are not due to a secondary fusion,
but to a commencing segmentation.
 
It further appears, as Miiller, Gegenbaur and others have
stated, that the ribs and haemal processes of the tail are serially
homologous structures ; but that the view maintained by Gotte
in his very valuable memoirs on the Vertebrate skeleton is also
correct to the effect that the h&mal arches of the tail are homologous throughout the series of Fishes.
 
To this subject we shall return again at the end of the
section.
 
Before leaving the haemal arches it may be mentioned that
behind the region of the ventral caudal fin the two haemal processes merge into one, and form an unpaired knob resting
on the ventral side of the notochord, and not perforated by
a canal.
 
There are now present well -developed intervertebral rings of
cartilage, each of which eventually becomes divided into two
parts, and converted into the adjacent faces of the contiguous
vertebrae. These rings are united with the neural and haemal
arches of the vertebrae in front and behind.
 
Each ring, as shewn by the transverse section (Plate 41, fig.
71), is not uniformly thick, but exhibits four projections, two
dorsal and two ventral. These four projections are continuous
with the bases of the neural and haemal arches of the adjacent
vertebrae, and afford presumptive evidence of the derivation of
the intervertebral rings from the neural and haemal arches; in
that had they so originated, it would be natural to anticipate
the presence of four thickenings indicating the four points from
which the cartilage had spread, while if the rings had originated
independently, it would not be easy to give any explanation of
the presence of such thickenings. Gegenbaur (No. 6), from the
investigation of a much older larva than that we are now describing, also arrived at the conclusion that the intervertebral cartilages were derived from the neural and haemal arches ; but as
doubts have been thrown upon this conclusion by Gotte, and
as it obviously required further confirmation, we have considered
it important to attempt to settle this point. From the description
given above, it is clear that we have not, however, been able
absolutely to trace the origin of this cartilage, but at the same
 
 
 
786 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
time we think that we have adduced weighty evidence in corroboration of Gegenbaur's view.
 
As shewn in longitudinal section (Plate 41, fig. 69, iv.r.}, the
intervertebral rings are thicker in the middle than at the two
ends. In this thickened middle part the division of the cartilage
into two parts to form the ends of two contiguous vertebrae is
subsequently effected. The curved line which this segmentation
will follow is, however, already marked out, and from surface
views it might be supposed that this division had actually
occurred.
 
The histological structure of the intervertebral cartilage is
very distinct from that of the cartilage of the bases of the
arches, the nuclei being much more closely packed. In parts,
indeed, the intervertebral cartilage has almost the character of
-fibre-cartilage. On each side of the line of division separating
two vertebrae it is invested by a superficial osseous deposit.
 
The next oldest larva we have had was 1 1 centims. in length.
The filamentous dorsal lobe of the caudal fin still projected far
beyond the permanent caudal fin (Plate 34; fig. 16).
 
The vertebral column was considerably less advanced in development than that dissected by Gegenbaur, though it shews a
great advance on the previous stage. Its features are illustrated
by two transverse sections, one through the median plane of a
vertebral region (Plate 42, fig. 78) and the other through that of
an intervertebral region (Plate 42, fig. 79), and by a horizontal
section (Plate 42, fig. 77).
 
In the last stage the notochord was only constricted vertebrally. Now, however, by the great growth of intervertebral
cartilage there have appeared (Plate 42, fig. 77) very wellmarked intervertebral constrictions, by the completion of which the
vertebrae of Lcpidosteus acquire their unique character amongst
Fishes.
 
These constrictions still, however, coexist with the earlier,
though at this stage relatively less conspicuous, vertebral constrictions.
 
The gelatinous body of the notochord retains its earlier
condition. The sheath has, however, undergone some changes.
In the vertebral regions there is present in any section of the
sheath (i) externally, the membrana elastica externa
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 787
 
then (2) the external layer of the sheath (sh.), which is, however,
less thick than before, and exhibits a very faint form of radial
striation ; and (3) internally, a fairly thick and concentrically
striated layer. The whole thickness is, on an average, O'l8
millim.
 
In the intervertebral regions the membrana elastica externa
is still present in most parts, but has become absorbed at the
posterior border of each vertebra, as shewn in longitudinal section
in Plate 42, fig. 77. It is considerably puckered transversely.
The sheath of the notochord within the membrana elastica
externa is formed of a concentrically striated layer, continuous
with the innermost layer of the sheath in the vertebral regions.
It is puckered longitudinally. Thus, curiously enough, the
membrana elastica externa and the sheath of the notochord
in the intervertebral regions are folded in different directions,
the folds of the one being only visible in transverse sections
(Plate 42, fig. 79), and those of the other in longitudinal sections
(Plate 42, fig. 77).
 
The osseous and cartilaginous structures investing the notochord may conveniently be dealt with in the same order as
before, viz. : the neural arches, the haemal arches, and the
intervertebral cartilages.
 
The cartilaginous portions of the neural arches are still
unossified, and form (Plate 42, fig. 78, n.a.) small wedge-shaped
masses resting on the sheath of the notochord. They are invested by a thick layer of bone prolonged upwards to meet
the dorsal processes (n.a'.}, which are still formed of cartilage
invested by bone.
 
It will be remembered that in the last stage there was no
key-stone closing in the neural arch above. This deficiency is
now however supplied, and consists of (i) two bars of cartilage
repeated for each vertebra, but intervertebral ly placed, which are
directly differentiated from the ligamentum longitudinale superius, into which they merge above ; and (2) two osseous plates
placed on the outer sides of these cartilages, which are continuous
with the lateral osseous bars of the neural arch. The former
of these elements gives rise to the cartilaginous elements above
the osseous bridge of the neural arch in the adult. The two
osseous plates supporting these cartilages clearly form what we
 
 
 
788 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
have called in our description of the adult the osseous roof of
the spinal canal.
 
A comparison of the neural arch at this stage with the arch
in the adult, and in the stage last described, shews that the
greater part of the neural arch of the adult is formed of membrane-bone, there being preformed in cartilage only a small basal
part, a dorsal process, and paired key-stones below the ligamentum longitudinale superius.
 
The haemal arches (Plate 42, fig. 78) are still largely cartilaginous, and rest upon the sheath of the notochord. They are
invested by a thick layer of bone. The bony layer investing
the neural and haemal arches is prolonged to form a continuous
investment round the vertebral portions of the notochord (Plate
42, fig. 78). This investment is at the sides prolonged outwards
into irregular processes (Plate 42, fig. 78), which form the commencement of the outer part of the thick but cellular osseous
cylinder forming the middle part of the vertebral body.
 
The intervertebral cartilages are much larger than in the
earlier stage (Plate 42, figs. 77 and 79), and it is by their growth
that the intervertebral constrictions of the notochord are produced. They have ceased to be continuous with the cartilage
of the arches, the intervening portion of the vertebral body
between the two being only formed of bone. They are not yet
divided into two masses to form the contiguous ends of adjacent
vertebrae.
 
Externally, the part of each cartilage which will form the
hinder end of a vertebral body is covered by a tube of bone,
having the form of a truncated funnel, shewn in longitudinal
section in Plate 42, fig. 77, and in transverse section in Plate 42,
 
fig- 79
At each end, the intervertebral cartilages are becoming
penetrated and replaced by beautiful branched processes from
the homogeneous bone which was first of all formed in the perichondrium (Plate 42, fig. 77).
 
This constitutes the latest stage which we have had.
 
Gegenbaur (No. 6) has described the vertebral column in
a somewhat older larva of 18 centims.
 
The chief points in which the vertebral column of this larva
differed from ours are : (i) the disappearance of all trace of the
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 789
 
 
 
primitive vertebral constriction of the notochord ; (2) the nearly
completed constriction of the notochord in the intervertebral
regions ; (3) the complete ossification of the vertebral portions
of the bodies of the vertebrae, the terminal so-called intervertebral
portions alone remaining cartilaginous ; (4) the complete ossification of the basal portions of the haemal and neural processes
included within the bodies of the vertebrae, so that in the case
of the neural arch all trace of the fact that the greater part
was originally not formed in cartilage had become lost. The
cartilage of the dorsal spinous processes was, however, still
persistent.
 
The only points which remain obscure in the later history
of the vertebral column are the history of the notochord and of
its sheath. We do not know how far these are either simply
absorbed or partially or wholly ossified.
 
Gotte in his memoir on the formation of the vertebral bodies
of the Teleostei attempts to prove (i) that the so-called membrana elastica externa of the Teleostei is not a homogeneous
elastica, but is formed of cells, and (2) that in the vertebral regions
ossification first occurs in it.
 
In Lepidosteus we have met with no indication that the membrana elastica externa is composed of cells ; though it is fair to
Gotte to state that we have not examined such isolated portions
of it as he states are necessary in order to make out its structure.
But further than this we have satisfied ourselves that during
the earlier stage of ossification this membrane is not ossified,
and indeed in part becomes absorbed in proximity to the intervertebral cartilages ; and Gegenbaur met with no ossification of
this membrane in the later stage described by him.
 
 
 
Summary of the development of the vertebral column and ribs,
 
A mesoblastic investment is early formed round the notochord, which is produced into two dorsal and two ventral ridges,
the former uniting above the neural canal. Around the cuticular
sheath of the notochord an elastic membrane, the membrana
elastica externa, is next developed. The neural ridges become
enlarged at each inter-muscular septum, and these enlargements
 
 
 
7QO STRUCTURE 'AND DEVELOPMENT OF LEPIDOSTEUS.
 
soon become converted into cartilage, thus forming a series of
neural processes riding on the membrana elastica externa, and
extending about two-thirds of the way up the sides of the neural
canal. The haemal processes arise simultaneously with, and in
the same manner as, the neural. They are small in the trunk,
but at the front end of the anal fin they suddenly enlarge and
extend ventralwards. Each succeeding pair of hsemal arches
becomes larger than the one in front, each arch finally meeting its
fellow below the caudal vein, thus forming a completely closed
haemal canal. These arches are moreover produced into long
spines supporting the fin-rays of the caudal fin, which thus
differs from the other unpaired fins in being supported by parts
of the vertebral column, and not by separately formed skeletal
elements.
 
In the next stage which we have had the opportunity of studying (larva of 5^ centims.), a series of very well-marked vertebral
constrictions are to be seen in the notochord. The sheath is now
much thicker in the vertebral than in the intervertebral regions :
this is due to a special differentiation of a superficial part of
the sheath, which appears more granular than the remainder.
This granular part of the sheath thus forms a cylinder in each
vertebral region. Between it and the gelatinous tissue of the
notochord there remains a thin unmodified portion of the sheath,
which is continuous with the intervertebral parts of the sheath.
The neural and haemal arches are seen to be continuous with a
cartilaginous tube embracing the intervertebral regions of the
notochord, and continuous from one vertebra to the next. A
delicate layer of bone, developed in the perichondrium, invests
the cartilaginous neural arches, and this bone grows upwards
so as to unite above with the osseous investment of separately
developed bars of cartilage, which are directed obliquely backwards. These bars, or dorsal processes, may be reckoned as
parts of the neural arches. Between the dorsal processes of the
two sides is placed a median rod of cartilage, which is developed
separately from the true neural arches, and which constitutes
the median spinous element of the adult. Immediately below
this rod is placed the ligamentum longitudinale superius. There
is now a commencement of separation between the dorsal and
ventral parts of the haemal arches, not only in the tail, but also
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 79 1
 
in the trunk, where they pass ventralwards on each side of the
body-cavity, immediately outside the peritoneal membrane, along
the lines of insertion of the intermuscular septa. These are
obviously the ribs of the adult, and there is no break of continuity of structure between the haemal processes of the tail and
the ribs. In the anterior part of the trunk the ribs pass outwards along the intermuscular septa till they reach the epidermis.
Thus the ribs are originally continuous with the haemal processes. Behind the region of the ventral caudal fin the two
haemal processes merge into one, which is not perforated by
a canal.
 
Each of the intervertebral rings of cartilage becomes eventually
divided into two parts, and converted into the adjacent faces of
contiguous vertebrae, the curved line where this will be effected
being plainly marked out. These rings are united with the
neural and haemal arches of the vertebrae next in front and
behind. As these rings are formed originally by the spreading
of the cartilage from the primitive neural and haemal processes,
the intervertebral cartilages are clearly derived from the neural
and haemal arches. The intervertebral cartilages are thicker in
the middle than at their two ends.
 
In our latest stage (11 centims.), the vertebral constrictions
of the notochord are rendered much less conspicuous by the
growth of the intervertebral cartilages giving rise to marked
intervertebral constrictions. In the intervertebral regions the
membrana elastica externa has become aborted at the posterior
border of each vertebra, and the remaining part is considerably
puckered transversely. The inner sheath of the notochord is
puckered longitudinally in the intervertebral regions. The
granular external layer of the sheath in the vertebral regions is
less thick than in the last stage, and exhibits faint radial
striations.
 
Two closely approximated cartilaginous elements now form
a key-stone to the neural arch above : these are directly differentiated from the ligamentum longitudinale superius, into which
they merge above. An osseous plate is formed on the outer side
of each of these cartilages. These plates are continuous with
the lateral osseous bars of the neural arches, and also give rise
to the osseous roof of the spinal canal of the adult.
 
 
 
792 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
Thus the greater part of the neural arches is formed of membrane bone. The haemal arches are invested by a thick layer of
bone, and there is also a continuous osseous investment round
the vertebral portions of the notochord. The intervertebral
cartilages become penetrated by branched processes of bone.
 
 
 
Comparison of the vertebral column of Lepidosteus with that of
 
other forms.
 
The peculiar form of the articulatory faces of the vertebrae of
Lepidosteus caused L. Agassiz (No. 2) to compare them with the
vertebrae of Reptiles, and subsequent anatomists have suggested
that they more nearly resemble the vertebrae of some Urodelous
Amphibia than those of any other form.
 
If, however, Gotte's account of the formation of the amphibian vertebrae is correct, there are serious objections to a
comparison between the vertebrae of Lepidosteus and Amphibia
on developmental grounds. The essential point of similarity
supposed to exist between them consists in the fact that in both
there is a great development of intervertebral cartilage which
constricts the notochord intervertebrally, and forms the articular
faces of contiguous vertebrae.
 
In Lepidosteus this cartilage is, as we have seen, derived from
the bases of the arches ; but in Amphibia it is held by Gotte to
be formed by a special thickening of a cellular sheath round the
notochord which is probably homologous with the cartilaginous
sheath of the notochord of Elasmobranchii, and therefore with
part of the notochordal sheath placed within the membrana
elastica externa.
 
If the above statements with reference to the origin of the
intervertebral cartilage in the two types are true, it is clear that
no homology can exist between structures so differently developed. Provisionally, therefore, we must look elsewhere
than in Lepidosteus for the origin of the amphibian type of
vertebrae.
 
The researches which we have recorded demonstrate, however, in a very conclusive manner that the vertebrae of Lepidosteus have very close affinities with those of Teleostei.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 793
 
 
 
In support of this statement we may point: (i) To the
structure of the sheath of the notochord ; (2) to the formation of
the greater part of the bodies of the vertebrae from ossification
in membrane around the notochord ; (3) to the early biconcave
form of the vertebras, only masked at a later period by the development of intervertebral cartilages ; (4) to the character of
the neural arches.
 
This latter feature will be made very clear if the reader will
compare our figures of the sections of later vertebrae (Plate 42,
fig. 78) with Gotte's 1 figure of the section of the vertebra of a
Pike (Plate 7, fig. i). In Gotte's figure there are shewn similar
intercalated pieces of cartilage to those which we have found,
and similar cartilaginous dorsal processes of the vertebras. Thus
we are justified in holding that whether or no the opisthoccelous
form of the vertebrae of Lepidostens. is a commencement of a
type of vertebrae inherited by the higher forms, yet in any case
the vertebrae are essentially built on the type which has become
inherited by the Teleostei from the bony Ganoids.
 
 
 
PART III. The ribs of Fishes.
 
The nature and homologies of the ribs of Fishes have long
been a matter of controversy ; but the subject has recently been
brought forward in the important memoirs of Gotte 2 on the
Vertebrate skeleton. The alternatives usually adopted are,
roughly speaking, these : Either the haemal arches of the tail
are homologous throughout the piscine series, while the ribs
of Ganoids and Teleostei are not homologous with those of
Elasmobranchii ; or the ribs are homologous in all the piscine
groups, and the haemal arches in the tail are differently formed
in the different types. Gotte has brought forward a great body
of evidence in favour of the first view; while Gegenbaur 3 may
 
1 "Beitrage zur vergl. Morphol. d. Skeletsystems d. Wirbelthiere." Archiv f.
Mikr. Anat. Vol. xvi. 1879.
 
2 " Beitrage z. vergl. Morph. d. Skeletsystems d. Wirbelthiere. II. Die Wirbelsaule u. ihre Anhange." Archvo /. Mikr. Anat., Vol. xv., 1878, and Vol. xvi.,
1879.
 
3 " Ueb. d. Entwick. d. Wirbelsaule d. Lepidosteus, mit. vergl. Anat. Bemerkungen. "Jena ische Zeitschrift, Bd. in., 1863.
 
B. 51
 
 
 
794 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
be regarded as more especially the champion of the second
view.
 
One of us held in a recent publication 1 that the question was
not yet settled, though the view that the ribs are homologous
throughout the series was provisionally accepted.
 
It is admitted by both Gegenbaur and Gotte that in Lepidosteus the ribs, in the transition from the trunk to the tail, bend
inwards, and finally unite in the region of the tail to form the
ventral parts of the haemal arches, and our researches have
abundantly confirmed this conclusion.
 
Are the haemal arches, the ventral parts of which are thus
formed by the coalescence of the ribs, homologous with the
haemal arches in Elasmobranchii ? The researches recorded in
the preceding pages appear to us to demonstrate in a conclusive
manner that they are so. .
 
The development of the haemal arches in the tail in these two
groups is practically identical ; they are formed in both as simple
elongations of the primitive haemal processes, which meet below
the caudal vein. In the adult there is an apparent difference
between them, arising from the fact that in Lepidosteus the
peripheral parts of the haemal processes are only articulated with
the basal portions, and not, as in Elasmobranchii, continuous
with them. This difference does not, however, exist in the early
larva, since in the larval Lepidosteus the haemal arches of the tail
are unsegmented cartilaginous arches, as they permanently are
in Elasmobranchii. If, however, the homology between the
haemal arches of the two types should still be doubted, the fact
that in both types the haemal arches are similarly modified to
support the fin-rays of the ventral lobe of the caudal fin, while in
neither type are they modified to support the anal fin, may
be pointed out as a very strong argument in confirmation of
their homology.
 
The demonstration of the homology of the haemal arches of
the tail in Lepidosteus and Elasmobranchii might at first sight be
taken as a conclusive argument in favour of Gotte's view, that
the ribs of Elasmobranchii are not homologous with those of
Ganoidei. This view is mainly supported by two facts :
 
1 Comparative Embryology, Vol. II., pp. 462, 463 [the original edition].
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 795
 
(1) In the first place, the ribs in Elasmobranchii do not at
first sight appear to be serially homologous with the ventral
parts of the haemal arches of the tail, but would rather seem to
be lateral offshoots of the haemal processes, while the haemal
arches of the tail appear to be completed by the coalescence of
independent ventral prolongations of the haemal processes.
 
(2) In the second place, the position of the ribs is different
in the two groups. In Elasmobranchii they are situated between
the dorso-lateral and ventro- lateral muscles (woodcut, fig. i, rb.},
 
FIG. i.
 
 
 
 
II,
 
 
 
m.el
 
 
 
Diagrammatic section through the trunk of an advanced embryo of Scyllium, to shew
the position of the ribs.
 
ao., aorta; c. sh., cartilaginous notochordal sheath; cv., cardinal vein; hp., hremal
process; k., kidney; /.j., ligamentum longitudinale superius ; m.el., membrana
elastica externa; na., neural arch; no., notochord ; //., lateral line; rb., rib;
sp.c., spinal cord.
 
while in Lepidosteus and other Ganoids they immediately girth
the body-cavity.
 
There is much, therefore, to be said in favour of Gotte's view.
At the same time, there is another possible interpretation of the
facts which would admit the homology of the ribs as well as of
the haemal arches throughout the Pisces.
 
512
 
 
 
796 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
Let us suppose, to start with, that the primitive arrangement
of the parts is more or less nearly that found in Lepidosteus,
where we have well-developed ribs in the region of the trunk,
girthing the body-cavity, and uniting in the caudal region to
form the ventral parts of the haemal arches. It is easy to conceive that the ribs in the trunk might somewhat alter their
position by passing into the muscles, along the inter-muscular
septa, till they come to lie between the dorso-lateral and ventrolateral muscles, as in Elasmobranchii. Lepidosteus itself affords
'a proof that such a change in the position of the ribs is not
impossible, in that it differs from other Ganoids and from Teleostci
in the fact that the free ends of the ribs leave the neighbourhood
of the body-cavity and penetrate into the muscles.
 
If it be granted that the mere difference in position between
the ribs of Ganoids and Elasmobranchii is not of itself sufficient
to disprove their homology, let us attempt to picture what would
take place at the junction of the trunk and tail in a type in
which the ribs had undergone the above change in position. On
nearing the tail it may be supposed that the ribs would gradually
become shorter, and at the same time alter their position, till
finally they shaded off into ordinary haemal processes. If, however, the haemal canal became prolonged forwards by the formation of some additional complete or nearly complete haemal
arches, an alteration in the relation of the parts would necessarily
take place. Owing to the position of the ribs, these structures
could hardly assist in the new formation of the anterior part of
the haemal canal, but the continuation forwards of the canal
would be effected by prolongations of the haemal processes
supporting the ribs. The new arches so formed would naturally
be held to be homologous with the haemal arches of the tail,
though really not so, while the true nature of the ribs would
also be liable to be misinterpreted, in that the ribs would appear
to be lateral outgrowths of the haemal processes of a wholly
different nature to the ventral parts of the haemal arches of the
tail.
 
In some Elasmobranchii, as shewn in the accompanying
woodcut (fig. 2), in the transitional vertebrae between the trunk
and the tail, the ribs are supported by lateral outgrowths of the
haemal processes, while the wholly independent prolongations of
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 797
 
the haemal processes appear to be about to give rise to the
haemal arches of the tail.
 
This peculiar state of things led Gotte, and subsequently one
of us, to deny for Elasmobranchs all homology between the ribs
and any part of the haemal arches of the tail ; but in view of the
explanation just suggested, this denial was perhaps too hasty.
 
FIG. 2.
 
 
 
r.p
 
 
 
. . . V. etuis.
 
 
 
Transverse section through the ventral part of the notochord, and adjoining structures
of an advanced Scyllium embryo at the root of the tail.
 
Vb., cartilaginous sheath of the notochord; ka., haemal process; r.p., process to
which the rib is articulated ; m.el., membrana elastica externa ; ck., notochord ;
ao., aorta; V.cau., caudal vein.
 
We are the more inclined to take this view because the researches of Gotte appear to shew that an occurrence, in manyrespects analogous, has taken place in some Teleostei.
 
In Teleostei, Johannes Muller, and following him Gegenbaur,
do not admit that the haemal arches of the tail are in any part
formed by the ribs. Gegenbaur (Elements of Comp. Anat., translation, p. 431) says, "In the Teleostei, the costiferous transverse
processes" (what we have called the haemal processes) "gradually
 
 
 
798 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
 
 
converge in the caudal region, and form inferior arches, which
are not homologous with those of Selachii and Ganoidei, although
they also form spinous processes."
 
The opposite view, that the haemal arches of the tail in Teleostei contain parts serially homologous with the basal parts of
the haemal processes as well as with the ribs, has been also
maintained by many anatomists, e.g., Meckel, Aug. Muller, &c.,
and has recently found a powerful ally in Gotte.
 
In many cases, the relations of the parts appear to be fundamentally those found in Lepidosteus and Amia, and Gotte has
shewn by his careful embryological investigations on Esox and
Anguilla, that in these two forms there is practically conclusive
evidence that the ribs as well as the haemal costiferous processes of Gegenbaur, which support them, enter into the formation of the haemal arches of the tail.
 
In a great number of Teleostei, e.g., the Salmon and most
Cyprinoids, &c., the haemal arches in the region of transition
from the trunk to the tail have 'a structure which at first sight
appears to support Johannes Miiller's and Gegenbaur's view.
The haemal processes grow larger and meet each other ventrally;
while the ribs articulated to them gradually grow smaller and
disappear.
 
The Salmon is typical in this respect, and has been carefully
studied by Gotte, who attempts to shew (with, in our opinion,
complete success) that the anterior haemal arches are really not
entirely homologous with the true haemal arches behind, but
that in the latter, the closure of the arch below is effected by the
haemal spine, which is serially homologous with a pair of coalesced ribs, while in the anterior haemal arches, i.e., those of the
trunk, the closure of the arch is effected by a bridge of bone
uniting the haemal processes.
 
The arrangement of the parts just described, as well as the
view of Gotte with reference to them, will be best understood
from the accompanying woodcut (fig. 3), copied from Gotte's
memoir.
 
Gotte sums up his own results on this point in the following
words (p. 138): "It follows from this, that the half rings, forming
the haemal canal in the hindermost trunk vertebrae of the Salmon, are not (with the exception of the last) completely homo
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 799
 
 
 
logous with those of the tail, but are formed by a connecting
piece between the basal stumps (haemal processes), which originates as a paired median process of these stumps."
 
The incomplete homology between the anterior haemal arches
and the true caudal haemal arches which follow them is exactly
what we suggest may be the case in Elasmobranchii, and if it be
admitted in the one case, we see no reason why it should not
also be admitted in the other.
 
If this admission is made, the only ground for not regarding
the ribs of Elasmobranchii as homologous with those of Ganoids
 
FIG. 3 .
 
 
 
 
Semi-diagrammatic transverse sections through the first caudal vertebra (A), the last
trunk vertebra (B), and the two trunk vertebrae in front (C and D), of a Salmon
embryo of 2-3 centims. (From Gotte.)
 
ub., haemal arch; ub'., haemal process; ud"., rib; c., notochord ; a., aorta; v. , vein;
^., connecting pieces between haemal processes ; u., kidney ; d., intestine ;
sp'., haemal spine ; m',, muscles.
 
is their different position, and we have already attempted to
prove that this is not a fundamental point.
 
The results of our researches appear to us, then, to leave two
alternatives as to the ribs of Fishes. One of these, which may
be called Gotte's view, may be thus stated: The haemal arches
 
 
 
800 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
are homologous throughout the Pisces: in Teleostei, Ganoidei,
and Dipnoi 1 , the ribs, placed on the inner face of the body-wall,
are serially homologous with the ventral parts of the haemal
arches of the tail ; in Elasmobranchii, on the other hand, the ribs
are neither serially homologous with the haemal arches of the
tail nor homologous with the ribs of Teleostei and Ganoidei, but
are outgrowths of the haemal processes into the space between
the dorso-lateral and ventro-lateral muscles, which may perhaps
have their homologues in Teleostei and Ganoids in certain
accessory processes of the vertebrae.
 
The other view, which we are inclined to adopt, and the
arguments for which have been stated in the preceding pages, is
as follows: The Teleostei, Ganoidei, Dipnoi, and Elasmobranchii are provided with homologous haemal arches, which are
formed by the coalescence below the caudal vein of simple prolongations of the primitive haemal processes of the embryo. The
canal enclosed by the haemal arches can be demonstrated embryologically to be the aborted body-cavity.
 
In the region of the trunk the haemal processes and their
prolongations behave somewhat differently in the different types.
 
In Ganoids and Dipnoi, in which the most primitive arrangement is probably retained, the ribs are attached to the haemal
processes,and are placed immediately without the peritoneal membrane at the insertions of the intermuscular septa. These ribs are
in many instances (Lepidosteus, Acipenser], and very probably in
all, developed continuously with the haemal processes, and become subsequently segmented from them. They are serially
homologous with the ventral parts of the haemal arches of the
tail, which, like them, are in many instances (Ceratodus, Lepidosteus, Polypterus, and to some extent in Amia) segmented off
from the basal parts of the haemal arches.
 
In Teleostei the ribs have the same position and relations as
those in Ganoids and Dipnoi, but their serial homology with the
ventral parts of the haemal processes of the tail, is often (e.g., the
Salmon) obscured by some of the anterior haemal arches in the
posterior part of the trunk being completed, not by the ribs, but
 
1 We .find the serial homology of the ribs and ventral parts of the haemal arches to
be very clear in Ceratodus. Wiedersheim states that it is not clear in Protopterus,
although he holds that the facts are in favour of this view.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 8oi
 
by independent outgrowths of the basal parts of the haemal processes.
 
In Elasmobranchii a still further divergence from the primitive arrangement is present. The ribs appear to have passed
outwards along the intermuscular septa into the muscles, and are
placed between the dorso-lateral and ventro-lateral muscles (a
change of position of the ribs of the same nature, but affecting
only their ends, is observable in Lepidosteus). This change of
position, combined probably with the secondary formation of a
certain number of anterior haemal arches similar to those in the
Salmon, renders their serial homology with the ventral parts of
the haemal processes of the tail far less clear than in other types,
and further proof is required before such homology can be considered as definitely established.
 
This is not the place to enter into the obscure question as to
how far the ribs of the Amphibia and Amniota are homologous
with those of Fishes. It is to be remarked, however, that the
ribs of the Urodela (i) occupy the same position in relation to
the muscles as the Elasmobranch ribs, (2) that they are connected with the neural arches, and (3) that they coexist in the
tail with the haemal arches, and seem, therefore, to be as different as possible from the ribs of the Dipnoi.
 
 
 
PART IV. The skeleton of the ventral lobe of the tail fin, and its
bearing on the nature of the tail fin of the various types of Pisces.
 
In the embryos or larvae of all the Elasmobranchii, Ganoidei,
and Teleostei which have up to this time been studied, the unpaired fins arise as median longitudinal folds of the integument
on the dorsal and ventral sides of the body, which meet at the
apex of the tail. The tail at first is symmetrical, having a form
which has been called diphycercal or protocercal. At a later
stage, usually, though not always, parts of these fins atrophy,
while other parts undergo a special development and constitute
the permanent unpaired fins.
 
Since the majority of existing as well as extinct Fishes are
provided with discontinuous fins, those forms, such as the Eel
(Anguilla), in which the fins are continuous, have probably re
 
 
802 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
 
 
verted to an embryonic condition : an evolutional process which
is of more frequent occurrence than has usually been admitted.
 
In the caudal region there is almost always developed in the
larvae of the above groups a special ventral lobe of the embryonic fin a short distance from the end of the tail. In Elasmobranchii and Chondrostean Ganoids the portion of the embryonic tail behind this lobe persists through life, and a special
type of caudal fin, which is usually called heterocercal, is thus
produced. This type of caudal fin appears to have been the
most usual in the earlier geological periods.
 
Simultaneously with the formation of the ventral lobe of the
heterocercal caudal fin, the notochord with the vertebral tissues
surrounding it, becomes bent somewhat dorsalwards, and thus
the primitive caudal fin forms a dorsally directed lobe of the
heterocercal tail. We shall call this part the dorsal lobe of the
tail-fin, and the secondarily formed lobe the ventral lobe.
 
Lepidosteus and Amia (Wilder, No. 15) amongst the bony
Ganoids, and, as has recently been shewn by A. Agassiz 1 , most
Teleostei acquire at an early stage of their development heterocercal caudal fins, like those of Elasmobranchii and the Chondrostean Ganoids ; but in the course of their further growth the
dorsal lobe partly atrophies, and partly disappears as such,
owing to the great prominence acquired by the ventral lobe. A
portion of the dorsally flexed notochord and of the cartilage or
bone replacing or investing it remains, however, as an indication
of the original dorsal lobe, though it does not project backwards
beyond the level of the end of the ventral lobe, which in these
types forms the terminal caudal fin.
 
The true significance of the dorsally flexed portion of the
vertebral axis was first clearly stated by Huxley 2 , but as
A. Agassiz has fairly pointed out in the paper already quoted,
this fact does not in any way militate against the view put
forward by L. Agassiz that there is a complete parallelism between the embryonic development of the tail in these Fishes
and the palseontological development of this organ. We think
 
1 " On the Young Stages of some Osseous Fishes. I. The Development of the
Tail," Proc. of the American Academy of Arts and Sciences, Vol. XIII., 1877.
 
2 "Observations on the Development of some Parts of the Skeleton of Fishes,"
Quart. Journ. of Micr. Science, Vol. vil., 1859.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 803
 
that it is moreover convenient to retain the term homocercal for
those types of caudal fin in which the dorsal lobe has atrophied
so far as not to project beyond the ventral lobe.
 
We have stated these now well-known facts to enable the
reader to follow us in dealing with the comparison between the
skeleton supporting the fin-rays of the ventral lobe of the caudal
fin, and that supporting the fin-rays of the remaining unpaired
fins.
 
It has been shewn that in Lepidosteus the unpaired fins fall
into two categories, according to the nature of the skeletal parts
supporting them. The fin-rays of the true ventral lobe of the
caudal fin are supported by the spinous processes of certain of
the haemal arches. The remaining unpaired fins, including the
anal fin, are supported by the so-called interspinous bones,
which are developed independently of the vertebral column and
its arches.
 
The question which first presents itself is, how far does this
distinction hold good for other Fishes ? This question, though
interesting, does not appear to have been greatly discussed by
anatomists. Not unfrequently the skeletal supports of the
ventral lobe of the caudal fin are assumed to be the same as
those of the other fins.
 
Davidoff 1 , for instance, in speaking of the unpaired fins of
Elasmobranch embryos, says (p. 514): "The cartilaginous rays
of the dorsal fins agreed not only in number with the spinous
processes (as indeed is also found in the caudal fin of the fullgrown Dog-fish)," &c.
 
Thacker 2 , again, in his memoir on the Median and Paired
Fins, states at p. 284 : " We shall here consider the skeleton of
the dorsal and anal fins alone. That of the caudal fin has
undergone peculiar modifications by the union of fin-rays with
haemal spines."
 
Mivart 3 goes into the question more fully. He points out
(p. 471) that there is an essential difference between the dorsal
and ventral parts of the caudal fin in Elasmobranchs, in that in
 
1 " Beitrage z. vergl. Anat. d. hinteren Gliedmassen d. Fische," Morph. Jahrbuch,
Vol. v., 1879.
 
* Trans, of the Connecticut Acad., Vol. in., 1877.
 
3 St George Mivart, "Fins of Elasmobranchs, " Zool, Trans., Vol. x.
 
 
 
804 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
the former the radials are more numerous than the vertebrae and
unconformable to them, while in the latter they are equal in
number to the vertebras and continuous with them. "This," he
goes on to say, "seems to point to a difference in nature between the dorsal and ventral portions of the caudal fin, in at
least most Elasmobranchs." He further points out that Polyodon
resembles Elasmobranchs. As to Teleostei, he does not express
himself decidedly except in the case of Murcena, to which we
shall return.
 
Mivart expresses himself as very doubtful as to the nature of
the supports of the caudal fin, and thinks " that the caudal fin of
different kinds of Fishes may have arisen in different ways in
different cases."
 
An examination of the ventral part of the caudal fin in various
Ganoids, Teleostei, and Elasmobranchii appears to us to shew
that there can be but little doubt that, in the majority of the
members of these groups at any rate, and we believe in all, the
same distinction between the ventral lobe of the caudal fin and
the remaining unpaired fins is found as in Lepidosteus.
 
In the case of most Elasmobranchii, a simple inspection of
the caudal fin suffices to prove this, and the anatomical features
involved in this fact have usually been recognized ; though, in the
absence of embryological evidence, the legitimate conclusion has
not always been drawn from them.
 
The difference between the ventral lobe of the caudal fin and
the other fins in the mode in which the fin-rays are supported is
as obvious in Chondrostean Ganoids as it is in Elasmobranchii ;
it would appear also to hold good for Amia. Polypterus we have
had no opportunity of examining, but if, as there is no reason to
doubt, the figure of its skeleton given by Agassiz (Poissons
Fossiles) is correct, there can be no question that the ventral lobe
of the caudal fin is supported by the haemal arches, and not
by interspinous bones. In Calamoicthys, the tail of which we
have had an opportunity of dissecting through the kindness
of Professor Parker, the fin- rays of the ventral lobe of the
true caudal fin are undoubtedly supported by true haemal
arches.
 
There is no unanimity of opinion as to the nature of the
elements supporting the fin-rays of the caudal fin of Teleostei.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 805
 
Huxley 1 in his paper on the development of the caudal fin of
the Stickleback, holds that these elements are of the nature of
interhsemal bones. He says (p. 39) : " The last of these rings lay
just where the notochord began to bend up. It was slightly
longer than the bony ring which preceded it, and instead of
having its posterior margin parallel with the anterior, it sloped
from above downwards and backwards. Two short osseous
plates, attached to the anterior part of the inferior surface of the
penultimate ring, or rudimentary vertebral centrum, passed downwards and a little backwards, and abutted against a slender
elongated mass of cartilage. Similar cartilaginous bodies occupy
the same relation to corresponding plates of bone in the anterior
vertebrae in the region of the anal fin ; and it is here seen, that
while the bony plates coalesce and form the inferior arches of
the caudal vertebrae, the cartilaginous elements at their extremities become the interhaemal bones. The cartilage connected
with the inferior arch of the penultimate centrum is therefore an
" interhsemal " cartilage. The anterior part of the inferior surface
of the terminal ossification likewise has its osseous inferior arch,
but the direction of this is nearly vertical, and though it is connected below with an element which corresponds in position
with the interhaemal cartilage, this cartilage is five or six times
as large, and constitutes a broad vertical plate, longer than it is
deep, and having its longest axis inclined downwards and backwards. . . .
 
" Immediately behind and above this anterior hypural apophysis (as it may be termed) is another very much smaller vertical
cartilaginous plate, which may be called the posterior hypural
apophysis."
 
We have seen that Mivart expresses himself doubtful on the
subject. Gegenbaur 2 appears to regard them as haemal arches.
 
The latter view appears to us without doubt the correct one.
An examination of the tail of normal Teleostei shews that the
fin-rays of that part of the caudal fin which is derived from the
ventral lobe of the larva are supported by elements serially
homologous with the haemal arches, but in no way homologous
 
1 "Observations on the Development of some parts of the Skeleton of Fishes,"
Quart. Journ. Micr. Science, Vol. vn., 1859.
 
2 Elements of Comparative Anatomy. (Translation), p. 431.
 
 
 
806 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
with the interspinous bones of the anal fin. The elements in
question formed of cartilage in the larva, become ossified in the
adult, and are known as the hypural bones. They may appear
in the form of a series of separate haemal arches, corresponding
in number with the primitive somites of this region, which
usually, however, atrophy in the adult, or more often are from
the first imperfectly segmented, and have in the adult the form
of two or three or even of a single broad bony plate. The
transitional forms between this state of things and that, for
instance, in Lepidosteus are so numerous, that there can be no
doubt that even the most peculiar forms of the hypural bones of
Teleostei are simply modified haemal arches.
 
This view of the hypural bones is, moreover, supported by
embryological evidence, since Aug. MUller 1 (p. 205) describes
their development in a manner which, if his statements are to be
trusted, leaves no doubt on this point.
 
There are a considerable number of Fishes which are not
provided with an obvious caudal fin as distinct from the remaining unpaired fins, i.e. Chimaera, Eels, and various Eel-like forms
amongst Teleostei, and the Dipnoi. Gegenbaur appears to hold
that these Fishes ought to be classed together in relation to the
structure of the caudal portion of their vertebral column, as he
says on p. 431 of his Comparative Anatomy (English Translation):
" In the Chimaerae, Dipnoi, and many Teleostei, the caudal
portion of the vertebral column ends by gradually diminishing in
size, but in most Fishes, &c."
 
For our purpose it will, however, be advisable to treat them
separately.
 
The tail of Chimsera appears to us to be simply a peculiar
modification of the typical Elasmobranch heterocercal tail, in
which the true ventral lobe of the caudal fin may be recognized
in the fin-fold immediately in front of the filamentous portion of
the tail. In the allied genus Callorhynchus this feature is more
distinct. The filamentous portion of the tail of Chimaera constitutes, according to the nomenclature adopted above, the true
dorsal lobe, and may be partially paralleled in the filamentous
dorsal lobe of the tail of the larval Lepidosteus (Plate 34, fig. 16).
 
1 " Beobachtungen zur vergl. Anat. d. Wirbelsaule," Miiller's Archiv, 1853.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 807
 
The tail of the eel-like Teleostei is again undoubtedly a
modification of the normal form of tail characteristic of the
Teleostei, in which, however, the caudal fin has become very
much reduced and merged into the prolongations of the anal and
dorsal fins.
 
This can be very clearly seen in Siluroid forms with an Eellike tail, such as Cnidoglanis. Although the dorsal and ventral
fins appear to be continuous round the end of the tail, and
there is superficially no distinct caudal fin, yet an examination
of the skeleton of Cnidoglanis shews that the end of the vertebral
column is modified in the usual Teleostean fashion, and that the
haemal arches of the modified portion of the vertebral column
support a small number of fin-rays ; the adjoining ventral finrays being supported by independent osseous fin-supports (interspinous bones).
 
In the case of the Eel (Anguilla anguilld) Huxley (loc. cit.}
long ago pointed out that the terminal portion of the vertebral
column was modified in an analogous fashion to that of other
Teleostei, and we have found that the modified haemal arches of
this part support a few fin-rays, though a still smaller number
than in Cnidoglanis, The fin-rays so supported clearly constitute an aborted ventral lobe of the caudal fin.
 
Under these circumstances we think that the following statement by Mivart (ZooL Trans. Vol. X., p. 471) is somewhat misleading :
 
"As to the condition of this part (i.e. the ventral lobe of the
tail-fin) in Teleosteans generally, I will not venture as yet to
say anything generally, except that it is plain that in siich forms
as Murcena, the dorsal and ventral parts of the caudal fin are
similar in nattire and homotypal with ordinary dorsal and anal
fins 1 ."
 
The italicized portion of this sentence is only true in respect
to that part of the fringe of fin surrounding the end of the body,
which is not only homotypal with, but actually part of, the
dorsal and anal fins.
 
Having settled, then, that the tails of Chimaera and of Eellike Teleostei are simply special modifications of the typical
form of tail of the group of Fishes to which they respectively
 
1 The italics are ours.
 
 
 
8o8 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
belong, we come to the consideration of the Dipnoi, in which the
tail-fin presents problems of more interest and greater difficulty
than those we have so far had to deal with.
 
The undoubtedly very ancient and primitive character of the
Dipnoi has led to the view, implicitly if not definitely stated in
most text- books, that their tail-fin retains the character of the
piscine tail prior to the formation of the ventral caudal lobe, a
stage which is repeated embryologically in the pre-heterocercal
condition of the tail in ordinary Fishes.
 
Through the want of embryological data, and in the absence
of really careful histological examination of the tail of any of
the Dipnoi, we are not willing to speak with very great confidence as to its nature ; we are nevertheless of the opinion that
the facts we can bring forward on this head are sufficient to
shew that the tail of the existing Dipnoi is largely aborted, so
that it is more or less comparable with that of the Eel.
 
We have had opportunities of examining the structure of the
tail of Ceratodus and Protopterus in dissected specimens in the
Cambridge Museum. The vertebral axis runs to the ends of
the tail without shewing any signs of becoming dorsally flexed.
At some distance from the end of the tail the fin-rays are supported by what are apparently segmented spinous prolongations
of the neural and haemal arches. The dorsal elements are
placed above the longitudinal dorsal cord, and occupy therefore
the same position as the independent elements of the neural
arches of Lepidostetis. They are therefore to be regarded as
homologous with the dorsal fin-supports or interspinous bones
of other types. The corresponding ventral elements are therefore also to be regarded as interspinous bones.
 
In view of the fact that the fin-supports, whenever their
development has been observed, are found to be formed independently of the neural and haemal arches, we may fairly assume
that this is also true for what we have identified as the interspinous elements in the Dipnoi.
 
The interspinous elements become gradually shorter as the
end of the tail is approached, and it is very difficult from a
simple examination of dissected specimens to make out how far
any of the posterior fin-rays are supported by the haemal arches
only. To this question we shall return, but we may remark
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 809
 
that, although there is a prolongation backwards of the vertebral axis beyond the last interspinous elements, composed it
would seem of the coalesced neural and haemal arches but
without the notochord, yet by far the majority of the fin-rays
which constitute the apparent caudal fin are supported by interspinous elements.
 
The grounds on which we hold that the tail of the Dipnoi is
to be regarded as a degenerate rather than primitive type of tail
are the following :
 
(1) If it be granted that a diphycercal or protocercal form
of tail must have preceded a heterocercal form, it is also clear
that the ventral fin-rays of such a tail must have been supported,
as in Polypterus and Calamoicthys, by haemal arches, and not by
interspinous elements ; otherwise, a special ventral lobe, giving
a heterocercal character to the tail, and provided with fin-rays
supported only by haemal arches, could never have become
evolved from the protocercal tail-fin. Since the ventral fin-rays
of the tail of the Dipnoi are supported by interspinous elements
and not by haemal arches, this tail-fin cannot claim to have the
character of that primitive type of diphycercal or protocercal
tail from which the heterocercal tail must be supposed to have
been evolved.
 
(2) Since the nearest allies of the Dipnoi are to be found in
Polypterus and the. Crossopterygidae of Huxley, and since in
these forms (as evinced by the structure of the tail-fin of Polypterus, and the transitional type between a heterocercal and
diphycercal form of fin observable in fossil Crossopterygidae) the
ventral fin-rays of the caudal fin were clearly supported by
haemal arches and not by interspinous elements, it is rendered
highly probable that the absence of fin-rays so supported in the
Dipnoi is a result of degeneration of the posterior part of the
tail.
 
[We use this argument without offering any opinion as to
whether the diphycercal character of the tail of many Crossopterygidae is primary or secondary.]
 
(3) The argument just used is supported by the degenerate
and variable state of the end of the vertebral axis in the Dipnoi
a condition most easily explained by assuming that the terminal
part of the tail has become aborted.
 
B. 52
 
 
 
8 10 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
(4) We believe that in Ceratodus we have been able to trace
a small number of the ventral fin-rays supported by haemal
arches only, but these rays are so short as not to extend so
far back as some of the rays attached to the interspinous elements
in front. These rays may probably be interpreted, like the more
or less corresponding rays in the tail of the Eel, as the last
remnant of a true caudal fin.
 
The above considerations appear to us to shew with very
considerable probability that the true caudal fin of the Dipnoi
has become all but aborted like that of various Teleostei ; and
that the apparent caudal fin is formed by the anal and dorsal fins
meeting round the end of the stump of the tail.
 
From the adult forms of Dipnoi we are, however, of opinion
that no conclusion can be drawn as to whether their ancestors
were provided with a diphycercal or a heterocercal form of
caudal fin.
 
The general conclusions with reference to the tail-fin at which
we have arrived are the following :
 
(1) The ventral lobe of the tail-fin of Pisces differs from the
other unpaired fins in the fact that its fin- rays are directly
supported by. spinous processes of certain of the haemal arches
instead of independently developed interspinous bones.
 
(2) The presence or absence of fin-rays in the tail-fin
supported by haemal arches may be used in deciding whether
apparently diphycercal tail-fins are aborted or primitive.
 
 
 
EXCRETORY AND GENERATIVE ORGANS.
 
I. Anatomy.
 
The excretory organs of Lepidostens have been described by
MUller (No. 13) and Hyrtl (No. n). These anatomists have
given a fairly adequate account of the generative ducts in the
female, and Hyrtl has also described the male generative ducts
and the kidney and its duct, but his description is contradicted
by our observations in some of the most fundamental points.
 
In the female example of 100*5 centims. which we dissected,
the kidney forms a paired gland, consisting of a narrow strip of
glandular matter placed on each side of the vertebral column, on
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 8ll
 
the dorsal aspect of the body-cavity. It is covered on its
ventral aspect by the oviduct and by its own duct, but is separated from both of these by a layer of the tough peritoneal
membrane, through which the collecting tubes pass. It extends
forwards from the anus for about three-fifths of the length of
the body-cavity, and in our example had a total length of about
28 centims. (Plate 39, fig. 60, k). Anteriorly the two kidneys
are separated by a short interval in the median line, but posteriorly they come into contact, and are so intimately united as
almost to constitute a single gland.
 
A superficial examination might lead to the supposition that
the kidney extended forwards for the whole length of the bodycavity up to the region of the branchial arches, and Hyrtl appears
to have fallen into this error ; but what appears to be its anterior
continuation is really a form of lymphatic tissue, something like
that of the spleen, filled with numerous cells. This matter
(Plate 39, fig. 60, fy.) continues from the kidney forwards without any break, and has a colour so similar to that of the kidney
as to be hardly distinguishable from it with the naked eye. The
true anterior end of the kidney is placed about 3 centims. in
front on the left side, and on the same level on the right side
as the wide anterior end of the generative duct (Plate 39, fig.
60, od.}. It is not obviously divided into segments, and is richly
supplied with malpighian bodies.
 
It is clear from the above description that there is no trace of
head-kidney or pronephros visible in the adult. To this subject
we shall, however, again return.
 
As will appear from the embryological section, the ducts
of the kidneys are probably simply the archinephric ducts, but
to avoid the use of terms involving a theory, we propose in the
anatomical part of our work to call them kidney ducts. They
are thin-walled widish tubes coextensive with the kidneys. If
cut open there may be seen on their inner aspect the numerous
openings of the collecting tubes of the kidneys. They are
placed ventrally to and on the outer border of the kidneys
(Plate 39, fig. 60, s.g.}. Posteriorly they gradually enlarge, and
approaching each other in the median line, coalesce, forming
an unpaired vesicle or bladder (/.) about 6 centims. long in
our example opening by a median pore on a more or less
 
52 2
 
 
 
8l2 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
prominent papilla (u.g.} behind the anus. The dilated portions
of the two ducts are called by Hyrtl the horns of the bladder.
 
The sides of the bladder and its so-called horns are provided with lateral pockets into which the collecting tubes of the
kidney open. These pockets, which we have found in two
female examples, are much larger in the horns of the bladder
than in the bladder itself. Similar pockets, but larger than
those we have found, have been described by Hyrtl in the male,
but are stated by him to be absent in the female. It is clear
from our examples that this is by no means always the case.
 
Hyrtl states that the wide kidney ducts, of which his description differs in no material point from our own, suddenly
narrow in front, and, perforating the peritoneal lining, are continued forwards to supply the anterior part of the kidney. We
have already shewn that the anterior part of the kidney has no
existence, and the kidney ducts supplying it are, according to
our investigations, equally imaginary.
 
It was first shewn by Miiller, whose observations on this point
have been confirmed by Hyrtl, &c., that the ovaries of Lepidosteus
are continuous with their ducts, forming in this respect an
exception to other Ganoids.
 
In our example of Lepidosteus the ovaries (Plate 39, fig. 60, ov.)
were about 1 8 centims. in length. They have the form of simple
sacks, filled with ova, and attached about their middle to their
generative duct, and continued both backwards and forwards
from their attachment into a blind process.
 
With reference to these sacks Miiller has pointed out and
the importance of this observation will become apparent when
we deal with the development that the ova are formed in the
thickness of the inner wall of the sack. We hope to shew that
the inner wall of the sack is alone equivalent to the genital ridge
of, for instance, the ovary of Scyllium. The outer aspect of
this wall i.e., that turned towards the interior of the sack is
equivalent to the outer aspect of the Elasmobranch genital ridge,
on which alone the ova are developed 1 . The sack into which
the ova fall is, as we shall shew in the embryological section, a
special section of the body-cavity shut off from the remainder,
 
1 Treatise on Comparative Embryology, Vol. I., p. 43 [the original edition].
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 813
 
 
 
and the dehiscence of the ova into this cavity is equivalent to
their discharge into the body-cavity in other forms.
 
The oviduct (Plate 39, fig. 60, od.} is a thin-walled duct of
about 21 centims. in length in the example we are describing,
continuous in front with the ovarian sack, and gradually tapering
behind, till it ends (od'.} by opening into the dilated terminal
section of the kidney duct on 'the inner side, a short distance
before the latter unites with its fellow. It is throughout closely
attached to the ureter and placed on its inner, and to some
extent on its ventral, aspect. The hindermost part of the oviduct
which runs beside the enlarged portion of the kidney duct
that portion called by Hyrtl the horn of the urinary bladder is
so completely enveloped by the wall of the horn of the urinary
bladder as to appear like a projection into the lumen of the
latter structure, and the somewhat peculiar appearance which
it presents in Hyrtl's figure is due to this fact. In our examples
the oviduct was provided with a simple opening into the kidney
duct, on a slight papilla ; the peculiar dilatations and processes
of the terminal parts of the oviduct, which have been described
by Hyrtl, not being present.
 
The results we have arrived at with reference to the male
organs are very different indeed from those of our predecessor,
in that we find the testicular products to be carried off by a series
of vasa efferentia, which traverse the mesorchium, and are continuous with the uriniferous tubuli ; so that the semen passes
through the uriniferous tubuli into the kidney duct and so to the
exterior. We have moreover been unable to find in tJu male a duct
homologous with the oviduct of the female.
 
This mode of transportation outwards of the semen has not
hitherto been known to occur in Ganoids, though found in all
Elasmobranchii, Amphibia, and Amniota. It is not, however,
impossible that it exists in other Ganoids, but has hitherto been
overlooked.
 
Our male example of Lepidosteus was about 60 centims. in
length, and was no doubt mature. It was smaller than any
of our female examples, but this according to Garman (vide,
p. 361) is usual. The testes (Plate 39, fig. 58 A. A) occupied
a similar position to the ovaries, and were about 21 centims.
long. They were, as is frequently the case with piscine testes,
 
 
 
8 14 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
divided into a series of lobes (10 12), and were suspended by
a delicate mesentery (mesorchium) from the dorsal' wall of the
abdomen on each side of the dorsal aorta. Hyrtl (No. n)
states that air or quicksilver injected between the limbs of the
mesentery, passed into a vas deferens 'homologous with the
oviduct which joins the ureter. We have been unable to find
such a vas deferens ; but we have found in the mesorchium a
number of tubes of a yellow colour, the colour being due to
a granular substance quite unlike coagulated blood, but which
appeared to us from microscopic examination to be the remains
of spermatozoa 1 . These tubes to the number of 40 50 constitute, we believe, the vasa efferentia. Along the line of suspension of the testis on its inner border these tubes unite to form
an elaborate network of tubes placed on the inner face of the
testis an arrangement very similar to that often found in Elasmobranchii (vide F. M. Balfour, Monograph on tJie Development of
Elasmobranch Fishes, plate 20, figs. 4 and 8).
 
We have figured this network on the posterior lobe of the
testis (fig. 58 B), and have represented a section through it
(fig. 59 A, n.v.e.}, and through one of the vasa efferentia (v.e.)
in the mesorchium. Such a section conclusively demonstrates
the real nature of these passages : they are filled with sperm
like that in the body of the testis, and are, as may be seen
from the section figured, continuous with the seminal tubes of the
testis itself.
 
At the attached base of the mesorchium the vasa efferentia
unite into a longitudinal canal, placed on the inner side of the
kidney duct (Plate 39, fig. 58 A, t.c., also shewn in section in
Plate 39, fig. 59 B, I.e.). From this canal tubules pass off which
are continuous with the tubuli uriniferi, as may be seen from
fig. 59 B, but the exact course of these tubuli through the kidney
could not be made out in the preparations we were able to
make of the badly conserved kidney. Hyrtl describes the
arrangement of the vascular trunks in the mesorchium in the
following way (No. 11, p. 6): "The mesorchium contains vascular trunks, viz., veins, which through their numerous anasto
1 The females we examined, which were no doubt procured at the same time as
the male, had their oviducts filled with ova : and it is therefore not surprising that
the vasa efferentia should be naturally injected with sperm.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 815
 
moscs form a plexus at the hilus of the testis, whose efferent
trunks, 13 in number, again unite into a plexus on the vertebral
column, which is continuous with the cardinal veins." The
arrangement (though not the number) of Hyrtl's vessels is
very similar to that of our vasa efferentia, and we cannot help
thinking that a confusion of the two may have taken place ;
which, in badly conserved specimens, not injected with semen,
would be very easy.
 
We have, as already stated, been unable to find in our dissections any trace of a duct homologous with the oviduct of
the female, and our sections through the kidney and its ducts
equally fail to bring to light such a duct. The kidney ducts are
about 19 centims. in length, measured from the genital aperture
to their front end. These ducts are generally similar to those
in the female ; they unite about 2 centims. from the genital
pore to form an unpaired vesicle. Their posterior parts are
considerably enlarged, forming what Hyrtl calls the horns of
the urinary bladder. In these enlarged portions, and in the
wall of the unpaired urinary bladder, numerous transverse
partitions are present, as correctly described by Hyrtl, which are
similar to those in the female, but more numerous. They give
rise to a series of pits, at the blind ends of which are placed the
openings of the kidney tubules. The kidney duct without doubt
serves as vas deferens, and we have found in it masses of yellowish
colour similar to the substance in the vasa efferentia identified
by us as remains of spermatozoa.
 
 
 
1 1 . Development.
 
In the general account of the development we have already
called attention to the earliest stages of the excretory system.
 
We may remind the reader that the first part of the system
to be formed is the segmental or archinephric duct (Plate 36,
figs. 28 and 29, .$-.). This duct arises, as in Teleostei and
Amphibia, by the constriction of a hollow ridge of the somatic
mesoblast into a canal, which is placed in contiguity with the
epiblast, along the line of junction between the mesoblastic
somites and the lateral plates of mesoblast. Anteriorly the duct
 
 
 
8l6 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
does not become shut off from the body-cavity, and also bends
inwards towards the middle line. The inflected part of the duct
is the first rudiment of the pronephros, and very soon becomes
considerably dilated relatively to the posterior part of the duct.
 
The posterior part of each segmental duct acquires an opening
into the cloacal section of the alimentary tract. Apart from
this change, the whole of the ducts, except their pronephric
sections, remain for a long time unaltered, and the next changes
we have to speak of concern the definite establishment of the
pronephros.
 
The dilated incurved portion of each segmental duct soon
becomes convoluted, and by the time the embryo is about 10
milling in length, but before the period of hatching, an important
change is effected in the relations of their peritoneal openings 1 .
 
Instead of leading into the body-cavity, they open into an
isolated chamber on each side (Plate 38, fig. $i,pr. c.}, which we
will call t\\Q pronephric chamber. The pronephric chamber is not,
however, so far as we can judge, completely isolated from the
body-cavity. We have not, it is true, detected with certainty at
this stage a communication between the two ; but in later stages,
in larvae of from 1 1 to 26 millims., we have found a richly ciliated
passage leading from the body-cavity into the pronephros on
each side (Plate 38, fig. 52, p.f.pl). We have not succeeded in
determining with absolute certainty the exact relations between
this passage and the tube of the pronephros, but we are inclined
to believe that it opens directly into the pronephric chamber just
spoken of.
 
As we hope to shew, this chamber soon becomes largely
filled by a vascular glomerulus. On the accomplishment of
these changes, the pronephros is essentially provided with all
the parts typically present in a segment of the mesonephros
(woodcut, fig. 4). There is a peritoneal tube (/) 2 , opening into
a vesicle (v) ; from near the neck of the peritoneal tube there
 
1 The change is probably effected somewhat earlier than would appear from our
description, but our specimens were not sufficiently well preserved to enable us to
speak definitely as to the exact period.
 
2 We feel fairly confident that there is only one pronephric opening on each side,
though we have no single series of sections sufficiently complete to demonstrate this
fact with absolute certainty.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 817
 
comes off a convoluted tube (pr.n.}, forming the main mass of
the pronephros, and ending in the segmental duct (sd.\
 
 
 
 
Diagrammatic views of the pronephros of Lepidosteus.
 
A, pronephros supposed to be isolated and seen from the side ; B, section through
the vesicle of the pronephros and the ciliated peritoneal funnel leading into it ;
pr.n., coiled tube of pronephros; sd., segmental or archinephric duct ; f., peritoneal funnel ; v., vesicle of pronephros ; bv., blood vessel of glomerulus ;
/., glomerulus.
 
The different parts do not, however, appear to have the same
morphological significance as those in the mesonephros.
 
Judging from the analogy of Teleostei, the embryonic structure
of whose pronephros is strikingly similar to that of Lepidosteus,
the two pronephric chambers into which the segmental ducts
open are constricted off sections of the body-cavity.
 
 
 
8l8 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
With the formation of the convoluted duct opening into the
isolated section of the body-cavity we may speak of a definite
pronephros as having become established. The pronephros is
placed, as can be made out in later stages, on the level of the
opening of the air-bladder into the throat.
 
The pronephros increases in size, so far as could be determined,
by the further convolution of the duct of which it is mainly
formed ; and the next change of importance which we have
noticed is the formation of a vascular projection into the pronephric chamber, forming the glomerulus already spoken of
(vide woodcut, fig. 4,gl.), which is similar to that of the pronephros
of Teleostei. We first detected these glomeruli in an embryo of
about 15 millims., some days after hatching (Plate 38, fig. 52, gl.},
but it is quite possible that they may be formed considerably
earlier.
 
In the same embryo in which the glomeruli were found we
also detected for the first time a mesonephros consisting of a
series of isolated segmental or nephridial tubes, placed posteriorly
to the pronephros along the dorsal wall ot' the abdomen.
 
These were so far advanced at this stage that we are not in a
position to give any account of their mode of origin. They are,
however, formed independently of the segmental ducts, and in
the establishment of the junction between the two structures,
there is no outgrowth from the segmental duct to meet the
segmental tubes. We could not at this stage find peritoneal
funnels of the segmental tubes, though we have met with them
at a later stage (Plate 38, fig. 53, //.), and our failure to find
them at this stage is not to be regarded as conclusive against
their existence.
 
A very considerable space exists between the pronephros
and the foremost segmental tube of the mesonephros. The
anterior mesonephric tubes are, moreover, formed earlier than
the posterior.
 
In the course of further development, the mesonephric tubules
increase in size, so that there ceases to be an interval between
them, the mesonephros thus becoming a continuous gland. In
an embryo of 26 millims. there was no indication of the formation of segmental tubes to fill up the space between the pronephros
and mesonephros.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 819
 
The two segmental ducts have united behind into an unpaired
structure in an embryo of 1 1 millims. This structure is no doubt
the future unpaired urinogenital chamber (Plate 39, figs. 58 A,
and 60, bl.}. Somewhat later, the hypoblastic cloaca becomes
split into two sections, the hinder one receiving the coalesced
segmental ducts, and the anterior remaining continuous with the
alimentary tract. The opening of the hinder one forms the
urinogenital opening, and that of the anterior the anus.
 
In an older larva of about 5*5 centims. the pronephros did
not exhibit any marked signs of atrophy, though the duct between
it and the mesonephros was somewhat reduced and surrounded
by the trabecular tissue spoken of in connection with the adult.
In the region between the pronephros and the front end of the
fully developed part of the mesonephros very rudimentary tubules
had become established.
 
The latest stage of the excretory system which we have studied
is in a young Fish of about 1 1 centims. in length. The special
interest of this stage depends upon the fact that the ovary is
already developed, and not only so, but the formation of the
oviducts has commenced, and their condition at this stage throws
considerable light on the obscure problem of their nature in the
Ganoids.
 
Unfortunately, the head of the young Fish had been removed
before it was put into our hands, so that it was impossible for us
to determine whether the pronephros was still present ; but as we
shall subsequently shew, the section of the segmental duct,
originally present between the pronephros and the front end of
the permanent kidney or mesonephros, has in any case disappeared.
 
In addition to an examination of the excretory organs in
situ, which shewed little except the presence of the generative
ridges, we made a complete series of sections through the excretory organs for their whole length (Plate 39, figs. 54 57).
 
Posteriorly these sections shewed nothing worthy of note,
the excretory organs and their ducts differing in no important
particular from these organs as we have described them in the
adult, except in the fact that the segmental ducts are not joined
by the oviducts.
 
Some little way in front of the point where the two segmental
 
 
 
820 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
ducts coalesce to form the urinary bladder, the genital ridge
comes into view. For its whole extent, except near its anterior
part (of which more hereafter) this ridge projects freely into the
body-cavity, and in this respect the young Fish differs entirely
from the adult. As shewn in Plate 39, figs. 56 and 57 (g.r.), it
is attached to the abdominal wall on the ventral side of, and near
the inner border of each kidney. The genital ridge itself has a
structure very similar to that which is characteristic of young
Elasmobranchii, and it may be presumed of young Fishes
generally. The free edge of the ridge is swollen, and this part
constitutes the true generative region of the ridge, while its dorsal
portion forms the supporting mesentery. The ridge itself is
formed of a central stroma and a germinal epithelium covering
it. The epithelium is thin on the whole of the inner aspect of
the ridge, but, just as in Elasmobranchii, it becomes greatly
thickened for a band-like strip on the outer aspect. Here, the
epithelium is several layers deep, and contains numerous primitive
germinal cells (p.o.}.
 
Though the generative organs were not sufficiently advanced
for us to decide the point with certainty, the structure of the
organ is in favour of the view that this specimen was a female,
and, as will be shewn directly, there can on other grounds be no
doubt that this is so. The large size of the primitive germinal
cells (primitive ova) reminded us of these bodies in Elasmobranchii.
 
In the region between the insertion of the genital ridge (or
ovary, as we may more conveniently call it) and the segmental
duct we detected the openings of a series of peritoneal funnels of
the excretory tubes (Plate 39 , fig. 57, /./!), which clearly therefore persist till the young Fish has reached a very considerable
size.
 
As we have already said, the ovary projects freely into the
body-cavity for the greater part of its length. Anteriorly, however, we found that a lamina extended from the free ventral
edge of the ovary to the dorsal wall of the body-cavity, to which
it was attached on the level of the outer side of the segmental
duct. A somewhat triangular channel was thus constituted, the
inner wall of which was formed by the ovary, the outer by the
lamina just spoken of, and the roof by the strip of the peritoneum
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 821
 
of the abdominal wall covering that part of the ventral surface
of the kidney in which the openings of the peritoneal funnels of
the excretory tubes are placed. The structure of this canal
will be at once understood by the section of it shewn in Plate 39,
 
% 55
There can be no doubt that this canal is the commencing
ovarian sack. On tracing it backwards we found that the lamina
forming its outer wall arises as a fold growing upwards from the
free edge of the genital ridge meeting a downward growth of the
peritoneal membrane from the dorsal wall of the abdomen ; and
in Plate 39, fig. 56, these two laminae may be seen before they
have met. Anteriorly the canal becomes gradually smaller and
smaller in correlation with the reduced size of the ovarian ridge,
and ends blindly nearly on a level with the front end of the
excretory organs.
 
It should be noted that, owing to the mode of formation of
the ovarian sack, the outer side of the ovary with the band of
thickened germinal epithelium is turned towards the lumen of
the sack; and thus the fact of the ova being formed on the
inner wall of the genital sack in the adult is explained, and the
comparison which we instituted in our description of the adult
between the inner wall of the genital sack and the free genital
ridge of Elasmobranchs receives its justification.
 
It is further to be noticed that, from the mode of formation
of the ovarian sack, the openings of the peritoneal funnels of the
excretory organs ought to open into its lumen ; and if these
openings persist in the adult, they will no doubt be found in
this situation.
 
Before entering on further theoretical considerations with
reference to the oviduct, it will be convenient to complete our
description of the excretory organs at this stage.
 
When we dissected the excretory organs out, and removed
them from the body of the young Fish, we were under the impression that they extended for the whole length of the bodycavity. Great was our astonishment to find that slightly in
front of the end of the ovary both excretory organs and segmental ducts grew rapidly smaller and finally vanished, and that
what we had taken to be the front part of the kidney was
nothing else but a linear streak of tissue formed of cells with
 
 
 
822 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
peculiar granular contents supported in a trabecular work
(Plate 39, fig. 54). This discovery first led us to investigate
histologically what we, in common with previous observers, had
supposed to be the anterior end of the kidneys in the adult, and
to shew that they were nothing else but trabecular tissue with
cells like that of lymphatic glands. The interruption of the
segmental duct at the commencement of this tissue demonstrates
that if any rudiment of the pronephros still persists, it is quite
functionless, in that it is not provided with a duct.
 
 
 
Ill . Theoretical considerations.
 
There are three points in our observations on the urinogenital system which appear to call for special remark. The
first of these concerns the structure and fate of the pronephros,
the second the nature of the oviduct, and the third the presence
of vasa efferentia in the male.
 
Although the history we have been able to give of the pronephros is not complete, we have nevertheless shewn that in most
points it is essentially similar to the pronephros of Teleostei.
In an early stage we find the pronephros provided with a peritoneal funnel opening into the body-cavity. At a later stage we
find that there is connected with the pronephros on each side, a
cavity the pronephric cavity into which a glomerulus projects.
This cavity is in communication on the one hand with the lumen
of the coiled tube which forms the main mass of the pronephros,
and on the other hand with the body-cavity by means of a
richly ciliated canal (woodcut, fig. 4, p. 817).
 
In Teleostei the pronephros has precisely the same characters, except that the cavity in which the glomerulus is placed is
without a peritoneal canal.
 
The questions which naturally arise in connection with the
pronephros are: (i) what is the origin of the above cavity with
its glomerulus ; and (2) what is the meaning of the ciliated canal
connecting this cavity with the peritoneal cavity ?
 
We have not from our researches been able to answer the
first of these questions. In Teleostei, however, the origin of this
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 823
 
 
 
cavity has been studied by Rosenberg 1 and Gotte*. According
to the account of the latter, which we have not ourselves confirmed but which has usually been accepted, the front end of the
segmental duct, instead of becoming folded off from the bodycavity, becomes included in a kind of diverticulum of the bodycavity, which only communicates with the remainder of the
body-cavity by a narrow opening. On the inner wall of this
diverticulum a projection is formed which becomes a glomerulus.
At this stage in the development of the pronephros we have
essentially the same parts as in the fully formed pronephros of
Lepidosteus, the only difference being that the passage connecting the diverticulum containing the glomerulus with the
remainder of the body-cavity is short in Teleostei, and in Lepidosteus forms a longish ciliated canal. In Teleostei the opening
into the body-cavity becomes soon closed. If the above comparison is justified, and if the development of these parts in
Lepidosteus takes place as it is described as doing in Teleostei, there can, we think, be no doubt that the ciliated canal
of Lepidosteus , which connects the pronephric cavity with
the body-cavity, is a persisting communication between this
cavity and the body-cavity; and that Lepidostetis presents
in this respect a more primitive type of pronephros than
Teleostei.
 
It may be noted that in Lepidosteus the whole pronephros
has exactly the character of a single segmental tube of the
mesonephros. The pronephric cavity with its glomerulus is
identical in structure with a malpighian body. The ciliated
canal is similar in its relations to the peritoneal canal of such a
segmental tube, and the coiled portion of the pronephros resembles the secreting part of the ordinary segmental tube. This
comparison is no doubt an indication that the pronephros is
physiologically very similar to the mesonephros, and so far
justifies Sedgwick's 3 comparison between the two, but it does
not appear to us to justify the morphological conclusions at
 
1 Rosenberg, Untersuch. ueb. d. Entwick. d. Teleostiemiere, Dorpat, 1867.
 
2 Gotte, Entwick. d. Unke, p. 826.
 
3 Seclgwick, " Early Development of the Wolffian Duct and anterior Wolffian
Tubules in the Chick; with some Remarks on the Vertebrate Excretory System,"
Quart. Journ. of Micros. Science, Vol. xxi., 1881.
 
 
 
824 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
which he has arrived, or to necessitate any modification in the
views on this subject expressed by one of us l .
 
The genital ducts of Ganoids and Teleostei have for some
time been a source of great difficulty to morphologists ; and any
contributions with reference to the ontogeny of these structures
are of interest.
 
The essential point which we have made out is that the anterior part of the oviduct of Lepidosteus arises by a fold of the
peritoneum attaching itself to the free edge of the genital ridge.
We have not, unfortunately, had specimens old enough to decide
how the posterior part of the oviduct is formed ; and although
in the absence of such stages it would be rash in the extreme to
speak with confidence as to the nature of this part of the duct, it
may be well to consider the possibilities of the case in relation
to other Ganoids and Teleostei.
 
The simplest supposition would be that the posterior part of
the genital duct had the same origin as the anterior, i. e., that it
was formed for its whole length by the concrescence of a peritoneal fold with the genital ridge, and that the duct so formed
opened into the segmental duct.
 
The other possible supposition is that a true Miillerian duct
i.e., a product of the splitting of the segmental duct is subsequently developed, and that the open end of this duct coalesces
with the duct which has already begun to be formed in our
oldest larva.
 
In attempting to estimate the relative probability of these
two views, one important element is the relation of the oviducts
of Lepidosteus to those of other Ganoids.
 
In all other Ganoids (vide Hyrtl, No. 1 1) there are stated to
be genital ducts in both sexes which are provided at their anterior extremities with a funnel-shaped mouth open to the abdominal cavity. At first sight, therefore, it might be supposed
that they had no morphological relationship with the oviducts
of Lepidosteus, but, apart from the presence of a funnel-shaped
mouth, the oviducts of Lepidosteus are very similar to those of
Chondrostean Ganoids, being thin-walled tubes opening on a
projecting papilla into the dilated kidney ducts (horns of the
 
1 F. M. Balfour, Comparative Embryology, Vol. n., pp. 600 603 [the original
edition].
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 825
 
urinary bladder, Hyrtl). These relations seem to prove beyond
a doubt that the oviduct of Lepidosteus is for its major part
homologous with the genital ducts of other Ganoids.
 
The relationship of the genital ducts to the kidney ducts in
Amia and Polypterns is somewhat different from that in the
Chondrostei and Lepidosteus. In Amia the ureters are so small
that they may be described rather as joining the coalesced
genital ducts than vice versa, although the apparent coalesced
portion of the genital ducts is shewn to be really part of the
kidney ducts by receiving the secretion of a number of mesonephric tubuli. In Polyptenis the two ureters are stated to
unite, and open by a common orifice into a sinus formed by the
junction of the two genital ducts, which has not been described
as receiving directly the secretion of any part of the mesonephros.
 
It has been usual to assume that the genital ducts of Ganoids
are true Mullerian ducts in the sense above defined, on the
ground that they are provided with a peritoneal opening and
that they are united behind with the kidney ducts. In the
absence of ontological evidence this identification is necessarily
provisional. On the assumption that it is correct we should
have to accept the second of the two alternatives above suggested as to the development of the posterior parts of the oviduct
in Lepidosteus.
 
There appear to us, however, to be sufficiently serious objections to this view to render it necessary for us to suspend our
judgment with reference to this point. In the first place, if the
view that the genital ducts are Mullerian ducts is correct, the
true genital ducts of Lepidosteus must necessarily be developed
at a later period than the secondary attachment between their
open mouths and the genital folds, which would, to say the least
of it, be a remarkable inversion of the natural order of development. Secondly, the condition of our oldest larva shews that
the Mullerian duct, if developed later, is only split off from quite
the posterior part of the segmental duct ; yet in all types in
which the development of the Mullerian duct has been followed,
its anterior extremity, with the abdominal opening, is split off
from either the foremost or nearly the foremost part of the segmental duct.
 
B- S3
 
 
 
826 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
Judging from the structure of the adult genital ducts of other
Ganoids they must also be developed only from the posterior
part of the segmental duct, and this peculiarity so struck one of
us that in a previous paper 1 the suggestion was put forward that
the true Ganoid genital ducts were perhaps not Miillerian ducts,
but enlarged segmental tubes with persisting abdominal funnels
belonging to the mesonephros.
 
If the possibility of the oviduct of Lepidosteus not being
a Miillerian duct is admitted, a similar doubt must also exist as
to the genital ducts of other Ganoids, and we must be prepared
to shew that there is a reasonable ground for scepticism on this
point. We would in this connexion point out that the second
of the two arguments urged against the view that the genital
duct of Lepidosteus is not a Miillerian duct applies with equal
force to the case of all other Ganoids.
 
The short funnel-shaped genital duct of the Chondrostei is
also very unlike undoubted Miillerian ducts, and could moreover
easily be conceived as originating by a fold of the peritoneum,
a slight extension of which would give rise to a genital duct like
that of Lepidosteus.
 
The main difficulty of the view that the genital ducts of
Ganoids are not Miillerian ducts lies in the fact that they open
into the segmental duct. While it is easy to understand the
genesis of a duct from a folding of the peritoneum, and also easy
to understand how such a duct might lead to the exterior by
coalescing, for instance, with an abdominal pore, it is not easy
to see how such a duct could acquire a communication with the
segmental duct.
 
We do not under these circumstances wish to speak dogmatically, either in favour of or against the view that the genital
ducts of Ganoids are Miillerian ducts. Their ontogeny would
be conclusive on this matter, and we trust that some of the
anatomists who have the opportunity of studying the development of the Sturgeon will soon let us know the facts of the case.
If there are persisting funnels of the mesonephric segmental
tubes in adult Sturgeons, some of them ought to be situated
within the genital ducts, if the latter are not Mullerian ducts ;
 
1 F. M. Balfour, "On the Origin and History of the Urinogenital Organs of
Vertebrates," Journ. of Anat. and Phys., Vol. X., 1876 [This edition, No. VII].
 
 
 
STRUCTURE AND DEVELOPMENT OF I-EPIDOSTEUS. 827
 
and naturalists who have the opportunity ought also to look out
for such openings.
 
The mode of origin of the anterior part of the genital duct
of Lepidosteus appears to us to tell strongly in favour of the
view, already regarded as probable by one of .us 1 , that the
Teleostean genital ducts are derived from those of Ganoids ;
and if, as appears to us indubitable, the most primitive type of
Ganoid genital ducts is found in the Chondrostei, it is interesting
to notice that the remaining Ganoids present in various ways
approximations to the arrangement typically found in Teleostei.
Lepidosteus obviously approaches Teleostei in the fact of the
ovarian ridge forming part of the wall of the oviduct, but differs
from the Teleostei in the fact of the oviduct opening into the
kidney ducts, instead of each pair of du^ts having an independent opening in the cloaca, and in the fact that the male genital
products are not carried to the exterior by a duct homologous
with the oviduct. Amia is closer to the Teleostei in the arrangement of the posterior part of the genital ducts, in that the two
genital ducts coalesce posteriorly ; while Polypterus approaches
still nearer to the Teleostei in the fact that the two genital ducts
and the two kidney ducts unite with each other before they
join ; and in order to convert this arrangement into that characteristic of the Teleostei we have only to conceive the coalesced
ducts of the kidneys acquiring an independent opening into the
cloaca behind the genital opening.
 
The male genital ducts. The discovery of the vasa efferentia
in Lepidosteus, carrying off the semen from the testis, and transporting it to the mesonephros, and thence through the mesonephric tubes to the segmental duct, must be regarded as the most
important of our results on the excretory system.
 
It proves in the first place that the transportation outwards
of the genital products of both sexes by homologous ducts,
which has been hitherto held to be universal in Ganoids, and
which, in the absence of evidence to the contrary, must still
be assumed to be true for all Ganoids except Lepidosteus, is
a secondary arrangement. This conclusion follows from the
fact that in Elasmobranchs, &c., which are not descendants of
 
1 F. M. Balfour, Comparative Embryology, Vol. II., p. 605 [the original edition].
 
532
 
 
 
828 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
the Ganoids, the same arrangement of seminal ducts is found
as in Lepidostens, and it must therefore have been inherited from
an ancestor common to the two groups.
 
If, therefore, the current statements about the generative
ducts of Ganoids are true, the males must have lost their vasa
efferentia, and the function of vas deferens must have been taken
by the homologue of the oviduct, presumably present in the
male. The Teleostei must, moreover, have sprung from Ganoidei
in which the vasa efferentia had become aborted.
 
Considerable phylogenetic difficulties as to the relationships
of Ganoidei and Elasmobranchii are removed by the discovery
that Ganoids were originally provided with a system of vasa
efferentia like that of Elasmobranchii.
 
 
 
THE ALIMENTARY CANAL AND ITS APPENDAGES.
 
I. -Anatomy.
 
Agassiz (No. 2) gives a short description with a figure of the
viscera of Lepidosteus as a whole. Van der Hceven has also
given a figure of them in his memoir on the air-bladder of this
form (No. 8), and Johannes Muller first detected the spiral valve
and gave a short account of it in his memoir (No. 13). Stannius, again, makes several references to the viscera of Lepidosteus in his anatomy of the Vertebrata, and throws some doubt
on Miiller's determination of the spiral valve.
 
The following description refers to a female Lepidosteus of
IOO'5 centims. (Plate 40, fig. 66).
 
With reference to the mouth and pharynx, we have nothing
special to remark. Immediately behind the pharynx there
comes an elongated tube, which is not divisible into stomach
and oesophagus, and may be called the stomach (j/.). It is about
44*6 centims. long, and gradually narrows from the middle towards the hinder or pyloric extremity. It runs straight backwards for the greater part of its length, the last 3*8 centims.,
however, taking a sudden bend forwards. For about half its
length the walls are thin, and the mucous membrane is smooth ;
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 829
 
in the posterior half the walls are thick, and the mucous membrane is raised into numerous longitudinal ridges. The peculiar
glandular structure of the epithelium of this part in the embryo
is shewn in Plate 40, fig. 62 (st.}. Its opening into the duodenum is provided with a very distinct pyloric valve (Py}.
This valve projects into a kind of chamber, freely communicating with the duodenum, and containing four large pits (c'},
into each of which a group of pyloric caeca opens. These caeca
form a fairly compact gland (c.) about 6-5 centims. long, which
overlaps the stomach anteriorly, and the duodenum posteriorly.
 
Close to the pyloric valve, on its right side, is a small papilla,
on the apex of which the bile duct opens (b.d'}.
 
A small, apparently glandular, mass closely connected with
the bile duct, in the position in which we have seen the pancreas
in the larva (Plate 40, figs. 62 and 63, /.), is almost certainly a
rudimentary pancreas, like that of many Teleostei ; but its
preservation was too bad for histological examination. We believe that the pancreas of Lepidosteus has hitherto been overlooked.
 
The small intestine passes straight backwards for about
8 centims., and then presents three compact coils. From the
end of these a section, about 5 centims. long, the walls of which
are much thicker, runs forwards. The intestine then again turns
backwards, making one spiral coil. -This spiral part passes
directly, without any sharp line of demarcation, into a short and
straight tube, which tapers slightly from before backwards, and
ends at the anus. The mucous membrane of the intestine for
about the first 3^5 centims. is smooth, and the muscular walls
thin : the rest of the small intestine has thick walls, and the
mucous membrane is reticulated.
 
A short spiral valve (sp. v.}, with a very rudimentary epithelial
fold, making nearly two turns, begins in about the posterior half
of the spiral coil of the intestine, extending backwards for
slightly less than half the straight terminal portion of the intestine, and ending 4 centims. in front of the anus. Its total
length in one example was about 4'5 centims.
 
The termination of the spiral valve is marked by a slight
constriction, and we may call the straight portion of the intestine behind it the rectum (re.}.
 
 
 
830 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
The posterior part of the intestine, from the beginning of the
spiral valve to the anus, is connected with the ventral wall of the
abdomen by a mesentery.
 
The air-bladder (a.b.} is 45 centims. long, and opens into the
alimentary canal by a slit-like aperture (a.fr.) on the median
dorsal line, immediately behind the epipharyngeal teeth. Each
lip of this aperture is largely formed by a muscular cushion,
thickest at its posterior end, and extending about 6 millims.
behind the aperture itself. A narrow passage is bounded by
these muscular walls, which opens dorsally into the air-bladder.
 
The air-bladder is provided with two short anterior cornua,
and tapers to a point behind : it shews no indication of any
separation into two parts. A strong band of connective tissue
runs along the inner aspect of its whole dorsal region, from
which there are given off on each side at intervals of about
12 millims. anteriorly, gradually increasing to 18 millims. posteriorly bands of muscle, which pass outwards towards its side
walls, and then spread out into the numerous reticulations with
which the air-bladder is lined throughout. By the contraction
of these muscles the cavity of the air-bladder can doubtless be
very much diminished.
 
The main muscular bands circumscribe a series of more or
less complete chambers, which were about twenty-seven in
number on each side in our example. The chambers are confined to the sides, so that there is a continuous cavity running
through the central part of the organ. The whole organ has the
characteristic structure of a simple lung.
 
The liver (lr.} consists of a single elongated lobe, about 32
centims. long, tapering anteriorly and posteriorly, the anterior
half being on the average twice as thick as the posterior half.
The gall-bladder (g.b.} lies at its posterior end, and is of considerable size, tapering gradually so as to pass insensibly into
the bile duct. The hepatic duct (kp.d) opens into the gallbladder at its anterior end.
 
The spleen (s.) is a large, compact, double gland, one lobe
lying in the turn of the intestine immediately above the spiral
valve, and the other on the opposite side of the intestine, so that
the intestine is nearly embraced between the two lobes.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 831
 
 
 
1 1. Development.
 
We have already described in detail the first formation of
the alimentary tract so far as we have been able to work it out,
and we need only say here that the anterior and posterior ends
of the canal become first formed, and that these two parts
gradually elongate, so as to approach each other ; the growth of
the posterior part is, however, the most rapid. The junction of
the two parts takes place a very short distance behind the
opening of the bile duct into the intestine.
 
For some time after the two parts of the alimentary tract
have nearly met, the ventral wall of the canal at this point is
not closed ; so that there is left a passage between the alimentary
canal and the yolk-sack, which forms a vitelline duct.
 
After the yolk-sack has ceased to be visible as an external
appendage it still persists within the abdominal cavity. It has,
however, by this stage ceased to communicate with the gut, so
that the eventual absorption of the yolk is no doubt entirely
effected by the vitelline vessels. At these later stages of development we have noticed that numerous yolk nuclei, like
those met with in Teleostei and Elasmobranchii 1 , are still to be
found in the yolk.
 
It will be convenient to treat the history of sections of the
alimentary tract in front of and behind the vitelline duct
separately. The former gives rise to the pharyngeal region, the
oesophagus, the stomach, and the duodenum.
 
The pharyngeal region, immediately after it has become
established, gives rise to a series of paired pouches. These may
be called the branchial pouches, and are placed between the
successive branchial arches. The first or hyomandibular pouch,
placed between the mandibular and hyoid arches, has rather
the character of a double layer of hypoblast than of a true
pouch, though in parts a slight space is developed between its
two walls. It is shewn in section in Plate 37, fig. 43 (h.m), from
an embryo of about 10 millims., shortly before hatching. It
 
1 For a history of similar nuclei, vide Comp. Embryol., Vol. II., chapters III.
and IV.
 
 
 
832 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
does not appear to undergo any further development, and, so far
as we can make out, disappears shortly after the embryo is
hatched, without acquiring an opening to the exterior.
 
It is important to notice that this cleft, which in the cartilaginous Ganoids and Polypterus remains permanently open as the
spiracle, is rudimentary even in the embryo of Lepidosteus.
 
The second pouch is the hyobranchial pouch : its outer end
meets the epiblast before the larva is hatched, and a perforation
is effected at the junction of the two layers, converting the pouch
into a visceral cleft.
 
Behind the hyobranchial pouch there are four branchial
pouches, which become perforated and converted into branchial
clefts shortly after hatching.
 
The region of the oesophagus following the pharynx is not
separated from the stomach, unless a glandular posterior region
(vide description of adult) be regarded as the stomach, a nonglandular anterior region forming the oesophagus. The lumen
of this part appears to be all but obliterated in the stages immediately before hatching, giving rise for a short period to a
solid oesophagus like that of Elasmobranchii and Teleostei 1 .
 
From the anterior part of the region immediately behind the
pharynx the air-bladder arises as a dorsal unpaired diverticulum.
From the very first it has an elongated slit-like mouth (Plate 40,
fig. 64, a.b'-.}, and is placed in the mesenteric attachment of the
part of the throat from which it springs.
 
We have first noticed it in the stages immediately after
hatching. At first very short and narrow, it grows in succeeding
stages longer and wider, making its way backwards in the
mesentery of the alimentary tract (Plate 40, fig. 65, a.b.}. In
the larva of a month and a half old (26 millims.) it has still a
perfectly simple form, and is without traces of its adult lung-like
structure ; but in the larva of 1 1 centims. it has the typical adult
structure.
 
The stomach is at first quite straight, but shortly after the
larva is hatched its posterior end becomes bent ventralwards and
forwards, so that the flexure of its posterior end (present in the
adult) is very early established. The stomach is continuous be
1 Vide Coinp. Embryo!., Vol. II., pp. 50 63 [the original edition].
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 833
 
hind with the duodenum, the commencement of which is indicated
by the opening of the bile duct.
 
The liver is the first-formed alimentary gland, and is already
a compact body before the larva is hatched. We have nothing
to say with reference to its development, except that it exhibits
the same simple structure in the embryo that it does in the
adult.
 
A more interesting glandular body is the pancreas. It has
already been stated that in the adult we have recognized a small
body which we believe to be the pancreas, but that we were
unable to study its histological characters.
 
In the embryo there is a well-developed pancreas which
' arises in the same position and the same manner as in those
Vertebrata in which the pancreas is an important gland in the
adult.
 
We have first noticed the pancreas in a stage shortly after
hatching (Plate 40, fig. 6i,/.). It then has the form of a funnelshaped diverticulum of the dorsal wall of the duodenum, immediately behind the level of the opening of the bile duct. From
the apex of this funnel numerous small glandular tubuli soon
sprout out.
 
The similarity in the development of the pancreas in Lepidosteus to that of the same gland in Elasmobranchii is very
striking 1 .
 
The pancreas at a later stage is placed immediately behind
the end of the liver in a loop formed by the pyloric section of the
stomach (Plate 40, fig. 62,/.). During larval life it constitutes a
considerable gland, the anterior end of which partly envelopes
the bile duct (Plate 40, fig. 63,/.).
 
Considering the undoubted affinities between Lepidosteus and
the Teleostei, the facts just recorded with reference to the
pancreas appear to us to demonstrate that the small size and
occasional absence (?) of this gland in Teleostei is a result of the
degeneration of this gland ; and it seems probable that the
pancreas will be found in the larvae of most Teleostei. These
conclusions render intelligible, moreover, the great development
of the pancreas in the Elasmobranchii.
 
1 Vide F. M. Balfour, "Monograph on Development of Elasmobranch Fishes,"
p. 226 [This edition, No. X., p. 454].
 
 
 
834 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
We have first noticed the pyloric caeca arising as outgrowths
of the duodenum in larvae of about three weeks old, and they
become rapidly longer and more prominent (Plate 40, fig. 62, .).
 
The portion of the intestine behind the vitelline duct is, as in
all the Vertebrata, at first straight. In Elasmobranchs the lumen
of the part of the intestine in which a spiral valve is present in
the adult, very early acquires a more or less semilunar form by
the appearance of a fold which winds in a long spiral. In Lepidosteus there is a fold similar in every respect (Plate 38, fig. 53,
sp.v.\ forming an open spiral round the intestine. This fold is
the first indication of the spiral valve, but it is relatively very
much later in its appearance than in Elasmobranchs, not being
formed till about three weeks after hatching. It is, moreover, in
correlation with the small extent of the spiral valve of the adult,
confined to a much smaller portion of the intestine than in
Elasmobranchii, although owing to the relative straightness of
the anterior part of the intestine it is proportionately longer in
the embryo than in the adult.
 
The similarity of the embryonic spiral valve of Lepidosteus to
that of Elasmobranchii shews that Stannius' hesitation in accepting Miiller's discovery of the spiral valve in Lepidosteus is not
justified.
 
J. Mliller (Ban u. Entwick. d. Myxinoideii) holds that the socalled bursa entiana of Elasmobranchii (i.e., the chamber placed
between the part of the intestine with the spiral valve and the
end of the pylorus) is the homologue of the more elongated
portion of the small intestine which occupies a similar position
in the Sturgeon. This portion of the small intestine is no doubt
homologous with the still more elongated and coiled portion of
the small intestine in Lepidosteus placed between the chamber
into which the pyloric caeca, &c., .open and the region of the
spiral valve. The fact that the vitelline duct in the embryo
Lepidosteus is placed close to the pyloric end of the stomach, and
that the greater portion of the small intestine is derived from
part of the alimentary canal behind this, shews that Miiller is
mistaken in attempting to homologise the bursa entiana of
Elasmobranchii, which is placed in front of the vitelline duct,
with the coiled part of the small intestine of the above forms.
The latter is either derived from an elongation of the very short
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 835
 
portion of the intestine between the vitelline duct and the primitive spiral valve, or more probably by the conversion of the
anterior part of the intestine, originally provided with a spiral
valve into a coiled small intestine not so provided.
 
We have already called attention to the peculiar mesentery
present in the adult attaching the posterior straight part of the
intestine to the ventral wall of the body. This mesentery, which
together with the dorsal mesentery divides the hinder section of
the body-cavity into two lateral compartments is, we believe, a
persisting portion of the ventral mesentery which, as pointed out
by one of us 1 , is primitively present for the whole length of the
body-cavity. The persistence of such a large section of it as
that found in the adult Lcpidosteus is, so far as we know, quite
exceptional. This mesentery is shewn in section in the embryo
in Plate 38, fig. 53 (v.tnt^. The small vessel in it appears to be
the remnant of the subintestinal vein.
 
 
 
THE GILL ON THE HYOID ARCH.
 
It is well known that Lepidosteus is provided with a gill on
the hyoid arch, divided on each side into two parts. An excellent
figure of this gill is given by Miiller (No. 13, plate 5, fig. 6), who
holds from a consideration of the vascular supply that the two
parts of this gill represent respectively the hyoid gill and the
mandibular gill (called by MUller pseudobranch). Miiller's views
on this subject have not usually been accepted, but it is the
fashion to regard the whole of the gill as the hyoid gill divided
into two parts. It appeared to us not improbable that embryology might throw some light on the history of this gill, and
accordingly we kept a look out in our embryos for traces of gills
on the hyoid and mandibular arches. The results we have arrived
at are purely negative, but are not the less surprising for this
fact. The hyomandibular cleft as shewn above, is never fully
developed, and early undergoes a complete atrophy a fact which
is, on the whole, against Muller's view ; but what astonished us
most in connection with the gill in question is that we have been
 
1 Comparative Embryology, Vol. II. p. 514 [the original edition].
 
 
 
836 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
unable to find any trace of it even in the oldest larva whose head
we have had (26 millims.), and at a period when the gills on the
hinder arches have reached their full development.
 
We imagined the gill in question to be the remnant of a gill
fully formed in extinct Ganoid types, and therefore expected to
find it better developed in the larva than in the adult. That the
contrary is the fact appears to us fairly certain, although we cannot at present offer any explanation of it.
 
 
 
SYSTEMATIC POSITION OF LEPIDOSTEUS.
 
A. Agassiz concludes his memoir on the development of
Lepidosteus by pointing out that in spite of certain affinities in
other directions this form is " not so far removed from the bony
Fishes as has been supposed." Our own observations go far to
confirm Agassiz' opinion.
 
Apart from the complete segmentation, the general development of Lepidosteus is strikingly Teleostean. In addition to the
general Teleostean features of the embryo and larva, which can
only be appreciated by those who have had an opportunity of
practically working at the subject, we may point to the following
developmental features 1 as indicative of Teleostean affinities :
 
(1) The formation of the nervous system as a solid keel of
the epiblast.
 
(2) The division of the epiblast into a nervous and epidermic
stratum.
 
(3) The mode of development of the gut (vide pp. 752 754).
 
(4) The mode of development of the pronephros ; though,
as shewn on p. 822, the pronephros of Lepidosteus has primitive
characters not retained by Teleostei.
 
(5) The early stages in the development of the vertebral
column (vide p. 779).
 
In addition to these, so to speak, purely embryonic characters
there are not a few important adult characters :
 
(i) The continuity of the oviducts with the genital glands.
 
1 The features enumerated above are not in all cases confined to Lepidosteus and
Teleostei, hut are always eminently characteristic of the latter.
 
 
 
STRUCTURE AND DEVELOPMPINT OF LEPIDOSTEUS. 837
 
(2) The small size of the pancreas, and the presence of
numerous so-called pancreatic caeca.
 
(3) The somewhat coiled small intestine.
 
(4) Certain characters of the brain, e.g., the large size of
the cerebellum ; the presence of the so-called lobi inferiores
on the infundibulum ; and of tori semicirculares in the midbrain.
 
In spite of the undoubtedly important list of features to which
we have just called attention, a list containing not less important
characters, both embryological and adult, separating Lepidosteus
from the Teleostei, can be drawn up :
 
(1) The character of the truncus arteriosus.
 
(2) The fact of the genital ducts joining the ureters.
 
(3) The presence of vasa efferentia in the male carrying the
semen from the testes to the kidney, and through the tubules of
the latter into the kidney duct.
 
(4) The 'presence of a well-developed opercular gill.
 
(5) The presence of a spiral valve; though this character
may possibly break down with the extension of our knowledge.
 
(6) The typical Ganoid characters of the thalamencephalon
and the cerebral hemispheres (vide pp. 769 and 770).
 
(7) The chiasma of the optic nerves.
 
(8) The absence of a pecten, and presence of a vascular membrane between the vitreous humour and the retina.
 
(9) The opisthoccelous form of the vertebrae.
 
(10) The articulation of the ventral parts of the haemal arches
of the tail with processes of the vertebral column.
 
(u) The absence of a division of the muscles into dorsolateral and ventro-lateral divisions.
 
(12) The complete segmentation of the ovum.
 
The list just given appears to us sufficient to demonstrate
that Lepidosteus cannot be classed with the Teleostei ; and we
hold that Muller's view is correct, according to which Lepidosteus
is a true Ganoid.
 
The existence of the Ganoids as a distinct group has, however, recently been challenged by so distinguished an Ichthyologist as Glinther, and it may therefore be well to consider how
far the group as defined by Mliller is a natural one for living
 
 
 
838 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
forms 1 , and how far recent researches enable us to improve upon
Mtiller's definitions. In his classical memoir (No. 13) the characters of the Ganoids are thus shortly stated :
 
" These Fishes are either provided with plate-like angular or
rounded cement-covered scales, or they bear osseous plates, or
are quite naked. The fins are often, but not always, beset with
a double or single row of spinous plates or splints. The caudal
fin occasionally embraces in its upper lobe the end of the vertebral column, which may be prolonged to the end of the upper
lobe. Their double nasal openings resemble those of Teleostei.
The gills are free, and lie in a branchial cavity under an operculum, like those of Teleostei. Many of them have an accessory
organ of respiration, in the form of an opercular gill, which is
distinct from the pseudobranch, and can be present together
with the latter ; many also have spiracles like Elasmobranchii.
They have many valves in the stem of the aorta like the latter,
also a muscular coat in the stem of the aorta. Their ova are
transported from the abdominal cavity by oviducts. Their optic
nerves do not cross each other. The intestine is often provided
with a spiral valve, like Elasmobranchii. They have a swimming-bladder with a duct, like many Teleostei. Their pelvic
fins are abdominal.
 
" If we include in a definition only those characters which
are invariable, the Ganoids may be shortly defined as being
those Fish with numerous valves to the stem of the aorta, which
is also provided with a muscular coat ; with free gills and an
operculum, and with abdominal pelvic fins."
 
To these distinctive characters, he adds in an appendix to
his paper, the presence of the spiral valve, and the absence of a
processus falciformis and a choroid gland.
 
To the distinctive set of characters given by Miiller we may
probably add the following :
 
(1) Oviducts and urinary ducts always unite, and open by a
common urinogenital aperture behind the anus.
 
(2) Skull hyostylic.
 
1 We do not profess to be able to discuss this question for extinct forms of Fish,
though of course it is a necessary consequence of the theory of descent that the various
groups should merge into each other as we go back in geological time.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 839
 
(3) Segmentation complete in the types so far investigated,
though perhaps Amia may be found to resemble the Teleostei in
this particular.
 
(4) A pronephros of the Teleostean type present in the larva.
 
(5) Thalamencephalon very large and well developed.
 
(6) The ventricle in the posterior part of the cerebrum is not
divided behind into lateral halves, the roof of the undivided part
being extremely thin.
 
(7) Abdominal pores always present.
 
The great number of characters just given are amply sufficient
to differentiate the Ganoids as a group ; but, curiously enough,
the only characters amongst the whole series which have been
given, which can be regarded as peculiar to the Ganoids, are (i)
the characters of the brain, and (2) the fact of the oviducts and
kidney ducts uniting together and opening by a common pore to
the exterior.
 
This absence of characters peculiar to the Ganoids is an indication of how widely separated in organization are the different
members of this great group.
 
At the same time, the only group with which existing Ganoids
have close affinities is the Teleostei. The points they have in
common with the Elasmobranchii are merely such as are due to
the fact that both retain numerous primitive Vertebrate characters 1 , and the gulf which really separates them is very wide.
 
There is again no indication of any close affinity between the
Dipnoi and, at any rate, existing Ganoids.
 
Like the Ganoids, the Dipnoi are no doubt remnants of a
very primitive stock ; but in the conversion of the air-bladder
into a true lung, the highly specialized character of their limbs 2 ,
their peculiar autostylic skulls, the fact of their ventral nasal
openings leading directly into the mouth, their multisegmented
bars (interspinous bars), directly prolonged from the neural and
haemal arches and supporting the fin-rays of the unpaired dorsal
and ventral fins, and their well-developed cerebral hemispheres,
 
1 As instances of this we may cite (i) the spiral valve; (2) the frequent presence
of a spiracle; (3) the frequent presence of a communication between the pericardium
and the body-cavity ; (4) the heterocercal tail.
 
2 Vide F. M. Balfour, "On the Development of the Skeleton of the Paired Fins
of Elasmobranchs," Proc. Zool. Soc., 1881 [This edition, No. XX.].
 
 
 
840 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
very unlike those of Ganoids and approaching the Amphibian
type, they form a very well-defined group, and one very distinctly separated from the Ganoids.
 
No doubt the Chondrostean Ganoids are nearly as far removed from the Teleostei as from the Dipnoi, but the links
uniting these Ganoids with the Teleostei have been so fully preserved in the existing fauna of the globe, that the two groups
almost run into each other. If, in fact, we were anxious to make
any radical change in the ordinary classification of Fishes, it
would be by uniting the Teleostei and Ganoids, or rather constituting the Teleostei into one of the sub-groups of the Ganoids,
equivalent to the Chondrostei. We do not recommend such an
arrangement, which in view of the great preponderance of the
Teleostei amongst living Fishes would be highly inconvenient,
but the step from Amia to the Teleostei is certainly not so great
as that from the Chondrostei to Amia, and is undoubtedly less
than that from the Selachii to the Holocephali.
 
 
 
LIST OF MEMOIRS ON THE ANATOMY AND DEVELOPMENT OF
LEPIDOSTEUS.
 
1. Agassiz, A. "The Development of Lepidosteus? Part I., Proc.
Amer. A cad. Arts and Sciences, Vol. xiv. 1879.
 
2. Agassiz, L. Recherches s. I. Poissons Fossiles. Neuchatel. 1833
45
3. Boas, J. E. " Ueber Herz u. Arterienbogen bei Ceradotus u. Protopterus" Morphol. Jahrbitch, Vol. VI. 1880.
 
4. Davidoff, M. von. " Beitrage z. vergleich. Anat. d. hinteren Gliedmassen d. Fische," Morphol. Jahrbuch, Vol. vi. 1880.
 
5. Gegenbaur, C. Untersuch. z. vergleich. Anat. d. Wirbelthiere,
Heft II., Schultergiirtel d. Wirbelthiere. Brnstflosse der Fische. Leipzig,
1865.
 
6. Gegenbaur, C. "Zur Entwick. d. Wirbelsaule d. Lepidosteus, &c."
Jenaische Zeitschrift, Vol. ill. 1867.
 
7. Hertwig, O. "Ueber d. Hautskelet d. Fische (Lepidosteus u.
Polypterus)? Morphol. Jahrbuch, Vol. V. 1879.
 
8. H ceven, Van der. " Ueber d. zellige Schvvimmblase d. Lepidosteus."
M tiller's Archiv, 1841.
 
 
 
STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 84!
 
9. Hyrtl, J. "Ueber d. Schwimmblase von Lepidosteus osseus" Sitz.
d. Wiener Akad. Vol. vin. 1852.
 
10. Hyrtl, J. "Ueber d. Pori abdominales, d. Kiemen-Arterien, u. d.
Glandula thyroidea d. Ganoiden," Sitz. d. Wiener Akad. Vol. VIII. 1852.
 
u. H y r 1 1, J . Ueber d, Zussammenhang d. Geschlechts u. Harnwerkzeuge
bet d. Ganoiden, Wien, 1855.
 
12. Kolliker, A. Ueber d. Ende d. Wirbelsaitle b. Ganoiden, Leipzig,
1860.
 
13. M tiller, J. "Ueber d. Bau u. d, Grenzen d. Ganoiden," Berlin
Akad. 1844.
 
14. Schneider, H. "Ueber d. Augenmuskelnerven d. Ganoiden,"
Jcnaische Zeitschrift, Vol. XV. 1881.
 
15. Wilder, Burt G. " Notes on the North American Ganoils, Amia,
Lepidosteus, Acipenser, and Polyodon? Proc. Amer. Assoc.for the Advancement of Science, 1875.
 
 
 
LIST OF REFERENCE LETTERS.
 
a. Anus, a b. Air-bladder, a b'. Aperture of air-bladder into throat, ac. Anterior commissure, af. Anal fin. al. Alimentary canal, ao. Aorta, ar. Artery.
ati. Auditory pit. b. Brain, be. Body-cavity, bd. Bile duct. bd'. Aperture of
bile duct into duodenum, bl. Coalesced portion of segmental ducts, forming urinogenital bladder. bra. Branchial arches, brc. Branchial clefts. c. Pyloric caeca.
c'. Apertures of caeca into duodenum. cb. Cerebellum. c</v. Cardinal vein.
ce. Cerebrum : in figs. 47 A and B, anterior lobe of cerebrum, ce'. Posterior lobe of
cerebrum, cf. Caudal fin. en. Centrum, ch. Choroidal fissure, crv. Circular
vein of vascular membrane of eye. csh. Cuticular sheath of notochord. cv. Caudal
vein. d. Duodenum, d c. Dorsal cartilage of neural arch. df. Dermal fin-rays.
dl. Dorsal lobe of caudal fin. dlf. Dorsal fin. e. Eye. ed. Epidermis, ep. Epiblast. fb. Fore-brain, fe. Pyriform bodies surrounding the zona radiata of the
ovum, probably the remains of epithelial cells, gb. Gall-bladder, gd. Genital duct.
gl. Glomerulus. gr. Genital ridge. h. Heart, h a. Haemal arch. h b. Hindbrain, h c. Head-cavity, hp d. Hepatic duct, h m. Hyomandibular cleft, h op.
Operculum. hy. Hypoblast ; in fig. 10, hyoid arch. hyl. Hyaloid membrane, ic.
Intercalated cartilaginous elements of the neural arches, in. Infundibulum. ir. Iris.
is. Interspinous cartilage or bones, iv. Sub-intestinal vein. ivr. Intervertebral
ring of cartilage, k. Kidney. /. Lens. / c . Longitudinal canal, formed by union of
the vasa efferentia. I in. Lobi inferiores. //. Ligamentum longitudinale superius.
/;-. Liver. It. Lateral line. ly. Lymphatic body in front of kidney, m. Mouth.
m b. Mid-brain. m c. Medullary cord. m el. Membrana elastica externa. vies.
Mesorchium. mn. Mandible, md. and mo. Medulla oblongata. ms. Mesoblast.
na. Neural arch. na'. Dorsal element of neural arch. nc. Notochord. nve. Network formed by vasa efferentia on inner face of testis. od. Oviduct, oif. Aperture
of oviduct into bladder, ol. Nasal pit or aperture, olf. Olfactory lobe. op. Optic
vesicle, opch. Optic chiasma. op I. Optic lobes, opth. Optic thalami. or ep.
 
B. 54
 
 
 
842 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
Oral epithelium, ov. Ovary. p. Pancreas, pc. Pericardium, pcf. Pectoral fin.
/ ch. Pigmented layer of choroid. pf. Peritoneal funnel of segmental tube of mesonephros. pfp- Peritoneal funnel leading into pronephric chamber, p g. Pectoral
girdle, pi/. Pelvic fin. pn. Pineal gland, po. Primitive germinal cells, pr.
Mesoblastic somite, prc. Pronephric chamber, prn. Pronephros. prn'. Opening
of pronephros into pronephric chamber, ft. Pituitary body. py. Pyloric valve.
p z. Parietal zone of blastoderm, r. Rostrum. rb. Rib. re. Rectum, s. Spleen.
-s c. Seminal vessels passing from the longitudinal canal into the kidney, s d. Suctorial disc. sg. Segmental or archinephric duct, sg t. Segmental tubules, sh.
Granular outer portion of the sheath of the notochord in the vertebral regions, s mx.
Superior maxillary process. s nc. Sub-notochordal rod. so. Somatic mesoblast.
sp. Splanchnic mesoblast. sp n. Spinal nerve, sp v. Spiral valve, st. Stomach.
st. Seminal tubes of the testis. sup. Suctorial papillae, t. Testis. th. Thalamencephalon. thl. Lobes of the roof of the thalamencephalon. tr. Trabeculse. ug.
Urinogenital aperture, v. Ventricle, v e. Vasa efferentia. v h. Vitreous humour.
v 1. Ventral lobe of the caudal fin. v mi. Ventral mesentery, vn. Vein. vs. Bloodvessel, v sh. Vascular sheath between the hyaloid membrane and the vitreous
humour, v th. Vesicle of the thalamencephalon. x. Groove in epiblast, probably
formed in process of hardening, y. Yolk. z. Commissure in front of pineal gland.
zr. Outer striated portion of investing membrane (zona radiata) of ovum. zr 1 . Inner
non-striated portion of investing membrane of ovum. I. Olfactory nerve. II. Optic
nerve. III. Oculomotor nerve. V. Trigeminal nerve. VIII. Facial and auditory
nerves.
 
 
 
EXPLANATION OF PLATES 3442.
PLATE 34.
 
Figs, i 4. Different stages in the segmentation of the ovum.
Fig. T. Ovum with a single vertical furrow, from above.
Fig. i. Ovum with two vertical furrows, from above.
Fig. 3. Side view of an ovum with a completely formed blastodermic disc.
Fig. 4. The same ovum as fig. 3, from below, shewing four vertical furrows
nearly meeting at the vegetative pole.
 
Figs. 5 10. External views of embryos up to time of hatching.
 
Fig. 5. Embryo, 3^5 millims. long, third day after impregnation.
Fig. 6. Embryo on the fifth day after impregnation.
Fig. 7. Posterior part of same embryo as fig. 6, shewing tail swelling.
Fig. 8. Embryo on the sixth day after impregnation.
Fig. 9. Embryo on the seventh day after impregnation.
Fig. 10. Embryo on the eleventh day after impregnation (shortly before
hatching).
 
Fig. ii. Head of embryo about the same age as fig. 10, ventral aspect.
 
Fig. 12. Side view of a larva about n millims. in length, shortly after hatching.
 
Fig. 13. Head of a larva about the same age as fig. 12, ventral aspect.
 
 
 
EXPLANATION OF PLATES 35, 36. 843
 
Fig. 14. Side view of a larva about 15 millims. long, five days after hatching.
Fig. 15. Head of a larva 23 millims. in length.
Fig. 16. Tail of a larva n centims. in length.
 
Fig. 17. Transverse section through the egg-membranes of a just-laid ovum.
We are indebted to Professor W. K. Parker for figs. 12, 14 and 15.
 
PLATE 35.
 
Figs. 18 22. Transverse sections of embryo on the third day after impregnation.
 
Fig. 18. Through head, shewing the medullary keel.
 
Fig. 19. Through anterior part of trunk.
 
Fig. 20. Through same region as fig. 19, shewing a groove (x) in the
epiblast, probably artificially formed in the process of hardening.
 
Fig. 21. Through anterior part of tail region, shewing partial fusion of
layers.
 
Fig. 22. Through posterior part of tail region, shewing more complete
fusion of layers than fig. 2 1 .
 
Figs. 23 25. Transverse sections of an embryo on the fifth day after impregnation.
 
Fig. 23. Through fore-brain and optic vesicles.
 
Fig. 24. Through hind -brain and auditory pits.
 
Fig. 25. Through anterior part of trunk.
 
Figs. 26 27. Tranverse sections of the head of an embryo on the sixth day after
impregnation.
 
Fig. 26. Through fore-brain and optic vesicles.
 
Fig. 27. Through hind-brain and auditory pits.
 
PLATE 36.
 
Figs. 28 29. Transverse sections of the trunk of an embryo on the sixth day
after impregnation.
 
Fig. 28. Through anterior part of trunk (from a slightly older embryo than
 
the other sections of this stage).
 
Fig. 29. Slightly posterior to fig. 28, shewing formation of segmental duct
as a fold of the somatic mesoblast.
 
Fig. 30. Longitudinal horizontal section of embryo on the sixth day after impregnation, passing through the mesoblastic somites, notochord, and medullary canal.
 
Figs. 31 34. Transverse sections through an embryo on the seventh day after
impregnation.
 
Fig. 31. Through anterior part of trunk.
 
Fig. 32. Through the trunk somewhat behind fig. 31.
 
F'g- 33- Through tail region.
 
Fig- 34- Further back than fig. 33, shewing constriction of tail from the
yolk.
 
54-2
 
 
 
844 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
 
 
Figs- 35 37- Transverse sections through an embryo on the eighth day after
impregnation.
 
Fig- 35- Through fore-brain and optic vesicles.
 
Fig. 36. Through hind-brain, shewing closed auditory pits, &c.
 
Fig- 37- Through anterior part of trunk.
 
Fig. 38. Section through tail of an embryo on the ninth day after impregnation.
 
PLATE 37.
 
Fig- 39- Section through the olfactory involution and part of fore-brain of a
larva on the ninth day after impregnation, shewing olfactory nerve.
 
Fig. 40. Section through the anterior part of the head of the same larva, shewing
pituitary involution.
 
Figs. 41 43. Transverse sections through an embryo on the eleventh day after
impregnation.
 
Fig. 41. Through fore-part of head, shewing the pituitary body still con
nected with the oral epithelium.
Fig. 42. Slightly further back than fig. 41, shewing the pituitary body
 
constricted off from the oral epithelium.
Fig. 43. Slightly posterior to fig. 42, to shew olfactory involution, eye,
 
and hyomandibular cleft.
 
Fig. 44. Longitudinal section of the head of an embryo of 1 5 millims. in length,
a few days after hatching, shewing the structure of the brain.
 
Fig. 45. Longitudinal section of the head of an embryo, about five weeks after
hatching, 26 millims. in length, shewing the structure of the brain. In the front part
of the brain the section passes slightly to one side of the median line.
 
Figs. 46 A to 46 G. Transverse sections through the brain of an embryo 25
millims. in length, about a month after hatching.
 
Fig. 46 A. Through anterior lobes of cerebrum.
 
Fig. 46 B. Through posterior lobes of cerebrum.
 
Fig. 46 C. Through thalamencephalon.
 
Fig. 46 D. Through optic thalami and optic chiasma.
 
Fig. 46 E. Through optic lobes and infundibulum.
 
Fig. 46 F. Through optic lobes and cerebellum.
 
Fig. 46 G. Through optic lobes and cerebellum, slightly in front of fig. 46 F.
 
PLATE 38.
 
Figs. 47 A, B, C. Figures of adult brain.
Fig. 47 A. From the side.
Fig. 47 B. From above.
Fig. 47 C. From below.
 
Fig. 48. Longitudinal vertical section through the eye of an embryo, about a
week after hatching, shewing the vascular membrane surrounding the vitreous
humour.
 
 
 
EXPLANATION OF PLATES 38, 39. 845
 
Fig. 49. Diagram shewing the arrangement of the vessels in the vascular membrane of the vitreous humour of adult eye.
 
Fig. 50. Capillaries of the same vascular membrane.
 
Fig. 51. Transverse section through anterior part of trunk of an embryo on the
ninth day after impregnation, shewing the pronephros and pronephric chamber.
 
Fig. 52. Transverse section through the region of the stomach of an embryo
15 millims. in length, shortly after hatching, to shew the glomerulus and peritoneal
funnel of pronephros.
 
Fig. 53- Transverse section through posterior part of the body of an embryo,
about a month after hatching, shewing the structure of the mesonephros, the spiral
valve, &c.
 
 
 
PLATE 39.
 
Figs. 54, 55, 56, and 57 are a series of transverse sections through the genital
ridge and mesonephros of one side from a larva of 1 1 centims.
 
Fig. 54. Section of the lymphatic organ which lies in front of the mesonephros.
 
Fig. 55. Section near the anterior end of the mesonephros, where the
genital sack is completely formed.
 
Fig- 56. Section somewhat further back, shewing the mode of formation of
the genital sack.
 
Fig- 57- Section posterior to the above, the formation of the genital sack not
having commenced, and the genital ridge with primitive germinal cells projecting freely into the body-cavity.
 
Fig. 58 A. View of the testis, mesorchium, and duct of the kidney of the left side
of an adult male example of Lepidosteus, 60 centims. in length, shewing the vasa
efferentia and the longitudinal canal at the base of the mesorchium. The kidney
ducts have been cut open posteriorly to shew the structure of the interior.
 
Fig. 58 B. Inner aspect of the posterior lobe of the testis from the same example,
to shew the vasa efferentia forming a network on the face of the testis.
 
Figs- 59 A and B. Two sections shewing the structure and relations of the
efferent ducts of the testis in the same example.
 
Fig. 59 A. Section through the inner aspect of a portion of the testis and
mesorchium, to shew the network of the vasa efferentia (n v e)
becoming continuous with the seminal tubes (s t). The granular matter nearly filling the vasa efferentia and the seminal
tubes represent the spermatozoa.
 
Fig. 59 B. Section through part of the kidney and its duct and the longitudinal canal (Ic) at the base of the mesorchium. Canals (s c)
are seen passing off from the latter, which enter the kidney and
join the uriniferous tubuli. Some of the latter (as well as the
seminal tubes) are seen to be filled with granular matter,
which we believe to be the remains of spermatozoa.
 
 
 
846 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS.
 
Fig. 60. Diagram of the urinogenital organs of the left side of an adult female
example of Lepidosteus 100 centims. in length. This figure shews the oviduct (od)
continuous with the investment of the ovary, opening at oa into the dilated part of
the kidney duct (segmental duct). It also shews the segmental duct and the junction
of the latter with its fellow of the right side to form the so-called bladder, this part
being represented as cut open. The kidney (k) and lymphatic organ (/y).in front of it
are also shewn.
 
PLATE 40.
 
Fig. 61. Transverse section through the developing pancreas (/) of a larva n
millims. in length.
 
Fig. 62. Longitudinal section through portions of the stomach, liver, and duodenum of an embryo about a month after hatching, to shew the relations of the pancreas
(/) to the surrounding parts.
 
Fig. 63. External view of portions of the liver, stomach, duodenum, &c., of a
young Fish, u centims. in length, to shew the pancreas (/).
 
Fig. 64. Transverse section through the anterior part of the trunk of an embryo,
about a month after hatching, shewing the connection of the air-bladder with the
throat (a b').
 
Fig. 65. Transverse section through the same embryo as fig. 64 further back,
shewing the posterior part of the air-bladder (a b).
 
Fig. 66. Viscera of an adult female, 100 centims. in length, shewing the alimentary canal with its appended glands in natural position, and the air-bladder with its
aperture into the throat (a b'). The proximal part of the duodenum and the terminal
part of the intestine are represented as cut open, the former to shew the pyloric valve
and the apertures of the pyloric caeca and bile duct, and the latter to shew the spiral
valve.
 
This figure was drawn for us by Professor A. C. Haddon.
 
PLATE 41.
 
Fig. 67. Transverse section through the tail of an advanced larva, shewing the
neural and haemal processes, the independently developed interneural and interhaemal
elements (is), and the commencing dermal fin-rays (df).
 
Fig. 68. Side view of the tail of a larva, 21 millims. in length, dissected so as to
shew the structure of the skeleton.
 
Fig. 69. Longitudinal horizontal section through the vertebral column of a larva,
5-5 centims. in length, on the level of the haemal arches, shewing the intervertebral
rings of cartilage continuous with the arches, the vertebral constriction of the notochord, &c.
 
Figs. 70 and 71. Transverse sections through the vertebral column of a larva of
5 '5 centims. The red represents bone, and the blue cartilage.
 
Fig. 70. Through the vertebral region, shewing the neural and haemal
 
arches, the notochordal sheath, &c.
 
Fig. 71. Through the intervertebral region, shewing the intervertebral cartilage.
 
 
 
EXPLANATION OF PLATES 41, 43. 847
 
Figs. 72 and 73. Transverse sections through the trunk of a larva of 5-5 centims.
to shew the structure of the ribs and hremal arches.
 
Fig. 72. Through the anterior part of the trunk.
Fig. 73. Through the posterior part of the trunk.
 
PLATE 42.
 
Figs. 74 76. Transverse sections through the trunk of the same larva as figs. 72
and 73.
 
Fig. 74. Through the posterior part of the trunk (rather further back than
 
fig- 73)
Fig. 75. Through the anterior part of the tail.
Fig. 76. Rather further back than fig. 75.
 
Fig. 77. Longitudinal horizontal section through the vertebral column of a larva
of ii centims., passing through the level of the haemal arches, and shewing the intervertebral constriction of the notochord, the ossification of the cartilage, &c.
 
Fig. 78. Transverse section through a vertebral region of the vertebral column of
a larva 1 1 centims. in length.
 
Fig. 79. Transverse section through an intervertebral region of the same larva as
% 78.
 
Fig. 80. Side view of two trunk vertebrae of an adult Lepidosteus.
 
Fig. 8 1. Front view of a trunk vertebra of adult.
 
In figures 80 and 81 the red does not represent bone as in the other figures, but
simply the ligamentum longitudinale superius.
 
 
 
XXIII. ON THE NATURE OF THE ORGAN IN ADULT TE
LEOSTEANS AND GANOIDS, WHICH IS USUALLY REGARDED AS THE HEAD-KIDNEY OR PRONEPHROs 1 .
 
 
 
WHILE working at the anatomy of Lepidosteus I was led to
doubt the accuracy of the accepted accounts of the anterior part
of the kidneys in this' 2 and in allied species of Fishes. In order
to test my doubts I first examined the structure of the kidneys
in the Sturgeon (Acipenser), of which I fortunately had a wellpreserved specimen.
 
The bodies usually described as the kidneys consist of two
elongated bands, attached to the dorsal wall of the abdomen,
and extending for the greater part of the length of the abdominal cavity. In front each of these bands first becomes considerably narrowed, and then expands and terminates in a great
dilatation, which is usually called the head-kidney. Along the
outer border of the hinder part of each kidney is placed a wide
ureter, which ends suddenly in the narrow part of the body,
some little way behind the head-kidney. To the naked eye"
there is no distinction in structure between the part of the socalled kidney in front of the ureter and that in the region of the
ureter. Any section through the kidney in the region of the
ureter suffices to shew that in this part the kidney is really
formed of uriniferous tubuli with numerous Malpighian bodies.
Just in front, however, of the point where the ureter ends the
true kidney substance rapidly thins out, and its place is taken
by a peculiar tissue formed of a trabecular work filled with cells,
 
1 From the Quarterly Journal of Microscopical Science, Vol. XXII., 1882.
 
2 I am about to publish, in conjunction with Mr Parker, a full account of the
anatomy and development of Lepidosteus [No. XXII. of this edition], and shall
therefore in this paper make no further allusion to it.
 
 
 
HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS. 849
 
which I shall in future call lymphatic tissue. Thus the wliole
of that part of the apparent kidney in front of the ureter, including
the whole of the so-called head-kidney, is simply a great mass of
lymphatic tissue, and does not contain a single urinifcrous tubule
or MalpigJdan body,
 
The difference in structure between the anterior and posterior
parts of the so-called kidney, although not alluded to in most
modern works on the kidneys, appears to have been known to
Stannius, at least I so interpret a note of his in the second edition of his Comparative Anatomy, p. 263, where he describes the
kidney of the Sturgeon as being composed of two separate parts,
viz. a spongy vascular substance (no doubt the so-called headkidney) and a true secretory substance.
 
After arriving at the above results with reference to the
Sturgeon I proceeded to the examination of the structure of the
so-called head-kidney in Teleostei.
 
I have as yet only examined four forms, viz. the Pike (Esox
lucius), the Smelt (Osmerus eperlanus], the Eel (Anguilla anguilld), and the Angler (Lophius piscatorius).
 
The external features of the apparent kidney of the Pike
have been accurately described by Hyrtl 1 . He says: "The
kidneys extend from the second trunk vertebra to the end of
the abdominal cavity. Their anterior extremities, w r hich have
the form of transversely placed coffee beans, are united together,
and lie on the anterior end of the swimming bladder. The continuation of the kidney backwards forms two small bands, separated from each other by the whole breadth of the vertebral
column. They gradually, however, increase in breadth, so that
about the middle of the vertebral column they unite together
and form a single symmetrical, keel-shaped body," &c.
 
The Pike I examined was a large specimen of about 58
centimetres in length, and with an apparent kidney of about 25^
centimetres. The relations of lymphatic tissue and kidney
tissue were much as in the Sturgeon. The whole of the anterior swelling, forming the so-called head-kidney, together with
a considerable portion of the part immediately behind, forming
not far short of half the whole length of the apparent kidney,
 
1 "Das Uropoetische System der Knochenfische," Si'.z. d. Wieu. Akad., 1850.
 
 
 
850 HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS.
 
was entirely formed of lymphatic tissue. The posterior part of
the kidney was composed of true kidney substance, but even at
1 6 centimetres from the front end of the kidney the lymphatic
tissue formed a large portion of the whole.
 
A rudiment of the duct of the kidney extended forwards for
a short way into the lymphatic substance beyond the front part
of the functional kidney.
 
In the Smelt (Osmerus eperlamis] the kidney had the typical
Teleostean form, consisting of two linear bands stretching for
the whole length of the body-cavity, and expanding into a great
swelling in front on the level of the ductus Cuvieri, forming the
so-called head-kidney. The histological examination of these
bodies shewed generally the same features as in the case of the
Sturgeon and Pike. The posterior part was formed of the
usual uriniferous tubuli and Malpighian bodies. The anterior
swollen part of these bodies, and the part immediately following, were almost wholly formed of a highly vascular lymphatic
tissue ; but in a varying amount in different examples portions
of uriniferous tubules were present, mainly, however, in the
region behind the anterior swelling. In some cases I could find
no tubules in the lymphatic tissue, and in all cases the number
of them beyond the region of the well-developed part of the
kidney was so slight, that there can be little doubt that they are
functionless remnants of the anterior part of the larval kidney.
Their continuation into the anterior swelling, when present, consisted of a single tube only.
 
In the Eel (Anguilla anguilla), which, however, I have not
examined w r ith the same care as the Smelt, the true excretory
part of the kidney appears to be confined to the posterior portion, and to the portion immediately in front of the anus, the
whole of the anterior part of each apparent kidney, which is
not swollen in front, being composed of lymphatic tissue.
 
LopJiius piscatorius is one of the forms which, according to
Hyrtl 1 , is provided with a head-kidney only, i.e. with that part
of the kidney which corresponds with the anterior swelling of
the kidney of other types. For this reason I was particularly
anxious to investigate the structure of its kidneys.
 
1 "Das Uropoetische System der Knochenfische," Sitz. d. Wien. Akad., 1850.
 
 
 
HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS. 851
 
Each of these bodies forms a compact oval mass, with the
ureter springing from its hinder extremity, situated in a forward
position in the body-cavity. Sections through the kidneys
shewed that they were throughout penetrated by uriniferous
tubules, but owing to the bad state of preservation of my specimens I could not come to a decision as to the presence of
Malpighian bodies. The uriniferous tubules were embedded in
lymphatic tissue, similar to that which forms the anterior part of
the apparent kidneys in other Teleostean types.
 
With reference to the structure of the Teleostean kidneys,
the account given by Stannius is decidedly more correct than
that of most subsequent writers. In the note already quoted he
gives it as his opinion that there is a division of the kidney into
the same two parts as in the Sturgeon, viz. into a spongy
vascular part and a true secreting part ; and on a subsequent
page he points out the absence or poverty of the uriniferous
tubules in the anterior part of the kidney in many of our native
Fishes.
 
Prior to the discovery that the larvae of Teleosteans and
Ganoids were provided with two very distinct excretory organs,
viz. a pronephros or head-kidney, and a mesonephros or Wolffian body, which are usually separated from each other by a
more or less considerable interval, it was a matter of no very
great importance to know whether the anterior part of the socalled kidney was a true excretory organ. In the present state
of our knowledge the question is, however, one of considerable
interest.
 
In the Cyclostomata and Amphibia the pronephros is a
purely larval organ, which either disappears or ceases to be
functionally active in the adult state.
 
, Rosenberg, to whom the earliest satisfactory investigations
on the development of the Teleostean pronephros are due, stated
that he had traced in the Pike (Esox Indus) the larval organ into
the adult part of the kidney, called by Hyrtl the pronephros ;
and subsequent investigators have usually assumed that the socalled head-kidney of adult Teleosteans and Ganoids is the
persisting larval pronephros.
 
We have already seen that Rosenberg was entirely mistaken
on this point, in that the so-called head-kidney of the adult is
 
 
 
852 HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS.
 
not part of the true kidney. From my own studies on young
Fishes I do not believe that the oldest larvae investigated by
Rosenberg were sufficiently advanced to settle the point in
question ; and, moreover, as Rosenberg had no reason for doubting that the so-called head-kidney of the adult was part of the
excretory organ, he does not appear to have studied the histological structure of the organ which he identified with the embryonic pronephros in his oldest larva.
 
The facts to which I have called attention in this paper
demonstrate that in the Sturgeon the larval pronephros undoubtedly undergoes atrophy before the adult stage is reached.
The same is true for Lepidosteus, and may probably be stated
for Ganoids generally.
 
My observations on Teleostei are clearly not sufficiently extensive to prove that the larval pronephros never persists in this
group. They appear to me, however, to shew that in the normal
types of Teleostei the organ usually held to be the pronephros
is actually nothing of the kind.
 
A different interpretation might no doubt be placed upon
my observations on Lophius piscatorius, but the position of the
kidney in this species appears to me to be far from affording a
conclusive proof that it is homologous with the anterior swelling
of the kidney of more normal Teleostei.
 
When, moreover, we consider that Lophius, and the other
forms mentioned by Hyrtl as being provided with a head-kidney
only, are all of them peculiarly modified and specialized types
of Teleostei, it appears to me far more natural to hold that their
kidney is merely the ordinary Teleostean kidney, which, like
many of their other organs, has become shifted in position, than
to maintain that the ordinary excretory organ present in other
Teleostei has been lost, and that a larval organ has been retained,
which undergoes atrophy in less specialized Teleostei.
 
As the question at present stands, it appears to me that the
probabilities are in favour of there being no functionally active
remains of the pronephros in adult Teleostei, and that in any
case the burden of proof rests with those who maintain that
such remnants are to be foun,d.
 
The general result of my investigations is thus to render it
probable that the pronephros, though found in the larvce or em
 
 
HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS. 853
 
bryos of almost all the IchtJiyopsida, except the Elasmobranchii, is
always a purely larval organ, which never constitutes an active
part of the excretory system in the adult state.
 
This conclusion appears to me to add probability to the view
of Gegenbaur that the pronephros is the primitive excretory
gland of the Chordata ; and that the mesonephros or Wolffian
body, by which it is replaced in existing Ichthyopsida, is phylogenetically a more recent organ.
 
In the preceding pages I have had frequent occasion to
allude to the lymphatic tissue which has been usually mistaken
for part of the excretory organ. This tissue is formed of trabecular work, like that of lymphatic glands, in the meshes of
which an immense number of cells are placed, which may fairly
be compared with the similarly placed cells of lymphatic glands.
In the Sturgeon a considerable number of cells are found with
peculiar granular nuclei, which are not found in the Teleostei.
In both groups, but especially in the Teleostei, the tissue is
highly vascular, and is penetrated throughout by a regular
plexus of very large capillaries, which appear to have distinct
walls, and which pour their blood into the posterior cardinal
vein as it passes through the organ. The relation of this tissue
to the lymphatic system I have not made out.
 
The function of the tissue is far from clear. Its great
abundance, highly vascular character, and presence before the
atrophy of the pronephros, appear to me to shew that it cannot
be merely the non-absorbed remnant of the latter organ. From
its size and vascularity it probably has an important function ;
and from its structure this must either be the formation of lymph
corpuscles or of blood corpuscles.
 
In structure it most resembles a lymphatic gland, though, till
it has been shewn to have some relation to the lymphatic system,
this can go for very little.
 
On the whole, I am provisionally inclined to regard it as a
form of lymphatic gland, these bodies being not otherwise represented in fishes.
 
 
 
XXIV. A RENEWED STUDY OF THE GERMINAL LAYERS OF
THE CHICK. BY F. M. BALFOUR AND F. DEIGHTON'.
 
(With Plates 43, 44, 45-)
 
THE formation of the germinal layers in the chick has been
so often and so fully dealt with in recent years, that we consider
some explanation to be required of the reasons which have induced us to add to the long list of memoirs on this subject.
Our reasons are twofold. In the first place the principal results
we have to record have already been briefly put forward in a
Treatise on Comparative Embryology by one of us ; and it seemed
desirable that the data on which the conclusions there stated
rest should be recorded with greater detail than was possible in
such a treatise. In the second place, our observations differ
from those of most other investigators, in that they were primarily made with the object of testing a theory as to the nature
of the primitive streak. As such they form a contribution to
comparative embryology ; since our object has been to investigate how far the phenomena of the formation of the germinal
layers in the chick admit of being compared with those of lower
and less modified vertebrate types.
 
We do not propose to weary the reader by giving a new
version of the often told history of the views of various writers
on the germinal layers in the chick, but our references to other
investigators will be in the main confined to a comparison of
our results with those of two embryologists, who have published
their memoirs since our observations were made. One of them
is L. Gerlach, who published a short memoir 2 in April last, and
 
1 From the Quarterly Journal of Microscopical Science, Vol. xxn. N. S. 1882..
 
2 " Ueb. d. entodennale Entstehungsweise d. Chorda dorsal is," Biol. Ccntralblatt,
Vol. I. Nos. i and i.
 
 
 
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 855
 
the other is C. Roller, who has published his memoir l still more
recently. Both of them cover part of the ground of our investigations, and their results are in many, though not in all
points, in harmony with our own. Both of them, moreover, lay
stress on certain features in the development which have escaped
our attention. We desired to work over these points again, but
various circumstances have prevented our doing so, and we have
accordingly thought it best to publish our observations as they
stand, in spite of their incompleteness, merely indicating where
the most important gaps occur.
 
Our observations commence at a stage a few hours after
hatching, but before the appearance of the primitive streak.
 
The area pellucida is at this stage nearly spherical. In it
there is a large oval opaque patch, which is continued to the
hinder border of the area. This opaque patch has received the
name of the embryonic shield a somewhat inappropriate name,
since the structure in question has no very definite connection
with the formation of the embryo.
 
Roller describes, at this stage, in addition to the so-called
embryonic shield, a sickle-shaped opaque appearance at the
hinder border of the area pellucida.
 
We have not made any fresh investigations for the purpose
of testing Roller's statements on this subject.
 
Embryologists are in the main agreed as to the structure of
the blastoderm at this stage. There is (PL 43, Ser. A, I and 2)
the epiblast above, forming a continuous layer, extending over
the whole of the area opaca and area pellucida. In the former
its cells are arranged as a single row, and are cubical or slightly
flattened. In the latter the cells are more columnar, and form,
in the centre especially, more or less clearly, a double row ;
many of them, however, extend through the whole thickness of
the layer.
 
We have obtained evidence at this stage which tends to shew
that at its outer border the epiblast grows not merely by the
division of its own cells, but also by the addition of cells derived
from the yolk below. The epiblast has been observed to extend
itself over the yolk by a similar process in many invertebrate forms.
 
1 " Untersuch. lib. d. Blatterbildung im Hiihnerkeim," Archiv f. mikr. Atiat.
Vol. xx. 1 88 1.
 
 
 
856 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
 
Below the epiblast there is placed, in the peripheral part of
the area opaca, simply white yolk ; while in a ring immediately
outside and concentric with the area pellucida, there is a closelypacked layer of cells, known as the germinal wall. The constituent cells of this wall are in part relatively small, of a
spherical shape, with a distinct nucleus, and a granular and not
very abundant protoplasm ; and in part large and spherical,
filled up with highly refracting yolk particles of variable size,
which usually render the nucleus (which is probably present)
invisible (A, I and 2). This mass of cell rests, on its outer side,
on a layer of white yolk.
 
The sickle-shaped structure, visible in surface veins, is stated
by Koller to be due to a special thickening of the germinal wall.
We have not found this to be a very distinctly marked structure
in our sections.
 
In the region of the area pellucida there is placed below the
epiblast a more or less irregular layer of cells. This layer is
continuous, peripherally, with the germinal wall ; and is composed of cells, which are distinguished both by their flattened
or oval shape and more granular protoplasm from the epiblastcells above, to which, moreover, they are by no means closely
attached. Amongst these cells a few larger cells are usually
present, similar to those we have already described as forming
an important constituent of the germinal wall.
 
We have figured two sections of a blastoderm of this age
(Ser. A, i and 2) mainly to shew the arrangement of these cells.
A large portion of them, considerably more flattened than the
remainder, form a continuous membrane over the whole of the
area pellucida, except usually for a small area in front, where
the membrane is more or less interrupted. This layer is the
hypoblast (Jiy^). The remaining cells are interposed between
this layer and the epiblast. In front of the embryonic shield
there are either comparatively few or none of these cells present
(Ser. A, i), but in the region of the embryonic shield they are
very numerous (Ser. A, 2), and are, without doubt, the main
cause of the opacity of this part of the area pellucida. These
cells may be regarded as not yet completely differentiated segmentation spheres.
 
In many blastoderms, not easily distinguishable in surface
 
 
 
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 857
 
 
 
views from those which have the characters just described, the
hypoblastic sheet is often much less completely differentiated,
and we have met with other blastoderms, again, in which the
hypoblastic sheet was completely established, except at the
hinder part of the embryonic shield ; where, in place of it and
of the cells between it and the epiblast, there was only to be
found a thickish layer of rounded cells, continuous behind with
the germinal wall.
 
In the next stage, of which we have examined surface views
and sections, there is already a well-formed primitive streak.
 
The area pellucida is still nearly spherical, the embryonic
shield has either disappeared or become much less obvious, but
there is present a dark linear streak, extending from the posterior border of the area pellucida towards the centre, its total
length being about one third, or even less, of the diameter of
the area. This streak is the primitive streak. It enlarges considerably behind, where it joins the germinal wall. By Koller
and Gerlach it is described as joining the sickle-shaped structure already spoken of. We have in some instances found the
posterior end of the primitive streak extending laterally in the
form of two wings (PL 45, fig. L). These extensions are, no
doubt, the sickle ; but the figures given by Koller appear to us
somewhat diagrammatic. One or two of the figures of early
primitive streaks in the sparrow, given by Kupffer and Benecke 1 ,
correspond more closely with what we have found, except that
in these figures the primitive streak does not reach the end of
the area pellucida, which it certainly usually does at this early
stage in the chick.
 
Sections through the area pellucida (PL 43, Ser. B and c)
give the following results as to the structure of its constituent
parts.
 
The epiblast cells have undergone division to a considerable
extent, and in the middle part, especially, are decidedly more
columnar than at an earlier stage, and distinctly divided into two
rows, the nuclei of which form two more or less distinct layers.
 
In the region in front of the primitive streak the cells of the.
lower part of the blastoderm have arranged themselves as- a
1 " Photogramme d. Ontogcnie d. Vogel." Nova Acta. K. Leop. Carol, Dattschen Akad. d. Naturfor, Bd. x. 41, 1879.
 
B. 55
 
 
 
858 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
 
definite layer, the cells of which are not so flat as is the case
with the hypoblast cells of the posterior part of the blastoderm,
and in the older specimens of this stage they are very decidedly
more columnar than in the younger specimens.
 
The primitive streak is however the most interesting structure
in the area pellucida at this stage.
 
The feature which most obviously strikes the observer in
transverse sections through it is the fact, proved by Kolliker, that
it is mainly due to a proliferation of the epiblast cells along an
axial streak, which, roughly speaking, corresponds with the dark
line visible in surface views. In the youngest specimens and at
the front end of the primitive streak, the proliferated cells do not
extend laterally beyond the region of their origin, but in the
older specimens they have a considerable lateral extension.
 
The hypoblast can, in most instances, be traced as a distinct
layer underneath the primitive streak, although it is usually less
easy to follow it in that region than elsewhere, and in some
cases it can hardly be distinctly separated from the superjacent
cells.
 
The cells, undoubtedly formed by a proliferation of the epiblast, form a compact mass extending downwards towards the
hypoblast ; but between this mass and the hypoblast there are
almost always present along the whole length of the primitive
streak a number of cells, more or less loosely arranged, and
decidedly more granular than the proliferated cells. Amongst
these loosely arranged cells there are to be found a certain
number of large spherical cells rilled with yolk granules. Sometimes these cells are entirely confined to the region of the primitive streak, at other times they are continuous laterally with cells
irregularly scattered between the hypoblast and epiblast (Ser.C,2),
which are clearly the remnants of the undifferentiated cells of
the embryonic shield. The junction between these cells and
the cells of the primitive streak derived from the epiblast is
often obscure, the two sets of cells becoming partially intermingled. The facility with which the cells we have just spoken
of can be recognized varies moreover greatly in different instances. In some cases they are very obvious (Ser. C), while in
other cases they can only be distinguished by a careful examination of good sections.
 
 
 
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 859
 
The cells of the primitive streak between the epiblast and
the hypoblast are without doubt mesoblastic, and constitute the
first portion of the mesoblast which is established. The section
of these cells attached to the epiblast, in our opinion, clearly
originates from the epiblast ; while the looser cells adjoining
the hypoblast must, it appears to us, be admitted to have their
origin in the indifferent cells of the embryonic shield, placed
between the epiblast and the hypoblast, and also very probably
in a distinct proliferation from the hypoblast below the primitive
streak.
 
Posteriorly the breadth of the streak of epiblast which buds
off the cells of the primitive streak widens considerably, and in
the case of the blastoderm with the earliest primitive streaks
extends into the region of the area opaca. The widening of the
primitive streak behind is shewn in Ser. B, 3 ; Sen c, 2 ; and Ser.
E, 4. Where very marked it gives rise to the sickle-shaped
appearance upon which so much stress has been laid by. Roller
and Gerlach. In the case of one of the youngest of our blastoderms of this stage in which we found in surface views (PI. 45,
fig. L) a very well-marked sickle-shaped appearance at the hind
end of the primitive streak, the appearance was caused, as is
clearly brought out by our sections, by a thickening of the hypoblast of the germinal wall.
 
There is a short gap in our observations between the stage
with a young primitive streak and the first described stage in
which no such structure is present. This gap has been filled up
both by Gerlach and Koller.
 
Gerlach states that during this period a small portion of the
epiblast, within the region of the area opaca, but close to the
posterior border of the area pellucida, becomes thickened by a
proliferation of its cells. This portion gradually grows outwards laterally, forming in this way a sickle-shaped structure.
From the middle of this sickle a process next grows forward
into the area pellucida. This process is the primitive streak,
and it is formed, like the sickle, of proliferating epiblast cells.
 
Koller 1 described the sickle and the growth forwards from it
of the primitive streak in surface views somewhat before Gerlach;
 
1 " Beitr. z. Kenntniss d. Hiihnerkeims im Beginne cl. Bebriitung," Site. d. k.
Akad. IViss. iv. Abth. 1879.
 
552
 
 
 
860 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
 
and in his later memoir has entered with considerable detail
into the part played by the various layers in the formation of
this structure.
 
He believes, as already mentioned, that the sickle-shaped
structure, which appears according to him at an earlier stage
than is admitted by Gerlach, is in the first instance due to a
thickening of the hypoblast. At a later stage he finds that the
epiblast in the centre of the sickle becomes thickened, and that
a groove makes its appearance in this thickening which he calls
the "Sichel-rinne." This groove is identical with that first
described by Kupffer and Benecke 1 in the sparrow and fowl.
We have never, however, found very clear indications of it in
our sections.
 
In the next stage, Roller states that, in the region immediately in front of the "Sichel-rinne," a prominence appears which
he calls the Sichelknopf, and from this a process grows forwards
which constitutes the primitive streak. This structure is in main
derived from a proliferation of epiblast cells, but Koller admits
that some of the cells just above the hypoblast in the region of
the Sichelknopf are probably derived from the hypoblast. Since
these cells form part of the mesoblast it is obvious that Roller's
views on the origin of the mesoblast of the primitive streak
closely approach those which we have put forward.
 
The primitive streak starting, as we have seen, at the hinder
border of the area pellucida, soon elongates till it eventually
occupies at least two-thirds of the length of the area. As Roller
(loc. cit.} has stated, this can only be supposed to happen in one
of two ways, viz. either by a progression forward of the region
of epiblast budding off mesoblast, or by an interstitial growth of
the area of budding epiblast. Roller adopts the second of these
alternatives, but we cannot follow him in doing so. The simplest
method of testing the point is by measuring the distance between
the front end of the primitive streak and the front border of the
area pellucida at different stages of growth of the primitive
streak. If this distance diminishes with the elongation of the
primitive streak then clearly the second of the two alternatives
is out of the question.
 
1 Die erstc Entwick. an Eier d. Reptilien, Konigsberg, 1878.
 
 
 
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 86l
 
We have made measurements to test this point, and find that
the diminution of the space between the front end of the primitive streak and the anterior border of the area pellucida is very
marked up to the period in which the medullary plate first becomes established. We can further point in support of our view
to the fact that the extent of the growth lateralwards of the
mesoblast from the sides of the primitive streak is always less in
front than behind; which would seem to indicate that the front
part of the streak is the part formed latest. Our view as to the
elongation of the primitive streak appears to be that adopted by
Gerlach.
 
Our next stage includes roughly the period commencing
slightly before the first formation of a groove along the primitive streak, known as the primitive groove, and terminating
immediately before the first trace of the notochord makes its
appearance. After the close of the last stage the primitive
streak gradually elongates, till it occupies fully two-thirds of
the diameter of the area pellucida. The latter structure also
soon changes its form from a circular to an oval, and finally
becomes pyriform with the narrow end behind, while the primitive streak occupying two-thirds of its long axis becomes in most
instances marked by a light linear band along the centre, which
constitutes the primitive groove.
 
In surface views the primitive streak often appears to stop
short of the hinder border of the area pellucida.
 
During the period in which the external changes, which we
have thus briefly described, take place in the area pellucida,
great modifications are effected in the characters of the germinal
layers. The most important of these concern the region in front
of the primitive streak; but they will be better understood if we
commence our description with the changes in the primitive
streak itself.
 
In the older embryos belonging to our last stage we pointed
out that the mesoblast of the primitive streak was commencing
to extend outwards from the median line in the form of two
lateral sheets. This growth of the mesoblast is continued
rapidly during the present stage, so that during the latter part
of it any section through the primitive streak has approximately
the characters of Ser. I, 5
 
 
862 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
 
The mesoblast is attached in the median line to the epiblast.
Laterally it extends outwards to the edge of the area pellucida, and in older embryos may even form a thickening beyond
the edge (fig. G). Beneath the denser part of the mesoblast, and
attached to the epiblast, a portion composed of stellate cells
may in the majority of instances be recognized, especially in the
front part of the primitive streak. We believe these stellate
cells to be in the main directly derived from the more granular
cells of the previous stage. The hypoblast forms a sheet of
flattened cells, which can be distinctly traced for the whole
breadth of the area pellucida, though closely attached to the
mesoblast above.
 
In sections we find that the primitive streak extends back
to the border of the area pellucida, and even for some distance
bayond. The attachment to the epiblast is wider behind; but
the thickness of the mesoblast is not usually greater in the
median line than it is laterally, and for this reason probably the
posterior part of the streak fails to shew up in surface views.
The thinning out of the median portion of the mesoblast of the
primitive streak is shewn in a longitudinal section of a duck's
blastoderm of this stage (fig. D). The same figure also shews
that the hypoblastic sheet becomes somewhat thicker behind,
and more independent of the parts above.
 
A careful study of the peripheral part of the area pellucida,
in the region of the primitive streak, in older embryos of this
stage, shews that the hypoblast is here thickened, and that its
upjjer part, i.e. that adjoining the mesoblast, is often formed
of stellate cells, many of which give the impression of being
in the act of passing into the mesoblast above. At a later
stage the mesoblast of the vascular area undoubtedly receives
accessions of cells from the yolk below; so that we see no
grounds for mistrusting the appearances just spoken of, or for
doubting that they are to be interpreted in the sense suggested.
 
We have already stated that during the greater part of the
present stage a groove, known as the primitive groove, is to be
found along the dorsal median line of the primitive streak.
 
The extent to which this groove is developed appears to be
subject to very great variation. On the average it is, perhaps,
slightly deeper than it is represented in Ser. I, 5. In some cases
 
 
 
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 863
 
it is very much deeper. One of the latter is represented in
fig. G. It has here the appearance of a narrow slit, and sections of it give the impression of the mesoblast originating
from the lips of a fold; in fact, the whole structure appears
like a linear blastopore, from the sides of which the mesoblast
is growing out; and this as we conceive actually to be the true
interpretation of the structure. Other cases occur in which the
primitive groove is wholly deficient, or at the utmost represented by a shallow depression along the median axial line of a
short posterior part of the primitive streak.
 
We may now pass to the consideration of the part of the
area pellucida in front of the primitive streak.
 
We called attention to a change in the character of the hypoblast cells of this region as taking place at the end of the last
stage. During the very early part of this stage the change in
the character of these cells becomes very pronounced.
 
What we consider to be our earliest stage in this change we
have only so far met with in the duck, and we have figured a
longitudinal and median section to shew it (PI. 43, fig. D). The
hypoblast (hy) has become a thick layer of somewhat cubical
cells several rows deep. These cells, especially in front, are
characterized by their numerous yolk spherules, and give the
impression that part of the area pellucida has been, so to speak,
reclaimed from the area opaca. Posteriorly, at the front end of
the primitive streak, the thick layer of Jiypoblast, instead of being
continuous with the flattened hypoblast tinder the primitive streak,
falls, in the axial line, into the mesoblast of the primitive streak
(PL 43, fig. D).
 
In a slightly later stage, of which we have specimens both of
the duck and chick, but have only figured selected sections of a
chick series, still further changes have been effected in the constitution of the hypoblast (PI. 44, Ser. H, I and 2).
 
Near the front border of the area pellucida (i) it has the
general characters of the hypoblast of the duck's blastoderm just
described. Slightly further back the cells of the hypoblast have
become differentiated into stellate cells several rows deep, which
can hardly be resolved in the axial line into hypoblast and mesoblast, though one can fancy that in places, especially laterally,
they are partially differentiated into two layers. The axial
 
 
 
864 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
 
sheet of stellate cells is continuous laterally with cubical hypoblast cells.
 
As the primitive streak is approached an axial prolongation
forwards of the rounded and closely-packed mesoblastic elements of the primitive streak is next met with ; and at the front
end of the primitive streak, where this prolongation unites with
the epiblast, it also becomes continuous with the stellate cells
just spoken of. In fact, close to the end of the primitive streak
it becomes difficult to say which mesoblast cells are directly
derived from the primitive layer of hypoblast in front of the
primitive streak, and which from the forward growth of the
mesoblast of the primitive streak. There is, in fact, as in the
earlier stage, a fusion of the layers at this point.
 
Sections of a slightly older chick blastoderm are represented
in PI. 45, Ser.l, I, 2, 3, 4 and 5.
 
Nearly the whole of the hypoblast in front of the primitive
streak has now undergone a differentiation into stellate cells.
In the second section the products of the differentiation of this
layer form a distinct mesoblast and hypoblast laterally, while in
the median line they can hardly be divided into two distinct
layers.
 
In a section slightly further back the same is true, except
that we have here, in the axial line above the stellate cells,
rounded elements derived from a forward prolongation of the
cells of the primitive streak. In the next section figured, passing through the front end of the primitive streak, the axial cells
have become continuous with the axial mesoblast of the primitive streak, while below there is an independent sheet of flattened
hypoblast cells.
 
The general result of our observations on the part of the
blastoderm in front of the primitive streak during this stage is
to shew that the primitive hypoblast of this region undergoes
considerable changes, including a multiplication of its cells; and
that these changes result in its becoming differentiated on each
side of the middle line, with more or less distinctness, into (i) a
hypoblastic sheet below, formed of a single row of flattened cells,
and (2) a mesoblast plate above formed of stellate cells, while in
the middle line there is a strip of stellate cells in which there is
no distinct differentiation into two layers.
 
 
 
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 865
 
Since the region in which these changes take place is that in
which the medullary plate becomes subsequently formed, the
lateral parts of the mesoblast plate are clearly the permanent
lateral plates of the trunk, from which the mesoblastic somites,
&c., become subsequently formed ; so that the main part of the
'mesoblast of the trunk is not directly derived from the primitive
streak.
 
Before leaving this stage we would call attention to the presence, in one of our blastoderms of this stage, of a deep pit at
the junction of the primitive streak with the region in front of it
(PI. 44, Ser. F, I and 2). Such a pit is unusual, but we think
it may be regarded as an exceptionally early commencement
of that most variable structure in the chick, the neurenteric
canal.
 
The next and last stage we have to deal with is that during
which the first trace of the notochord and of the medullary plate
make their appearance.
 
In surface views this stage is marked by the appearance of a
faint dark line, extending forwards, from the front end of the
primitive streak, to a fold, which has in the mean time made its
appearance near the front end of the area pellucida, and constitutes the head fold.
 
PI. 45, Ser. K, represents a series of sections through a blastoderm of this stage, which have been selected to illustrate the
mode of formation of the notochord.
 
In a section immediately behind the head fold the median
part of the epiblast is thicker than the lateral parts, forming the
first indication of a medullary plate (Ser. K, i). Below the
median line of the epiblast is a small cord of cells, not divided
into two layers, but continuous laterally, both with the hypoblast and mesoblast, which are still more distinctly separated
than in the previous stage.
 
A section or so further back (Ser. K, 2) the axial cord, which
we need scarcely say is the rudiment of the notochord, is thicker,
and causes a slight projection in the epiblast above. It is, as
before, continuous laterally, both with the mesoblast and with
the hypoblast. The medullary plate is more distinct, and a
shallow but unmistakable medullary groove has made its appearance.
 
 
 
866 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
 
As we approach the front end of the primitive streak the
notochord becomes (Sen K, 3) very much more prominent,
though retaining the same relation to the germinal layers as in
front.
 
In the section immediately behind (Ser. K, 4) the convex
upper surface of the notochord has become continuous with the
epiblast for a very small region. The section, in fact, traverses
the front end of the primitive streak.
 
In the next section the attachment between the epiblast and
the cells below becomes considerably wider. It will be noticed
that this part of the primitive streak is placed on the floor of the
wide medullary groove, and there forms a prominence known as
the anterior swelling of the primitive streak.
 
It will further be noticed that in the two sections passing
through the primitive streak, the hypoblast, instead of simply
becoming continuous with the axial thickening of the cells, as in
front, forms a more or less imperfect layer underneath it. This
layer becomes in the sections following still more definite, and
forms part of the continuous layer of hypoblast present in the
region of the primitive streak.
 
A comparison of this stage with the previous one shews very
clearly that the notochord is formed out of the median plate of
cells of the earlier stage, which was not divided into mesoblast
and hypoblast, together with the short column of cells which
grew forwards from the primitive streak;
 
The notochord, from its mode of origin, is necessarily contios -behind -with the axial cells of the primitive streak.
 
The sections immediately behind the last we have represented
shew a rudiment of the neurenteric canal of the same form as
that first figured by Gasser, viz. a pit perforating the epiblast
with a great mass of rounded cells projecting upwards through it.
 
The observations just recorded practically deal with two
much disputed points in the ontogeny of birds, viz. the origin of
the mesoblast and the origin of the notochord.
 
With reference to the first of these our results are briefly as
follows :
 
The first part of the mesoblast to be formed is that which
arises in connection with the primitive streak. This part is in
 
 
 
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 867
 
the main formed by a proliferation from an axial strip of the
epiblast along the line of the primitive streak, but in part also
from a simultaneous differentiation of hypoblast cells also along
the axial line of the primitive streak. The two parts of the
mesoblast so formed become subsequently indistinguishable.
The second part of the mesoblast to be formed is that which
gives rise to the lateral plates of mesoblast of the head and
trunk of the embryo. This part appears as two plates one on
each side of the middle line which arise by direct differentiation from the hypoblast in front of the primitive streak. They
are continuous behind with the lateral wings of mesoblast
which grow out from the primitive streak, and on their inner
side are also at first continuous with the cells which form the
notochord.
 
In addition to the parts of mesoblast, formed as just described, the mesoblast of the vascular area is in a large measure
developed by a direct formation of cells round the nuclei of the
germinal wall.
 
The mesoblast formed in connection with the primitive
streak gives rise in part to the mesoblast of the allantois, and
ventral part of the tail of the embryo (?), and in part to the
vascular structures found in the area pellucida.
 
With reference to the formation of the mesoblast of the
primitive streak, our conclusions are practically in harmony
with those of Koller ; except that Koller is inclined to minimise the share taken by the hypoblast in the formation of the
mesoblast of the primitive streak.
 
Gerlach, with reference to the formation of this part of the
mesoblast, adopts the now generally accepted view of Kolliker,
according to which the whole of the mesoblast of the primitive
streak is derived from the epiblast.
 
As to the derivation of the lateral plates of mesoblast of the
trunk from the hypoblast of the anterior part of the primitive
streak, our general result is in complete harmony with Gerlach's
results, although in our accounts of the details of the process we
differ in some not unimportant particulars.
 
As to the origin of the notochord, our main result is that
this structure is formed as an actual thickening of the primitive
hypoblast of the anterior part of the area pellucida. We find
 
 
 
868 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
 
that it unites posteriorly with a forward growth of the axial
tissue of the primitive streak, while it is laterally continuous, at
first, both with the mesoblast of the lateral plates and with the
hypoblast. At a later period its connection with the mesoblast
is severed, while the hypoblast becomes differentiated as a continuous layer below it.
 
As to the hypoblastic origin of the notochord, we are again
in complete accord with Gerlach ; but we differ from him in
admitting that the notochord is continuous posteriorly with the
axial tissue of the primitive streak, and also at first continuous
with the lateral plates of mesoblast
 
The account we have given of the formation of the mesoblast
may appear to the reader somewhat fantastic, and on that account not very credible. We believe, however, that if the view
which has been elsewhere urged by one of us, that the primitive
streak is the homologue of the blastopore of the lower vertebrates is accepted, the features we have described receive an
adequate explanation.
 
The growth outwards of part of the mesoblast from the axial
line of the primitive streak is a repetition of the well-known
growth from the lips of the blastopore. It might have been
anticipated that all the layers would fuse along the line of the
primitive streak, and that the hypoblast as well as part of the
mesoblast would grow out from it. There is, however, clearly a
precocious formation of the hypoblast ; but the formation of the
mesoblast of the primitive streak, partly from the epiblast and
partly from the hypoblast, is satisfactorily explained by regarding the whole structure as the blastopore. The two parts
of the mesoblast subsequently become indistinguishable, and
their difference in origin is, on the above view, to be regarded as
simply due to a difference of position, and not as having a deeper
significance.
 
The differentiation of the lateral plates of mesoblast of the
trunk directly from the hypoblast is again a fundamental feature
of vertebrate embryology, occurring in all types from Amphioxus upwards, the meaning of which has been fully dealt
with in the Treatise on Comparative Embryology by one of us.
Lastly, the formation of the notochord from the hypoblast is
the typical vertebrate mode of formation of this organ, while
 
 
 
EXPLANATION OF PLATES. 869
 
the fusion of the layers at the front end of the primitive
streak is the universal fusion of the layers at the dorsal lip
of the blastopore, which is so well known in the lower vertebrate types.
 
 
 
EXPLANATION OF PLATES 4345.
N. B. The series of sections are in all cases numbered from before backwards.
 
LIST OF REFERENCE LETTERS.
 
a. p. Area pellucida. ep. Epiblast. ch. Notochord. gr. Germinal wall. hy.
Hypoblast. m. Mesoblast. o. p. Area opaca. pr. g. Primitive groove. pv s.
Primitive streak, yk. Yolk of germinal wall.
 
PLATE 43.
 
SERIES A, i and 2. Sections through the blastoderm before the appearance of
primitive streak.
 
I. Section through anterior part of area pellucida in front of embryonic
shield. The hypoblast here forms an imperfect layer. The figure represents about
half the section, i. Section through same blastoderm, in the region of the embryonic shield. Between the epiblast and hypoblast are a number of undifferentiated
cells. The figure represents considerably more than half the section.
 
SERIES B, i, 2 and 3. Sections through a blastoderm with a very young primitive streak.
 
i. Section through the anterior part of the area pellucida in front of the
primitive streak. 2. Section through about the middle of the primitive streak.
3. Section through the posterior part of the primitive streak.
 
SERIES C, i and 2. Sections through a blastoderm with a young primitive streak.
r. Section through the front end of the primitive streak. 2. Section through
the primitive streak, somewhat behind i. Both figures shew very clearly the difference in character between the cells of the epiblastic mesoblast of the primitive streak,
and the more granular cells of the mesoblast derived from the hypoblast.
 
FIG. D. Longitudinal section through the axial line of the primitive streak, and
the part of the blastoderm in front of it, of an embryo duck with a well-developed
primitive streak.
 
PLATE 44.
 
SERIES E, i, 2, 3 and 4. Sections through blastoderm with a primitive streak,
towards the end of the first stage.
 
i. Section through the anterior part of the area pellucida. 2. Section a little
way behind i shewing a forward growth of mesoblast from the primitive streak. 3.
Section through primitive streak. 4. Section through posterior part of primitive
streak, shewing the great widening of primitive streak behind.
 
 
 
8/0 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
 
SERIES F, i and 2. Sections through a blastoderm with primitive groove.
 
i. Section shewing a deep pit in front of primitive streak, probably an early
indication of the neurenteric canal. 2. Section immediately following i.
 
FIG. G. Section through blastoderm with well-developed primitive streak, shewing an exceptionally deep slit-like primitive groove.
 
SERIES H, i and 2. Sections through a blastoderm with a fully-developed primitive streak.
 
i. Section through the anterior part of area pellucida, shewing the cubical
granular hypoblast cells in this region. 2. Section slightly behind i, shewing the
primitive hypoblast cells differentiated into stellate cells, which can hardly be resolved
in the middle line into hypoblast and mesoblast.
 
PLATE 45.
 
SERIES I, i, 2, 3, 4 and 5. Sections through blastoderm somewhat older than
Series H.
 
i. Section through area pellucida well in front of primitive streak. 2. Section
through area pellucida just in front of primitive streak. 3. Section through the front
end of primitive streak. 4. Section slightly behind 3. 5. Section slightly behind 4.
 
SERIES K, 1,2, 3, 4 and 5. Sections through a blastoderm in which the first
traces of notochord and medullary groove have made their appearance. Rather more
than half the section is represented in each figure, but the right half is represented in
i and 3, and the left in 2 and 4.
 
i. Section through notochord immediately behind the head-fold. 2. Section
shewing medullary groove a little behind i. 3. Section just in front of the primitive
streak. 4 and 5. Sections through the front end of the primitive streak.
 
FlG. L. Surface view of blastoderm with a very young primitive streak.
 
 
 
XXV. THE ANATOMY AND DEVELOPMENT OF PERIPATUS
 
CAPENSIS 1 .
(With Plates 4653.)
 
INTRODUCTION.
 
THE late Professor Balfour was engaged just before his death
in investigating the structure and embryology of Peripatus
capensis, with the view of publishing a complete monograph
of the genus. He left numerous drawings intended to serve
as illustrations to the monograph, together with a series of
notes and descriptions of a large part of the anatomy of
Peripatus capensis. Of this manuscript some portions were
ready for publication, others were more or less imperfect ; while
of the figures many were without references, and others were
provided with only a few words of explanation.
 
It was obviously necessary that Professor Balfour's work
embodying as it did much important discovery should be published without delay; and the task of preparing his material
for the press was confided to us. We have printed all his
notes and descriptions without alteration 2 . Explanations which
appeared to be necessary, and additions to the text in cases in
which he had prepared figures without writing descriptions, together with full descriptions of all the plates, have been added
by us, and are distinguished by enclosure in square brackets 3 .
 
We have to thank Miss Balfour, Professor Balfour's sister,
for the important service which she has rendered by preparing
 
1 From the Quarterly Jotirnal of Microscopical Science, April, 1883.
 
2 Excepting in an unimportant matter of change of nomenclature used with regard
to the buccal cavity.
 
3 The account of the external characters, generative organs, and development, has
been written by the editors.
 
 
 
872 ANATOMY AND DEVELOPMENT
 
a large part of the beautiful drawings with which the monograph is illustrated. Many of these had been executed by her
under Professor Balfour's personal supervision ; and the knowledge of his work which she then acquired has been of the
greatest assistance to us in preparing the MSS. and drawings for
publication.
 
Since his death she has spared no pains in studying the
structure of Peripatus, so as to enable us to bring out the
first part of the monograph in as complete a state as possible.
It is due to her skill' that the first really serviceable and accurate
representation of the legs of any species of Peripatus available
for scientific purposes are issued with the present memoir 1 .
 
We have purposely refrained from introducing comments on
the general bearing of the new and important results set forth in
this memoir, and have confined ourselves to what was strictly
necessary for the presentation of Mr Balfour's discoveries in a
form in which they could be fully comprehended.
 
Mr Balfour had at his disposal numerous specimens of
Peripatus nova zealandia, collected for him by Professor Jeffrey
Parker, of Christchurch, New Zealand ; also specimens from
the Cape of Good Hope collected by Mr Lloyd Morgan,
and brought to England by Mr Roland Trimen in 1881 ; and
others given to him by Mr Wood Mason, together with all the
material collected by Mr Moseley during the "Challenger"
voyage.
 
A preliminary account of the discoveries as to the embryology of Peripatus has already been communicated to the
Royal Society 2 . It is intended that the present memoir shall
be followed by others, comprising a complete account of all the
species of the genus Peripatus.
 
H. M. MOSELEY.
A. SEDGWICK.
 
 
 
1 The drawings on PI. 47, figs. 9 and 10 on PI. 48, and the drawings of the
embryos (except fig. 37), have been made by Miss Balfour since Professor Balfour's
death.
 
5 Proc. Royal Soc. 1883.
 
 
 
OF PERIPATUS CAPENSTS. 873
 
PART I.
DESCRIPTION OF THE SPECIES.
 
Peripatus capensis (fig. i).
 
[The body is elongated, and slightly flattened dorso-ventrally.
The dorsal surface is arched, and darkly pigmented ; while the
ventral surface is nearly flat, and of a lighter colour.
 
The mouth is placed at the anterior end of the body, on the
ventral surface.
 
The anus is posterior and terminal.
 
The generative opening is single and median, and placed
in both sexes on the ventral surface, immediately in front of
the anus.
 
There are a pair of ringed antennae projecting from the anterior end of the head, and a pair of simple eyes, placed on the
dorsal surface at the roots of the antennae.
 
The appendages of the body behind the antennae are disposed in twenty pairs.
 
1. The single pair of jaws placed within the buccal cavity
in front of the true mouth opening, and consisting each of a
papilla, armed at its termination with two cutting blades.
 
2. The oral papillae placed on each side of the mouth. At
their apices the ducts of the slime glands open.
 
3. The seventeen pairs of ambulatory appendages, each provided with a pair of chitinous claws at its extremity.
 
4. The anal papillae placed on each side of the generative
opening.
 
Colour. The following statements on this head are derived
from observations of spirit specimens. The colour varies in
different individuals. It always consists of a groundwork of
green and bluish grey, with a greater or less admixture of
brown. The chief variations in the appearance of the animal,
so far as colour is concerned, depend on the shade of the green.
In some it is dark, as in the specimen figured (fig. i) ; in others
it is of a lighter shade.
 
There is present in most specimens a fairly broad light band
on each side of the body, immediately dorsal to the attachment
B. 56
 
 
 
8/4 ANATOMY AND DEVELOPMENT
 
of the legs. This band is more prominent in the lighter coloured
vaiieties than in the dark, and is especially conspicuous in large
individuals. It is due to a diminution in the green pigment, and
an increase in the brown.
 
There is a dark line running down the middle of the dorsal
surface, in the middle of which is a fine whitish line.
 
The ventral surface is almost entirely free from the green
pigment, but possesses a certain amount of light brown. This
brown pigment is more conspicuous and of a darker shade on
the spinous pads of the foot.
 
In parts of the body where the pigment is scarce, it is seen
to be confined to the papillae. This is especially evident round
the mouth, where the sparse green pigment is entirely confined
to the papillae.
 
In some specimens a number of white papillae, or perhaps
light brown, are scattered over the dorsal surface ; and sometimes there is a scattering of green papillae all over the ventral
surface. These two peculiarities are more especially noticeable
in small specimens.
 
Ridges and Papilla of the Skin. The skin is thrown into
a number of transverse ridges, along which the primary wartlike papillae are placed.
 
The papillae, which are found everywhere, are specially developed on the dorsal surface, less so on the ventral. The
papillae round the lips differ from the remaining papillae of the
ventral surface in containing a green pigment. Each papilla
bears at its extremity a well-marked spine.
 
The ridges of the skin are not continued across the dorsal
middle line, being interrupted by the whitish line already
mentioned. Those which lie in the same transverse line as
the legs are not continued on to the latter, but stop at the
junction of the latter with the body. All the others pass round
to the ventral surface and are continued across the middle line ;
they do not, however, become continuous with the ridges of the
other side, but passing between them gradually thin off and
vanish.
 
The ridges on the legs are directed transversely to their
long axes, i.e. are at right angles to the ridges of the rest of the
body.
 
 
 
OF PERIPATUS CAPENSIS. 875
 
The -antennae are ringed and taper slightly till near their
termination, where they present a slight enlargement in spirit
specimens, which in its turn tapers to its termination.
 
The rings consist essentially of a number of coalesced primary
papillae, and are, therefore, beset by a number of spines like
those of the primary papillae (described below). They are more
deeply pigmented than the rest of the antenna.
 
The free end of the antenna is covered by a cap of tissue like
that of the rings. It is followed by four or more rings placed
close together on the terminal enlargement. There appears to
be about thirty rings on the antennae of all adults of this species.
But they are difficult to count, and a number of small rings
occur between them, which are not included in the thirty.
 
The antennae are prolongations of the dorso-lateral parts of
the anterior end of the body.
 
The eyes are paired and are situated at the roots of the
antennae on the dorso-lateral parts of the head. Each is placed
on the side of a protuberance which is continued as the antenna, and presents the appearance of a small circular crystalline ball inserted on the skin in this region.
 
The rings of papillae on that part of the head from which
the antennae arise lose their transverse arrangement. They
are arranged concentrically to the antennal rings, and have a
straight course forwards between the antennae.
 
The oral papillae are placed at the side of the head. They
are attached ventro-Iaterally on each side of the lips. The
duct of the slime gland opens through their free end. They
possess two main rings of projecting tissue, which are especially
pigmented on the dorsal side ; and their extremities are covered
by papillae irregularly arranged.
 
The buccal cavity, jaws, and lips are described below.
 
The Ambulatory Appendages. The claw-bearing legs are
usually seventeen in number ; but in two cases of small females
we have observed that the anal papillae bear claws, and present all the essential features of the ambulatory appendages.
In one small female specimen there were twenty pairs of clawbearing appendages, the last being like the claw-bearing anal
papillae last mentioned, and the generative opening being placed
between them.
 
56 2
 
 
 
8/6 ANATOMY AND DEVELOPMENT
 
The ambulatory appendages, with the exception of the fourth
and fifth pairs in both sexes, and the last pair (seventeenth) in
the male, all resemble each other fairly closely. A typical appendage (figs. 2 and 3) will first be described, and the small
variations found in the appendages just mentioned will then
be pointed out. Each consists of two main divisions, a larger
proximal portion, the leg, and a narrow distal claw-bearing
portion, the foot.
 
The leg has the form of a truncated cone, the broad end of
which is attached to the ventro-lateral body-wall, of which it
appears to be, and is, a prolongation. It is marked by a number
of rings of primary papillae, placed transversely to the long axis
of the leg, the dorsal of which contain a green and the ventral a
brown pigment. These rings of papillae, at the attachment of
the leg, gradually change their direction and merge into the
body rings. At the narrow end of the cone there are three
ventrally placed pads, in which the brown pigment is dark, and
which are covered by a number of spines precisely resembling
the spines of the primary papillae. These spinous pads are continued dorsally, each into a ring of papillae.
 
The papillae of the ventral row next the proximal of these
spinous pads are intermediate in character between the primary
papillae and the spinous pads. Each of these papillae is larger
than a normal papilla, and bears several spines (fig. 2). This
character of the papilla of this row is even more marked in
some of the anterior legs than in the one figured ; it seems
probable that the pads have been formed by the coalescence of
several rows of papillae on the ventral surface of the legs. On
the outer and inner sides of these pads the spines are absent,
and secondary papillae only are present.
 
In the centre of the basal part of the ventral surface of the
foot there are present a group of larger papillae, which are of a
slightly paler colour than the others. They are arranged so as
to form a groove, directed transversely to the long axis of the
body, and separated at its internal extremity by a median papilla
from a deep pit which is placed at the point of junction of the
body and leg. The whole structure has the appearance, when
viewed with the naked eye, of a transverse slit placed at the base
of the leg. The segmental organs open by the deep pit placed
 
 
 
OF FERIPATUS CAPENSIS. 877
 
at the internal end of this structure. The exact arrangement of
the papillae round the outer part of the slit does not appear to be
constant.
 
The foot is attached to the distal end of the leg. It is
slightly narrower at its attached extremity than at its free end,
which bears the two claws. The integument of the foot is
covered with secondary papillae, but spines and primary papillae are absent, except at the points now to be described.
 
On each side of the middle ventral line of the proximal end
of the foot is placed an elliptical elevation of the integument
covered with spines. Attached to the proximal and lateral end
of this is a primary papilla. At the distal end of the ventral
side of the foot on each side of the middle line is a group of inconspicuous pale elevations, bearing spines.
 
On the front side of the distal end of the foot, close to the
socket in which the claws are placed, are two primary papillae,
one dorsal and the other ventral.
 
On the posterior side of the foot the dorsal of these only
is present. The claws are sickle-shaped, and placed on papillae
on the terminal portion of the foot. The part of the foot on
which they are placed is especially retractile, and is generally
found more or less telescoped into the proximal part (as in the
figure).
 
The fourth and fifth pairs of legs exactly resemble the others,
except in the fact that the proximal pad is broken up into
three, a small central and two larger lateral. The enlarged
segmental organs of these legs open on the small central division.
 
The last (17) leg of the male (PL 47, fig. 4) is characterized by possessing a well-marked white papilla on the ventral
surface. This papilla, which presents a slit-like opening at its
apex, is placed on the second row of papillae counting from the
innermost pad, and slightly posterior to the axial line of the leg.
 
The anal papillae, or as they should be called, generative
papillae, are placed one on each side of the generative aperture.
They are most marked in small and least so in large specimens.
That they are rudimentary ambulatory appendages is shewn by
the fact that they are sometimes provided with claws, and resemble closely the anterior appendages.]
 
 
 
8/8 ANATOMY AND DEVELOPMENT
 
 
 
*
 
PART II.
ALIMENTARY CANAL.
 
The alimentary canal of Peripatus capensis forms, in the
extended condition of the animal, a nearly straight tube, slightly
longer than the body, the general characters of which are shewn
in figs. 6 and 7.
 
For the purposes of description, it may conveniently be divided into five regions, viz. (i) the buccal cavity with the tongue,
jaws, and salivary glands, (2) pharynx, (3) the oesophagus, (4)
the stomach, (5) the rectum.
 
The Buccal Cavity. The buccal cavity has the form of a
fairly deep pit, of a longitudinal oval form, placed on the ventral
surface of the head, and surrounded by a tumid lip.
 
[The buccal cavity has been shewn by Moseley to be formed
in the embryo by the fusion of a series of processes surrounding
the true mouth-opening, and enclosing in their fusion the jaws.]
 
The 'lip is covered by a soft skin, in which are numerous
organs of touch, similar to those in other parts of the skin having
their projecting portions enclosed in delicate spines formed by
the cuticle. The skin of the lips differs, however, from the remainder of the skin, in the absence of tubercles, and in the great
reduction of the thickness of the dermis. It is raised into a
series of papilliform ridges, whose general form is shewn in fig. 5 ;
of these there is one unpaired and median behind, and a pair,
differing somewhat in character from the remainder, in front, and
there are, in addition, seven on each side.
 
The structures within the buccal cavity are shewn as they
appear in surface views in figs. 5 and 7, but their real nature is
best seen in sections, and is illustrated by PL 49, figs. II and 12,
representing the oral cavity in transverse section, and by PL 49,
figs. 17 and 1 8, representing it in horizontal longitudinal sections.
In the median line of the buccal cavity in front is placed a thick
muscular protuberance, which may perhaps conveniently be
called the tongue, though attached to the dorsal instead of
the ventral wall of the mouth. It has the form of an elongated
 
 
 
OF PERIPATUS CAPENSIS. 879
 
ridge, which ends rather abruptly behind, becoming continuous
with the dorsal wall of the pharynx. Its projecting edge is
armed by a series of small teeth, which are thickenings of the
chitinous covering, prolonged from the surface of the body over
the buccal cavity. Where the ridge becomes flatter behind, the
row of teeth divides into two, with a shallow groove between
them (PI. 48, fig. 7).
 
The surface of the tongue is covered by the oral epithelium,
in parts of which are organs of special sense, similar to those in
the skin; but its interior is wholly formed of powerful muscles.
The muscles form two groups, intermingled amongst each other.
There are a series of fibres inserted in the free edge of the
tongue, which diverge, more or less obliquely, towards the skin
at the front of the head anteriorly, and towards the pharynx
behind. The latter set of fibres are directly continuous with
the radial fibres of the pharynx. The muscular fibres just
described are clearly adapted to give a sawing motion to the
tongue, whose movements may thus, to a certain extent, be compared to those of the odontophor of a mollusc.
 
In addition to the above set of muscles, there are also transverse muscles, forming laminae between the fibres just described.
They pass from side to side across the tongue, and their action
is clearly to narrow it, and so cause it to project outwards from
the buccal cavity.
 
On each side of the tongue are placed the jaws, which are,
no doubt, a pair of appendages, modified in the characteristic
arthropodan manner, to subserve mastication. Their structure
has never been satisfactorily described, and is very complicated.
They are essentially short papillae, moved by an elaborate
and powerful system of muscles, and armed at their free extremities by a pair of cutting blades or claws. The latter structures are, in all essential points, similar to the claws borne by
the feet, and, like these, are formed as thickenings of the cuticle.
They have therefore essentially the characters of the claws and
jaws of the Arthropoda, and are wholly dissimilar to the setae of
Chsetopoda. The claws are sickle-shaped and, as shewn in PL
47, fig. 5, have their convex edge directed nearly straight forwards, and their concave or cutting edge pointed backwards.
Their form differs somewhat in the different species, and, as will
 
 
 
88O ANATOMY AND DEVELOPMENT
 
be shewn in the systematic part of this memoir 1 , forms a good
specific character. In Peripatus capensis (PI. 48, fig. 10) the
cutting surface of the outer blade is smooth and without teeth,
while that of the inner blade (fig. 9), which is the larger of the
two, is provided with five or six small teeth, in addition to the
main point. A more important difference between the two blades
than that in the character of the cutting edge just spoken of, is
to be found in their relation to the muscles which move them.
The anterior parts of both blades are placed on two epithelial
ridges, which are moved by muscles common to both blades (PI.
49, fig. 1 1). Posteriorly, however, the behaviour of the two blades
is very different. The epithelial ridge bearing the outer blade
is continued back for a short distance behind the blade, but
the cuticle covering it becomes very thin, and it forms a
simple epithelial ridge placed parallel to the inner blade. The
cuticle covering the epithelial ridge of the inner blade is, on the
contrary, prolonged behind the blade itself as a thick rod, which,
penetrating backwards along a deep pocket of the buccal epithelium, behind the main part of the buccal cavity for the whole
length of the pharynx, forms a very powerful lever, on which
a great part of the muscles connected with the jaws find their
insertion. The relations of the epithelial pocket bearing this
lever are somewhat peculiar.
 
The part of the epithelial ridge bearing the proximal part of
this lever is bounded on both its outer and inner aspect by a deep
groove. The wall of the outer groove is formed by the epithelial ridge of the outer blade, and that of the inner by a special
epithelial ridge at the side of the tongue. Close to the hinder
border of the buccal cavity (as shewn in PL 49, fig. 12, on the
right hand side), the outer walls of these two grooves meet over
the lever, so as completely to enclose it in an epithelial tube,
and almost immediately behind this point the epithelial tube is
detached from the oral epithelium, and appears in section as
a tube with a chitinous rod in its interior, lying freely in the
body-cavity (shewn in PI. 49, figs. 13 16 le). This apparent
tube is the section of the deep pit already spoken of. It may
 
1 Some material for this memoir was left by Prof. Balfour, which will be published
separately.
 
 
 
OF PERIPATUS CAPENSIS. 88 1
 
 
 
be traced back even beyond the end of the pharynx, and serves
along its whole length for the attachment of muscles.
 
The greater part of the buccal cavity is filled with the tongue
and jaws just described. It opens dorsally and behind by the
mouth into the pharynx, there being no sharp line of demarcation between the buccal cavity and the pharynx. Behind the
opening into the pharynx there is a continuation of the buccal
cavity shewn in transverse section in fig. 13, and in longitudinal
and horizontal section in fig. 17, into which there opens the
common junction of the two salivary glands. This diverticulum
is wide at first and opens by a somewhat constricted mouth into
the pharynx above (PL 49, fig. 13, also shewn in longitudinal
and horizontal section in fig. 17). Behind it narrows, passing
insensibly into what may most conveniently be regarded as a
common duct for the two salivary glands (PL 49, fig. 17).
 
The Salivary Glands, These two bodies were originally
described by Grube, by whom their nature was not made out,
and subsequently by Moseley, who regarded them as fat bodies.
They are placed in the lateral compartments of the body-cavity
immediately dorsal to the ventral nerve cords, and extend for
a very variable distance, sometimes not more than half the
length of the body, and in other instances extending for nearly
its whole length. Their average length is perhaps about twothirds that of the body. Their middle portion is thickest, and
they thin off very much behind and to a slight extent in front.
Immediately behind the mouth and in front of the first pair of
legs, they bend inwards and downwards, and fall (fig. 7) one on
each side into the hind end of the narrow section of the oral
diverticulum just spoken of as the common duct for the two
salivary glands. The glandular part of these organs is that
extending back from the point where they bend inwards. This
part (fig. 1 6) is formed of very elongated cells supported by
a delicate membrana propria. The section of this part is somewhat triangular, and the cells are so long as to leave a comparatively small lumen. The nuclei of the cells are placed close to
the supporting membrane, and the remainder of the cells arc
filled with very closely packed secretory globules, which have
a high index of refraction. It was the presence of these globules
which probably led Moseley to regard the salivary glands as fat
 
 
 
882 ANATOMY AND DEVELOPMENT
 
bodies. The part of each gland which bends inwards must be
regarded as the duct.
 
The cells lining the ducts are considerably less columnar
than those of the gland proper. Their nuclei (fig. 14) are
situated at the free extremities instead of at the base of the cells,
and they are without secretory globules. The cells lining the
ducts of the salivary glands pass, without any sharp line of
demarcation, into those of the oral epithelium, which are flatter
and have their nuclei placed in the middle.
 
The Pharynx. The Pharynx is a highly muscular tube (fig.
7) with a triangular lumen (figs. 14, 15), .which extends from
the mouth to about half way between the first and second pair
of legs. It is lined by a flattish epithelium bounded by a cuticle
continuous with that of the mouth. On the dorsal side is a
ridge projecting into the lumen of the pharynx. This ridge
may be traced forwards (PI. 49, figs. II 14) into the tongue,
and the two grooves at the side of this ridge, forming the two
upper angles of the triangular lumen, may be followed into those
at the sides of the tongue. The muscles of the pharynx are
very highly developed, consisting of an intrinsic and an extrinsic
set. The former consists, as is best seen in longitudinal sections,
of (PI. 51, fig. 23) radial fibres, arranged in somewhat wedgeshaped laminae, between which are rings of circular fibres. The
latter are thicker externally than internally, and so also appear
wedge-shaped in longitudinal sections. Very characteristic of
the pharynx are the two sympathetic nerves placed close to the
two dorsal angles of the triangular lumen (fig. 14, sy).
 
The pharynx of Peripatus is interesting in that it is unlike,
so far as I know, the pharynx of any true Arthropod, in all of
which the region corresponding with the pharynx of Peripatus
is provided with relatively very thin walls.
 
The pharynx of Peripatus has, on the other hand, a very
close and obvious resemblance to that of many of the Chaetopoda, a resemblance which is greatly increased by the characteristic course of the sympathetic nerves.
 
The form of the lumen, as already pointed out by Grube,
resembles that of the Nematoda.
 
Ttte (Esophagus. Behind the pharynx there follows a narrow
oesophagus (fig. 7, o e] shewn in section in fig. 16. It has some
 
 
OF PERIPATUS CAPENSIS. 883
 
 
 
what folded and fairly thick walls, and lies freely in the central
division of the body-cavity without any mesenteric support. Its
walls are formed of five layers, viz. from without inwards.
 
(1) A peritoneal investment.
 
(2) A layer of longitudinal fibres.
 
(3) A layer of circular fibres, amongst which are numerous
nuclei.
 
(4) A connective-tissue layer supporting (5) a layer of fairly
columnar hyaline epithelium, bounded on its inner aspect by
a cuticle continued from that of the pharynx. In front it passes
insensibly into the pharynx, and beyond the region where the
dorsal walls of the pharynx have clearly commenced, the ventral
walls still retain the characters of the cesophageal walls. The
oesophagus is vertically oval in front, but more nearly circular
behind. Characteristic of the cesophagus is the junction of the
two sympathetic nerves on its dorsal wall (fig. 16). These
nerves cannot be traced far beyond their point of junction.
 
The Stomach. The next section of -the alimentary tract is
the stomach or rnesenteron (fig. 6). It is by far the largest
part of the alimentary tract, commencing at about the second
pair of legs and extending nearly to the hind end of the body.
It tapers both in front and behind, and is narrowest in the
middle, and is marked off sharply both from the cesophagus in
front and the rectum behind, and is distinguished from both of
these by its somewhat pinker hue. In the retracted condition
of the animal it is, as pointed out by Moseley, folded in a single
short dorsal loop, at about the junction of its first with its second
third, and also, according to my observations, at its junction
with the rectum ; but in the extended condition it is nearly
straight, though usually the posterior fold at the junction of the
rectum is not completely removed. Its walls are always marked
by plications which, as both Moseley and Grube have stated, do
not in any way correspond with the segmentation of the body.
In its interior I have frequently found the chitinous remains of
the skins of insects, so that we are not justified in considering
that the diet is purely vegetable. It lies free, and is, like the
remainder of the alimentary tract, without a mesentery. The
structure of the walls of the stomach has not hitherto been very
satisfactorily described.
 
 
 
884 ANATOMY AND DEVELOPMENT
 
The connective tissue and muscular coats are extremely
thin. There is present everywhere a peritoneal covering, and
in front a fairly well-marked though very thin layer of muscles
formed of an external circular and an internal longitudinal
layer. In the middle and posterior parts, however, I was unable to recognize these two layers in section ; although in surface
view Grube found an inner layer of circular fibres and an outer
layer formed of bands of longitudinal fibres, which he regards as
muscular.
 
The layer supporting the epithelium is reduced to a basement membrane. The epithelial part of the wall of the stomach
is by far the thickest (fig. 20), and is mainly composed of enormously elongated, fibre-like cells, which in the middle part of
the stomach, where they are longest, are nearly half a millimetre
in length, and only about '006 mm. in breadth. Their nuclei, as
seen in fig. 20, are very elongated, and are placed about a quarter of the length from the base.
 
The cells are mainly filled with an immense number of
highly refracting spherules, probably secretory globules, but
held by Grube, from the fact of their dissolving in ether, to be
fat. The epithelial cells are raised into numerous blunt processes projecting into the lumen of the stomach.
 
In addition to the cells just described there are present in
the anterior part of the stomach a fair sprinkling of mucous
cells. There are also everywhere present around the bases of
the columnar cells short cells with spherical nuclei, which are
somewhat irregularly scattered in the middle and posterior parts
of the stomach, but form in the front part a definite layer. I
have not been able to isolate these cells, and can give no account of their function.
 
The rectum extends from the end of the stomach to the
anus. The region of junction between the stomach and the
rectum is somewhat folded. The usual arrangement of the
parts is shewn in fig. 6, where the hind end of the stomach is
seen to be bent upon itself in a U-shaped fashion, and the
rectum extending forwards under this bent portion and joining
the front end of the dorsal limb of the U. The structure of
the walls of the rectum is entirely different to that of the
stomach, and the transition between the two is perfectly sudden.
 
 
 
OF PERIPATUS CAPENSIS. 885
 
Within the peritoneal investment comes a well-developed muscular layer with a somewhat unusual arrangement of its layers,
there being an external circular layer and an internal layer
formed of isolated longitudinal bands. The epithelium is fairly
columnar, formed of granular cells with large nuclei, and is lined
by a prolongation of the external cuticle. It is raised into
numerous longitudinal folds, which are visible from the surface,
and give a very characteristic appearance to this part of the
alimentary tract. The muscular layers do not penetrate into
the epithelial folds, which are supported by a connective tissue
layer.
 
NERVOUS SYSTEM.
 
The central nervous system consists of a pair of supra-cesophageal ganglia united in the middle line, and of a pair of
widely divaricated ventral cords, continuous in front with the
supra-cesophageal ganglia.
 
It will be convenient in the first instance to deal with the
general anatomy of the nervous system and then with the
histology.
 
Ventral Cords. The ventral cords at first sight appear to be
without ganglionic thickenings, but on more careful examination they are found to be enlarged at each pair of legs (PI. 48,
fig. 8). These enlargements may be regarded as imperfect
ganglia. There are, therefore, seventeen such pairs of ganglia
corresponding to the seventeen pairs of legs. There is in addition a ganglionic enlargement at the commencement of the
cesophageal commissures, where the nerves to the oral papillae
are given off (PL 51, fig. 22 or. g.\ and the region of junction
between the cesophageal commissures with the supra-cesophageal
ganglia, where another pair of nerves are given off to the jaws
(PI. 51, fig. 22/0), may be regarded as the anterior ganglion of
the ventral cords. There are, therefore, according to the above
reckoning, nineteen pairs of ganglia connected with the ventral
cords.
 
The ventral cords are placed each in the lateral compartments of the body-cavity, immediately within the longitudinal
layer of muscles.
 
 
 
886 ANATOMY AND DEVELOPMENT
 
They are connected with each other, rather like the pedal
nerves of Chiton and the lower Prosobranchiata, by a number
of commissures. These commissures exhibit a fairly regular
arrangement from the region included between the first and the
last pair of true feet. There are nine or ten of them between
each pair of feet (PI. 52, fig. 26). They pass along the ventral
wall of the body, perforating the ventral mass of longitudinal
muscles. On their way they give off nerves which innervate
the skin.
 
In Peripatus nova zealandicz, and probably also in P. capensis, two of these nerves, coming off from each pair of ganglia,
are distinguished from the remainder by the fact that they are
provided with numerous nerve-cells, instead of being composed
of nerve-fibres only, like the remaining commissures (PL 52, fig.
26 g co). In correlation with the nerves given off from them to
the skin the commissures are smaller in the middle than at the
two ends.
 
Posteriorly the two nerve-cords nearly meet immediately in
front of the generative aperture, and between this aperture and
the last pair of feet there are about six commissures passing
between them (PL 48, fig. 8). Behind the generative aperture
the two cords bend upwards, and, as is shewn in fig. 8, fall into
each other dorsally to the rectum. The section of the two cords
placed dorsally to the rectum is solely formed of nerve-fibres;
the nerve-cells, present elsewhere, being here absent.
 
In front of the ganglion of the first foot the commissures
have a more dorsal situation than in the remainder of the body.
The median longitudinal ventral muscle here gradually thins
out and comes to an end, while the commissures pass immediately below the wall of the pharynx (PL 49, figs. 14, 15). The
ventral cords themselves at first approach very close to each
other in this region, separating again, however, to envelope between them the pharynx (PL 51, fig. 22).
 
There are eleven commissures in front of the first pair of legs
(PL 51, fig. 22). The three foremost of these are very close
together, the middle one arising in a more ventral position than
the other two, and joining in the median ventral line a peculiar
mass of cells placed in contact with the oral epithelium (fig. 14).
It is probably an organ of special sense.
 
 
 
OF PERIPATUS CAPENSIS. 887
 
 
 
The ventral cords give off a series of nerves from their outer
borders, which present throughout the trunk a fairly regular
arrangement. From each ganglion two large nerves (figs. 8, 22,
26) are given off, which, diverging somewhat from each other,
pass into the feet, and, giving off branches on their way, may be
traced for a considerable distance within the feet along their
anterior and posterior borders.
 
In front of each of the pair of pedal nerves a fairly large
nerve may be seen passing outwards towards the side of the
body (fig. 22). In addition to this nerve there are a number of
smaller nerves passing off from the main trunk, which do not
appear to be quite constant in number, but which are usually
about seven or eight. Similar nerves to those behind are given
off from the region in front of the first pair of legs, while at the
point where the two ventral cords pass into the oesophageal
commissures two large nerves (fig. 22), similar to the pairs of
pedal nerves, take their origin. These nerves may be traced
forwards into the oral papillae, and are therefore to be regarded
as the nerves of these appendages. On the ventral side of the
cords, where they approach most closely, between the oral
papillae and the first pair of legs, a number of small nerves are
given off to the skin, whose distribution appears to be to the
same region of the skin as that of the branches from the
commissures behind the first pair of legs.
 
From the cesophageal commissures, close to their junction
with the supra-cesophageal ganglia, a nerve arises on each side
which passes to the jaws, and a little in front of this, apparently
from the supra-cesophageal ganglion itself, a second nerve to the
jaws also takes its origin (PI. 51, fig. 22 j n}.. These two nerves
I take to be homologous with a pair of pedal nerves.
 
Between the nerves to the jaws and those to the oral papillae
a number of small nerves take their origin. Three of these on
each side pass in a dorsal direction and one or two in a ventral
one.
 
The Supra-cesophageal Ganglia. The supra-cesophageal ganglia (figs. 8 and 22) are large, somewhat oval masses, broader in
front than behind, completely fused in the middle, but free at
their extremities. Each of them is prolonged anteriorly into an
antennary nerve, and is continuous behind with one of the
 
 
 
888 ANATOMY AND DEVELOPMENT
 
cesophageal commissures. On the ventral surface of each, rather
behind the level of the eye, is placed a very peculiar protuberance (fig. 22 d], of which I shall say more in dealing with
the histology of the nervous system.
 
A number of nerves arise from the supra-cesophageal ganglia,
mainly from their dorsal surface.
 
In front are the immense antennary nerves extending along
the whole length of each antenna, and giving off numerous
lateral twigs to the sense organs. Near the origin of the antennary nerves, and rather on the dorsal surface, there spring
a few small twigs, which pass to the skin, and are presumably
sensory. The largest of them is shewn in PI. 50, fig. 19 A.
About one-third of the way back the two large optic nerves take
their origin, also arising laterally, but rather from the dorsal
surface (PL 50, fig. 19 D and E). Each of them joins a large
ganglionic mass placed immediately behind the retina. Nearly
on a level with the optic nerves and slightly nearer the middle
dorsal line a pair of small nerves (fig. 19 D) spring from the
brain and pass upwards, while nearly in the same line with the
optic nerves and a little behind them a larger pair of nerves take
their origin.
 
Behind all these nerves there arises from the line of suture
between the two supra-cesophageal ganglia a large median nerve
which appears to supply the integument of the dorsal part of
the head (PL 48, fig. 8 ; PL 49, figs. 11 14 d it).
 
Sympathetic System. In addition to the nerves just described there are two very important nerves which arise near
the median ventral line, close to the hind end of the supracesophageal ganglia. The origin of these two nerves is shewn
in the surface view (fig. 22 sy, and in section in fig. n). They
at first tend somewhat forwards and pass into the muscles near
the epithelium lining the groove on each side of the tongue.
Here they suddenly bend backwards again and follow the
grooves into the pharynx.
 
The two grooves are continuous with the two dorsal angles
of the pharynx ; and embedded in the muscles of the pharynx,
in juxtaposition with the epithelium, these two nerves may
easily be traced in sections. They pass backwards the whole
length of the pharynx till the latter joins the oesophagus.
 
 
 
OF PERIPATUS CAPENSIS. 889
 
Here they at once approach and shortly meet in the median
dorsal line (fig. 16). They can only be traced for a very short
distance beyond their meeting point. These nerves are, without
doubt, the homologues of the sympathetic system of Chaetopods,
occupying as they do the exact position which Semper has
shewn to be characteristic of the sympathetic nerves in that
group, and arising from an almost identical part of the brain 1 .
 
 
 
Histology of the Nervous System.
 
Ventral Cords. The histology of the ventral cords and
cesophageal commissures is very simple and uniform. They
consist of a cord almost wholly formed of nerve-fibres, placed
dorsally, and a ventral layer of ganglion cells (figs. 16 and 20).
 
The fibrous portion of the cord has the usual structure, being
formed mainly of longitudinal fibres, each probably being a
bundle of fibres of various sizes, enveloped in a sponge-work
of connective tissue. The larger bundles of fibres are placed
near the inner borders of the cords. In this part of the cord
there are placed a very small number of ganglion cells.
 
The layer of ganglion cells is somewhat crescent-shaped in
section, and, as shewn in figs. 16 and 20, envelopes the whole
ventral aspect of the fibrous parts of the cord, and even creeps
up slightly on to the dorsal side. It is thicker on the inner
than on the outer side, and increases considerably in bulk at
each ganglionic enlargement. The cells of which it is composed are for the most part of a nearly uniform size, but at the
border of the fibrous matter a fair sprinkling of larger cells is
found.
 
The tracheal vessels supplying the nervous system are placed
amongst the larger cells, at the boundary between the ganglionic
and fibrous regions of the cords.
 
With reference to the peripheral nerve-stems there is not
much to be said. They have for the most part a similar structure to the fibrous parts of the main cord, but are provided with
a somewhat larger number of cells.
 
1 Vide Spengel, " Oligognathus Boncllioc." Naples Mittheilungen, Bel. III. pi. iv.
fig- 52
B. 57
 
 
 
890 ANATOMY AND DEVELOPMENT
 
Sheath of tlie Ventral Cords. The ventral cords are enveloped by a double sheath, the two layers of which are often in
contact, while in other cases they may be somewhat widely
separated from each other. The inner layer is extremely thin
and always very closely envelopes the nerve-cords. The outer
layer is thick and fibrous, and contains a fair sprinkling of
nuclei.
 
Supra-cesophageal Ganglia. In the present state of our knowledge a very detailed description of the histology of the supracesophageal ganglia would be quite superfluous, and I shall
confine myself to a description of the more obvious features in
the arrangement of the ganglionic and fibrous portions (PI. 50,
fig. 19 A G).
 
The ganglion cells are in the first place confined, for the
most part, to the surface. Along the under side of each ganglion there is a very thick layer of cells, continuous behind,
with the layer of ganglion cells which is placed on the under
surface of the cesophageal commissures. These cells have,
moreover, an arrangement very similar to that in the ventral
cords, so that a section through the supra-cesophageal ganglia
has an obvious resemblance to what would be the appearance
of a section through the united ventral cords. On the outer
borders of the ganglia the cells extend upwards, but they end
on about the level of the optic nerve (fig. 19 D). Immediately
dorsal to this point the fibrous matter of the brain is exposed
freely on the surface (fig. 19 A, B, &c., a}. I shall call the region
of fibrous matter so exposed the dorso-lateral horn of white
matter.
 
Where the two ganglia separate in front the ganglion cells
spread up the inner side, and arch over so as to cover part of
the dorsal side. Thus, in the anterior part, where the two
ganglia are separate, there is a complete covering of ganglionic
substance, except for a narrow strip, where the dorso-lateral
lobe of white matter is exposed on the surface (fig. 19 A). From
the point where the two ganglia meet in front the nerve-cells
extend backwards as a median strip on the dorsal surface (fig.
19 D and E). This strip, becoming gradually smaller behind,
reaches nearly, though not quite, the posterior limit of the junction of the ganglia. Behind it there is, however, a region where
 
 
 
OF PERIPATUS CAPENSIS. 891
 
the whole dorsal surface of the ganglia is without any covering
of nerve-cells.
 
This tongue of ganglion cells sends in, slightly behind the
level of the eyes, a transverse vertical prolongation inwards into
the white matter of the brain, which is shewn in the series of
transverse sections in fig. 19 E, and also in the vertical longitudinal section (PL 51, fig. 21), and in horizontal section in
PL 51, fig. 22.
 
On the ventral aspect of each lobe of the brain there is present a very peculiar, bluntly conical protuberance of ganglion
cells (PL 51, fig. 22), which was first detected by Grube (No. 10),
and described by him as "a white thick body of a regular
tetrahedral form, and exhibiting an oval dark spot in the middle
of two of the faces." He further states that it is united by a
delicate nerve to the supra-cesophageal ganglion, and regards it
as an organ of hearing.
 
In Peripatus capensis the organ in question can hardly be
described as tetrahedral. It is rather, of a flattened oval form,
and consists, as shewn in sections (PL 50, fig. 19 C and D, d\
mainly of ganglion cells. In its interior is a cavity with a distinct
bounding membrane : the cells of which it is composed vary
somewhat in size, being smallest near the point of attachment.
At its free end is placed a highly refractive, somewhat oval
body, probably forming what Grube describes as a dark spot,
half embedded in its substance, and kept in place by the sheath
of nervous matter surrounding it. This body appears to have
fallen out in my sections. The whole structure is attached to
the under surface of the brain by a very short stalk formed of a
bundle of cells and nervous fibres.
 
It is difficult to offer any interpretation of the nature of this
body. It is removed considerably from the surface of the
animal, and is not, therefore, so far as I can see, adapted to serve
as an organ of hearing.
 
The distribution of the white or fibrous matter of the ganglia
is not very easy to describe.
 
There is a central lobe of white matter (fig. 19 E), which
is continuous from ganglion to ganglion, where the two are
united. It is smaller behind than in front. On its ventral side
it exhibits fairly well-marked transverse commissural fibres, con
572
 
 
 
892 ANATOMY AND DEVELOPMENT
 
necting the two halves of the ganglion. Laterally and somewhat ventrally it is prolonged into a horn (fig. 19 D, E, b], which
I propose calling the ventro-lateral horn. In front it is placed
in a distinct protuberance of the brain, which is placed ventrally
to and nearly in the same vertical plane as the optic nerve.
This protuberance is best shewn in the view of the brain from
below given in PL 51, fig. 22. This part of the horn is characterized by the presence of large vertically-directed bundles of
nerve-fibres, shewn in transverse section in fig. 190. Posteriorly
the diameter of this horn is larger than in front (fig. 19, E, F, G),
but does not give rise to a protuberance on the surface of the
brain owing to the smaller development of the median lobe
behind.
 
The median lobe of the brain is also prolonged into a dorsolateral lobe (fig. 19, a], which, as already mentioned, is freely
exposed on the surface. On its ventral border there springs the
optic nerve, and several pairs of sensory nerves already described (fig. 19 D, E), while from its dorsal border a pair of
sensory nerves also spring, nearly in the same vertical plane as
the optic nerves.
 
Posteriorly where the dorsal surface of the brain is not
covered in with ganglion cells the dorso-lateral horn and median
lobe of the brain become indistinguishable.
 
In the front part of the brain the median lobe of white matter
extends dorsalwards to the dorsal strip of ganglion cells, but
behind the region of the transverse prolongation of these cells,
into the white matter already described (p. 890), there is a more
or less distinctly defined lobe of white matter on the dorsal
surface, which I propose calling the postero-dorsal lobe of white
matter. It is shewn in the transverse sections (fig. 19 F and
G, c). It gradually thins away and disappears behind. It is
mainly characterized by the presence on the ventral border of
definite transverse commissural fibres.
 
 
 
OF PERIPATUS CAPENSIS. 893
 
 
 
THE SKIN.
 
The skin is formed of three layers.
 
1. The cuticle.
 
2. The epidermis or hypodermis.
 
3. The dermis.
 
The cuticle is a layer of about O'CO2 mm. in thickness. Its
surface is not, however, smooth, but is everywhere, with the
exception of the perioral region, raised into minute secondary
papillae, the base of which varies somewhat in diameter, but is
usually not far from O'O2 mm. On the ventral surface of the
body these papillae are for the most part somewhat blunt, but
on the dorsal surface they are more or less sharply pointed. In
most instances they bear at their free extremity a somewhat
prominent spine. The whole surface of each of the secondary
papillae just described is in its turn covered by numerous
minute spinous tubercles. In the perioral region, where the
cuticle is smooth, it is obviously formed of two layers which
easily separate from each other, and there is I believe a similar
division elsewhere, though it is not so easy to see. It is to be
presumed that the cuticle is regularly shed.
 
The epidermis, placed immediately within the cuticle, is
composed of a single row of cells, which vary, however, a good
deal in size in different regions of the body. The cells excrete
the cuticle, and, as shewn in fig. 32, they stand in a very remarkable relation to the secondary papillae of the cuticle just
described. Each epidermis cell is in fact placed within one of
these secondary papillae, so that the cuticle of each secondary
papilla is the product of a single epidermis cell. This relation
is easily seen in section, while it may also be beautifully shewn
by taking a part of the skin which is not too much pigmented,
and, after staining it, examining from the surface.
 
In fig. 32 a region of the epidermis is figured, in which the
cells are exceptionally columnar. The cuticle has, moreover,
in the process of cutting the section, been somewhat raised and
carried away from the subjacent cells. The cells of the epidermis are provided with large oval nuclei, which contain a well
 
 
894 ANATOMY AND DEVELOPMENT
 
developed reticulum, giving with low powers a very granular
appearance to the nuclei. The protoplasm of the cells is also
somewhat granular, and the granules are frequently so disposed
as to produce a very well-marked appearance of striation on
the inner end of the cells. The pigment which gives the characteristic colour to the skin is deposited in the protoplasm of the
outer ends of the cells in the form of small granules. An attempt is made to shew this in fig. 32.
 
At the apex of most, if not all, the primary wart-like papillae
there are present oval aggregations, or masses of epidermis
cells, each such mass being enclosed in a thickish capsule (fig.
31). The cells of these masses appear to form the wall of a
cavity which leads into the hollow interior of a long spine.
These spines when carefully examined with high objectives
present a rather peculiar structure. The base of the spine is
enveloped by the normal cuticle, but the spine itself, which
terminates in a very fine point, appears, as shewn in fig. 31, to
be continuous with the inner layer of the cuticle. In the
perioral region the outer layer of the cuticle, as well as the
inner, appear to be continued to the end of the spines. Within
the base of the spine there is visible a finely striated substance
which may often be traced into the cavity enclosed by the cells,
and appears to be continuous with the cells. Attached to the
inner ends of most of the capsules of these organs a delicate
fibrillated cord may be observed, and although I have not in any
instance succeeded in tracing this cord into one of the nervestems, yet in the antennas, where the nerve-stems are of an
enormous size, I have satisfied myself that the minute nerves
leaving the main nerve-stems and passing out towards the skin
are histologically not to be distinguished from these fibrillated
cords. I have therefore but little hesitation in regarding these
cords as nerves.
 
In certain regions of the body the oval aggregations of cells
are extremely numerous ; more especially is this the case in the
antennas, lips, and oral papillae. On the ventral surface of the
peripheral rings of the thicker sections of the feet they are
also very thick set (fig. 20 P). They here form a kind of pad,
and have a more elongated form than in other regions. In the
antennae they are thickly set side by side on the rings of skin
 
 
 
OF PERIPATUS CAPENSIS. 895
 
which give such an Arthropod appearance to these organs in
Peripatus.
 
The arrangement of the cells in the bodies just described led
me at first to look upon them as glands, but a further investigation induced me to regard them as a form of tactile organ.
The arguments for this view are both of a positive and a negative kind.
 
The positive arguments are the following :
 
(1) The organs are supplied with large nerves, which is distinctly in favour of their being sense organs rather than glands.
 
(2) The peculiar striae at the base of the spines appear to me
like the imperfectly preserved remains of sense hairs.
 
(3) The distribution of these organs favours the view that
they are tactile organs. They are most numerous on the antennas, where such organs would naturally be present, especially
in a case like that of Pe'ripatus, where the nerve passing to
the antennas is simply gigantic. On the other hand, the antennae would not be a natural place to look for an enormous
development of dermal glands.
 
The lips, oral papillae, and under surface of the legs, where
these bodies are also very numerous, are situations where tactile
organs would be of great use.
 
Under the head of negative arguments must be classed those
which tell against these organs being glandular. The most important of these is the fact that they have no obvious orifice.
Their cavities open no doubt into the spines, but the spines
terminate in such extremely fine points that the existence of an
orifice at their apex is hardly credible.
 
Another argument, from the distribution of these organs over
the body is practically the converse of that already used. The
distribution being as unfavourable to the view that they are
glands, as it is favourable to that of their being sense organs.
 
THE TRACHEAL SYSTEM.
 
The apertures of the tracheal system are placed in the depressions between the papillae or ridges of the skin. Each of
them leads into a tube, which I shall call the tracheal pit (fig.
30), the walls of which are formed of epithelial cells bounded
 
 
 
896 ANATOMY AND DEVELOPMENT
 
towards the lumen of the pit by a very delicate cuticular membrane continuous with the cuticle covering the surface of the
body. The pits vary somewhat in depth; the pit figured was
about O'CX) mm. It perforates the dermis and terminates in the
subjacent muscular layer. The investigation of the inner end of
the pit gave me some little trouble.
 
Transverse sections (fig. 30) through the trunk containing a
tracheal opening shew that the walls of the pit expanded internally in a mushroom-like fashion, the narrow part being, however, often excentric in relation to the centre of the expanded
part.
 
Although it was clear that the tracheae started from the expanded region of the walls of the pit, I could not find that the
lumen of the pit dilated into a large vesicle in this part, and
further investigation proved that the tracheae actually started
from the slightly swollen inner extremity of the narrow part of
the pit, the expanded walls of the pit forming an umbrella-like
covering for the diverging bundles of tracheae.
 
I have, in fig. 30, attempted to make clear this relation between the expanded walls of the tracheal pits and the tracheae.
In longitudinal sections of the trunk the tracheal pits do not
exhibit the lateral expansion which I have just described, which
proves that the divergence of the bundles of tracheae only takes
place laterally and not in an antero-posterior direction. Cells
similar in general character to those of the walls of the tracheal
pits are placed between the branches of tracheae, and somewhat
similar cells, though generally with more elongated nuclei, accompany the bundles of tracheae as far as they can be followed
in my sections. The structure of these parts in the adult would,
in fact, lead one to suppose that the tracheae had originated at
the expense of the cells of pits of the epidermis, and that the
cells accompanying the bundles of tracheae were the remains of
cords of cells which sprouted out from the blind ends of the
epidermis pits and gave rise in the first instance to the tracheae.
 
The tracheae themselves are extremely minute, unbranched
(so far as I could follow them) tubes. Each opening by a separate aperture into the base of the tracheal pit, and measuring
about O-QO2 mm. in diameter. They exhibit a faint transverse
striation, which I take to be the indication of a spiral fibre.
 
 
 
OF PERIPATUS CAPENSIS. 897
 
[Moseley (Phil. Trans., 1874, PI. 73, fig. i) states that the
tracheae branch, but only exceptionally.]
 
Situation of the tracheal apertures. Moseley states (No. 13)
that the tracheae arise from the skin all over the surface of the
body, but are especially developed in certain regions. He finds
"a row of minute oval openings on the ventral surface of the
body," the openings being "situate with tolerable regularity in
the centres of the interspaces between the pairs of members, but
additional ones occurring at irregular intervals. Other similar
openings occur in depressions on the inner side of the conical
foot protuberance." It is difficult in preserved specimens to
make out the exact distributions of the tracheal apertures, but I
have been able to make out certain points about them.
 
There is a double row of apertures on each side of the
median dorsal line, forming two sub-dorsal rows of apertures.
The apertures are considerably more numerous than the legs.
There is also a double row of openings, again more numerous
than the legs, on each side of the median ventral line between
the insertions of the legs. Moseley speaks of a median row in
this position. I think this must be a mistake.
 
Posteriorly the two inner rows approach very close to each
other in the median ventral line, but I have never seen them
in my section opening quite in the middle line. Both the dorsal
and ventral rows are very irregular.
 
I have not found openings on the ventral or dorsal side of
the feet but there are openings at the anterior and posterior
aspects of the feet. There are, moreover, a considerable number of openings around the base of the feet.
 
The dorsal rows of tracheal apertures are continued into
the head and give rise in this situation to enormous bundles of
tracheae.
 
In front of the mouth there is a very large median ventral
tracheal pit, which gives off tracheae to the ventral part of the
nervous system, and still more in front a large number of such
pits close together. The tracheae to the central nervous system
in many instances enter the nervous system bound up in the
same sheath as the nerves.
 
 
 
898 ANATOMY AND DEVELOPMENT
 
 
 
THE MUSCULAR SYSTEM.
 
The general muscular system consists of (i) the general
wall of the body; (2) the muscles connected with the mouth,
pharynx, and jaws; (3) the muscles of the feet; (4) the muscles
of the alimentary tract.
 
The muscular wall of the body is formed of (i) an external
layer of circular fibres; (2) an internal layer of longitudinal
muscles; (3) a layer of transverse fibres.
 
The layer which I have spoken of as formed of circular fibres
is formed of two strata of fibres which girth the body somewhat
obliquely (PI. 51, fig. 25). In the outer stratum the rings are
arranged so that their ventral parts are behind, while the ventral
parts of the rings of the inner stratum are most forward. Both
in the median dorsal and ventral lines the layer of circular fibres
become somewhat thinner, and where the legs are attached the
regularity of both strata is somewhat interfered with, and they
become continuous with a set of fibres inserted in the wall of the
foot.
 
The longitudinal muscles are arranged as five bands (vide
fig. 1 6), viz. two dorsal, two lateral, and three ventral. The
three ventral may be spoken of as the latero-ventral and medioventral bands.
 
The transverse fibres consist of (i) a continuous sheet on
each side inserted dorsally in the cutis, along a line opposite
the space between the dorsal bands of longitudinal fibres, and
ventrally between the ventro-median and ventro-lateral bands.
Each sheet at its insertion slightly breaks up into separate
bands. They divide the body-cavity into three regions a
median, containing the alimentary tract, slime glands, &c., and
two lateral, which are less well developed, and contain the nervous system, salivary glands, segmental organs, &c.
 
(2) Inserted a little dorsal to the transverse band just described is a second band which immediately crosses the first,
and then passes on the outer side of the nervous cord and
salivary gland, where such is present, and is inserted ventrally
in the space between the ventro-lateral and lateral longitudinal
band.
 
 
 
OF PERIPATUS CAPENSIS. 899
 
Where the feet are given off the second transverse band becomes continuous with the main retractor muscular fibres in the
foot, which are inserted both on to the dorsal side and ventral
side.
 
Muscular system of the feet. This consists of the retractors
of the feet connected with the outer transverse muscle and the
circular layer of muscles. In addition to these muscles there are
intrinsic transverse muscles which cross the cavity of the feet in
various directions (PI. 51, fig. 20). There is no special circular
layer of fibres.
 
Histology of the muscle, The main muscles of the body are
unstriated and divided into fibres, each invested by a delicate
membrane. Between the membrane and muscle are scattered
nuclei, which are never found inside the muscle fibres. The
muscles attached to the jaws form an exception in that they are
distinctly transversely striated.
 
THE BODY-CAVITY AND VASCULAR SYSTEM.
 
The body-cavity, as already indicated, is formed of three
compartments one central and two lateral. The former is by
far the largest, and contains the alimentary tract, the generative
organs, and the mucous glands. It is lined by a delicate endothelial layer, and is not divided into compartments nor traversed
by muscular fibres.
 
The lateral divisions are much smaller than the central, and
are shut .off from it by the inner transverse band of muscles.
They are almost entirely filled with the nerve-cord and salivary
gland in front and with the nerve-cord alone behind, and their
lumen is broken up by muscular bands. They further contain
the segmental organs which open into them. They are prolonged into the feet, as is the embryonic body-cavity of most
Arthropoda.
 
The vascular system is usually stated to consist of a dorsal
heart. I find between the dorsal bands of longitudinal fibres
a vessel in a space shut off from the body-cavity by a continuation of the endothelial. lining of the latter (fig. 16). The
vessel has definite walls and an endothelial lining, but I could
not make out whether the walls were muscular. The ventral
 
 
 
9OO ANATOMY AND DEVELOPMENT
 
part of it is surrounded by a peculiar cellular tissue, probably, as
suggested by Moseley, equivalent to the fat bodies of insects.
It is continued from close to the hind end of the body to the
head, and is at its maximum behind. In addition to this vessel
there is present a very delicate ventral vessel, by no means easy
to see, situated between the cutis and the outer layer of circular
muscles.
 
SEGMENTAL ORGANS.
 
A series of glandular organs are found in Peripatus which
have their external openings situated on the ventral surface of a
certain number of the legs, and which, to the best of my belief,
end internally by opening into the lateral compartments of the
body-cavity. These organs are probably of an excretory nature,
and I consider them homologous with the nephridia or segmental organs of the Chaetopoda.
 
In Peripatus capensis they are present in all the legs. In all
of them (except the first three) the following parts may be
recognized :
 
(1) A vesicular portion opening to the exterior by a narrow
passage.
 
(2) A coiled portion, which is again subdivided into several
sections.
 
(3) A terminal section ending by a somewhat enlarged opening into the lateral compartment of the body-cavity.
 
The last twelve pairs of these organs are all constructed in a
very similar manner, while the two pairs situated in the fourth
and fifth pairs of legs are considerably larger than those behind,
and are in some respects very differently constituted.
 
It will be convenient to commence with one of the hinder
nephridia. Such a nephridium from the ninth pair of legs is
represented in fig. 28. The external opening is placed at the
outer end of a transverse groove placed at the base of one of the
feet, while the main portion of the organ lies in the body-cavity
in the base of the leg, and extends into the trunk to about the
level of the outer edge of the nerv.e-cord of its side. The external opening (p s) leads into a narrow tube (s d\ which
gradually dilates into a large sack (s).
 
 
 
OF PERIPATUS CAPENS1S. QOI
 
The narrow part is lined by small epithelial cells, which are
directly continuous with and perfectly similar to those of the
epidermis (fig. 20). It is provided with a superficial coating
of longitudinal muscular fibres, which thins out where it passes
over the sack, along which it only extends for a short distance.
 
The sack itself, which forms a kind of bladder or collecting
vesicle for the organ, is provided with an extremely thin wall,
lined with very large flattened cells. These cells are formed of
granular protoplasm, and each of them is provided with a large
nucleus, which causes a considerable projection into the lumen
of the sack (figs. 20, 29 s). The epithelial wall of the sack is
supported by a membrana propria, over which a delicate layer
of the peritoneal epithelium is reflected.
 
The coiled tube forming the second section of the nephridium
varies in length, and by the character of the epithelium lining
it may be divided into four regions. It commences with a region
lined by a fairly columnar epithelium with smallish nuclei (fig.
28 s c i). The boundaries of the cells of this epithelium are
usually very indistinct, and the protoplasm contains numerous
minute granules, which are usually arranged in such a manner
as to give to optical or real sections of the wall of this part of
the tube a transversely striated appearance. These granules are
very probably minute balls of excretory matter.
 
The nuclei of the cells are placed near their free extremities,
contrary to what might have been anticipated, and the inner
ends of the cells project for very different lengths into the interior, so causing the inner boundary of the epithelium of this
part of the tube to have a very ragged appearance. This portion of the coiled tube is continuous at its outer end with the
thin-walled vesicle. At its inner end it is continuous with region
No. 2 of the coiled tube (fig. 28 s c 2), which is lined by small
closely-packed columnar cells. This portion is followed by
region No. 3, which has a very characteristic structure (fig.
28 s c 3). The cells lining this part are very large and flat, and
contain large disc-shaped nuclei, which are usually provided
with large nucleoli, and often exhibit a beautiful reticulum.
They may frequently be observed in a state of division. The
protoplasm of this region is provided with similar granules to
that in the first region, and the boundaries of the cells are usually
 
 
 
902 ANATOMY AND DEVELOPMENT
 
very indistinct. The fourth region is very short (fig. 28 s c 4),
and is formed of small columnar cells. It gradually narrows
till it opens suddenly into the terminal section (s o t], which
ends by opening into the body-cavity, and constitutes the most
distinct portion of the whole organ. Its walls are formed of
columnar cells almost filled by oval nuclei, which absorb
colouring matters with very great avidity, and thus renders
this part extremely conspicuous. The nuclei are arranged in
several rows.
 
The study of the internal opening of this part gave me some
trouble. No specimens ever shew it as rounded off in the
characteristic fashion of tubes ending in a cul-de-sac. It is
usually somewhat ragged and apparently open. In the best
preserved specimens it expands into a short funnel-shaped
mouth, the free edge of which is turned back. Sections confirm
the results of dissections. Those passing longitudinally through
the opening prove its edges are turned back, forming a kind of
rudimentary funnel. This is represented in fig. 29, from the last
leg of a female. I have observed remains of what I consider
to be cilia in this section of the organ. The fourth region of the
organ is always placed close to the thin-walled collecting vesicle
(figs. 28 and 29). In the whole of the coiled tube just described the epithelium is supported by a membrana propria,
which in its turn is invested by a delicate layer of peritoneal
epithelium.
 
The fourth and fifth pairs are very considerably larger than
those behind, and are in other respects peculiar. The great
mass of each organ is placed behind the leg, on which the external opening is placed, immediately outside one of the lateral
nerve-cords. Its position is shewn in fig. 8.
 
The external opening, instead of being placed near the base
of the leg, is placed on the ventral side of the third ring (counting from the outer end) of the thicker portion of the leg. It
leads (fig. 27) into a portion which clearly corresponds with the
collecting vesicle of the hinder nephridia. This part is not,
however, dilated into a vesicle in the same sort of way, and the
cells which form the lining epithelium have not the same characteristic structure, but are much smaller. Close to the point
where the vesicle joins the coiled section of the nephridium the
 
 
 
OF PERIPATUS CAPENSIS. 903
 
former has a peculiar nick or bend in it. At this nick it is firmly
attached to the ventral side of the foot by muscles and tracheae,
and when cut away from its attachment the muscles and tracheae
cannot easily be detached from it. The main part of the coils
are formed by region No. i, and the epithelial cells lining this
part present very characteristically the striated appearance which
has already been spoken of. The large-celled region of the
coiled tube (fig. 2 ; ") is also of considerable dimensions, and the
terminal portion is wedged in between this and the commencing
part of the coiled tube. The terminal portion with its internal
opening is in its histological characters exactly similar to the
homologous region in the hinder nephridia.
 
The three pairs of nephridia in the three foremost pairs of
legs are very rudimentary, consisting, so far as I have been
able to make out, solely of the collecting vesicle and the duct
leading from them to the exterior. The external opening is
placed on the ventral side of the base of the feet, in the same
situation as that of the posterior nephridia, but the histological
 
 
part of the body to which it belongs), does not acquire the
normal relations of a blastopore, but presents only those
rudimentary features (deep groove connected with origin of
mesoblast) which the whole blastopore of other tracheates
presents.
 
We think it probable that the larval anus eventually shifts
to the hind end of the body, and gives rise to the adult anus.
We reserve the account of the internal structure of these embryos (Stages A E) and of the later stages for a subsequent
memoir.
 
We may briefly summarise the more important facts of the
early development of Peripatus capensis, detailed in the preceding
account.
 
1. The greater part of the mesoblast is developed from the
walls of the archenteron.
 
2. The embryonic mouth and anus are derived from the
respective ends of the original blastopore, the middle part of the
blastopore closing up.
 
3. The embryonic mouth almost certainly becomes the
adult mouth, i.e. the aperture leading from the buccal cavity
into the pharynx, the two being in the same position. The
embryonic anus is in front of the position of the adult anus, but
in all probability. shifts back, and persists as the adult anus.
 
4. The anterior pair of mesoblastic somites gives rise to the
swellings of the praeoral lobes, and to the mesoblast of the
head 1 .
 
There is no need for us to enlarge upon the importance of
these facts. Their close bearing upon some of the most important problems of morphology will be apparent to all, and
we may with advantage quote here some passages from Balfour's Comparative Embryology, which shew that he himself
long ago had anticipated and in a sense predicted their discovery.
 
"Although the mesoblastic groove of insects is not a gastrula, it is quite possible that it is the rudiment of a blastopore, the gastrula corresponding to which has now vanished
 
1 We have seen nothing in any of our sections which we can identify as of socalled mesenchymatous origin.
 
 
 
OF PERIPATUS CAPENSIS. 913
 
from development." (Comparative Embryology, Vol. I. p. 378,
the original edition 1 .)
 
"TRACHEATA. Insecta. It (the mesoblast) grows inwards
from the lips of the germinal groove, which probably represents
the remains of a blastopore." (Comparative Embryology, Vol. II.
p. 291, the original edition 2 .)
 
"It is, therefore, highly probable that the paired ingrowths
of the mesoblast from the lips of the blastopore may have been,
in the first instance, derived from a pair of archenteric diverticula." (Comparative Embryology, Vol. II. p. 294, the original
edition 3 .)
 
The facts now recorded were discovered in June last, only
a short time before Balfour started for Switzerland ; we know
but little of the new ideas which they called up in his mind.
We can only point to passages in his published works which
seem to indicate the direction which his speculations would have
taken.
 
After speculating as to the probability of a genetic connection between the circumoral nervous system of the Ccelenterata,
and the nervous system of Echinodermata, Platyelminthes, Chaetopoda, Mollusca, &c., he goes on to say :
 
" A circumoral nerve-ring, if longitudinally extended, might
give rise to a pair of nerve-cords united in front and behind
exactly such a nervous system, in fact, as is present in many
Nemertines (the Enopla and Pelagonemertes), in Peripatus and
in primitive molluscan types (Chiton, Fissurella, &c.). From
the lateral parts of this ring it would be easy to derive the ventral
cord of the Chaetopoda and Arthropoda. It is especially deserving of notice, in connection with the nervous system of the
above-mentioned Nemertines and Peripatus, that the commissure connecting the two nerve-cords behind is placed on the
dorsal side of the intestines. As is at once obvious, by referring
to the diagram (fig. 231 B), this is the position this commissure
ought, undoubtedly, to occupy if derived from part of a nervering which originally followed more or less closely the ciliated
edge of the body of the supposed radiate ancestor." (Comparative Embryology, Vol. II. pp. 311, 312, the original edition 4 .)
 
1 This edition, Vol. n. p. 457. 2 This edition, Vol. III. p. 352.
 
3 This edition, Vol. m. p. 356. 4 This edition, Vol. in. pp. 378, 379.
 
 
 
9 14 ANATOMY AND DEVELOPMENT OF PERIPATUS CAPENSIS.
 
The facts of development here recorded give a strong additional support to this latter view, and seem to render possible
a considerable extension of it along the same lines.]
 
 
 
LIST OF MEMOIRS ON PERIPATUS.
 
1. M. Lansdown Guilding. "An Account of a New Genus of
Mollusca," Zoological Journal, Vol. II. p. 443, 1826.
 
2. M. Andouin and Milne-Edwards. " Classific. des Anndlides et
description de celles qui habitent les cotes de France," p. 411, Ann. Scien.
Nat. ser. I. Vol. xxx. 1833.
 
3. M. Gervais. "Etudes p. servir a 1'histoire naturelle des Myriapodes," Ann. Scien. Nat. ser. n. Vol. vn. 1837, p. 38.
 
4. Wiegmann. Wiegmann's Archiv, 1837.
 
5. H. Milne-Edwards. "Note sur le Peripate juluforme" Ann.
Scien. Nat. ser. n. Vol. xvm. 1842.
 
6. Blanchard. "Sur Forganisation des Vers," chap. IV. pp. 137 141,
Ann. Scien. Nat. ser. in. Vol. Vlll. 1847.
 
7. Quatrefages. " Anat. des Hermelles, note on," p. 57, Ann. Scien.
Nat. ser. in. Vol. x. 1848.
 
8. Quatrefages. Hist. Nat. des Anneles, 1865, Appendix, pp. 675 6.
 
9. De Blainville. SuppL au Diet, des Sc. Nat. Vol. I.
 
10. Ed. Grube. " Untersuchungen lib. d. Bau von Peripatus Edwardsii? Archiv fur Anat. und Physiol. 1853.
 
11. Saenger. " Moskauer Naturforscher Sammlung," Abth. Zool.
1869.
 
12. H. N. Moseley. "On the Structure and Development of Peripatus
capensis? Proc. Roy. Soc. N.O. 153, 1874.
 
13. H. N. Moseley. " On the Structure and Development of Peripatus
capensis," Phil. Trans. Vol. CLXIV. 1874.
 
14. H. N. Moseley. "Remarks on Observations by Captain Hutton,
Director of the Otago Museum, on Peripatus novce zealandice," Ann. and
Mag. of Nat. History, Jan. 1877.
 
15. Captain Hutton. " Observations on Peripatus novce sealandice,"
Ann. and Mag. of Nat. History, Nov. 1876.
 
16. F. M. Balfour. "On Certain Points in the Anatomy of Peripatus
capensis" Quart. Journ. of Micr. Science, Vol. xix. 1879.
 
17. A. Ernst. Nature, March loth, 1881.
 
 
 
EXPLANATION OF PLATES. 915
 
 
 
EXPLANATION OF PLATES 4653!.
 
 
 
COMPLETE LIST OF REFERENCE LETTERS.
 
A. Anus. a. Dorso-lateral horn of white matter in brain, a.g. Accessory gland
of male (modified accessory leg gland), at. Antenna, at. n. Antennary nerve, b.
Ventro-lateral horn of white matter of brain. b. c. Body-cavity. bl. Blastopore.
C. Cutis. c. Postero-dorsal lobe of white matter of brain. e.g. Supracesophageal
ganglia, cl. Claw. c. m. Circular layer of muscles, co. Commissures between the
ventral nerve-cords, co. i. Second commissure between the ventral nerve-cords.
co 1 . 2. Mass of cells developed on second commissure, cor. Cornea, c. s. d. Common duct for the two salivary glands. . cu. Cuticle, d. Ventral protuberance of
brain. d. 1. m. Dorsal longitudinal muscle of pharynx. d. n. Median dorsal nerve
to integument from supraoesophageal ganglia, d. o. Muscular bands passing from the
ventro-lateral wall of the pharynx at the region of its opening into the buccal cavity.
E. Eye. E. Central lobe of white matter of brain, e. n. Nerves passing outwards from
the ventral cords, ep. Epidermis, ep.c. Epidermis cells. F. i, F. a, &c. First and
second pair of feet, c. f. Small accessory glandular tubes of the male generative
apparatus. F.^. Ganglionic enlargement on ventral nerve-cord, from which a pair of
nerves to foot pass off. f. gl. Accessory foot-gland. F. n. Nerves to feet. g. co.
Commissures between the ventral nerve-cords containing ganglion cells, g. o. Generative orifice. H. Heart, h. Cells in lateral division of body-cavity. hy. Hypoblast, i.j. Inner jaw. j. Jaw. j. n. Nerves to jaws. L. Lips. /. Lens. /. b. c.
Lateral compartment of body-cavity, le. Jaw lever (cuticular prolongation of inner
jaw lying in a backwardly projecting diverticulum of the buccal cavity). /. m. Bands
of longitudinal muscles. M. Buccal cavity. M 1 . Median backward diverticulum of
mouth or common salivary duct which receives the salivary ducts, me. Mesenteron.
mes. Mesoblastic somite, m. 1. Muscles of jaw lever, m. s. Sheets of muscle passing
round the side walls of pharynx to dorsal body wall. od. Oviduct, ce. OZsophagus.
a's. co. OZsophageal commissures, o.f. g. Orifice of duct of foot-gland, o.j. Outer
jaw. op. Optic ganglion, op. n. Optic nerve, or.g. Ganglionic enlargements for
oral papillae, o r. n. Nerves to oral papillae, or. p. Oral papillas. o. s. Orifice of
duct of segmental organ, ov. Ovary, p. Pads on ventral side of foot. p. Common
duct into which the vasa deferentia open. p. c. Posterior lobe of brain. /. d. c.
Posterior commissure passing dorsal to rectum. /./. Internal opening of nephridium
into body cavity, ph. Pharynx, pi. Pigment in outer ends of epidermic cells, pi. r.
Retinal pigment, p. n. Nerves to feet. p.p. Primary papilla, pr. Prostate. R.
Rectum. Re. Retinal rods. R. m. Muscle of claw. s. Vesicle of nephridium. j 1 .
Part of 4th or 5th nephridium which corresponds to vesicle of other nephridia.
 
1 The explanations of the figures printed within inverted commas are by Professor
Balfour, the rest are by the Editors.
 
 
 
91 6 EXPLANATION OF PLATES.
 
s. c. i. Region No. i of coiled tube of nephridium. s. c. 2. Region No. i of ditto.
s. c. 3. Region No. 3 of ditto. s. c. 4. Region No. 4 of ditto, s. d. Salivary duct.
s. g. Salivary gland, si. d. Reservoir of slime gland, sl.g. Tubules of slime gland.
s. o. i, 2, 3, &c. Nephridia of ist, 2nd, &c., feet. s. o.f. Terminal portion of nephridium. s.p. Secondary papilla, st. Stomach, sf. e. Epithelium of stomach, sy.
Sympathetic nerve running in muscles of tongue and pharynx, sy 1 . Origin of pharyngeal sympathetic nerves. T. Tongue, t. Teeth on tongue, te. Testis. tr. Trach.e0e.
tr. c. Cells found along the course of the tracheae. tr. o. Tracheal stigma, tr. p.
Tracheal pit. tit. Uterus, v. c. Ventral nerve cord. v. d. Vas deferens. v. g.
Imperfect ganglia of ventral cord.
 
PLATE 46.
 
Fig. i. Peripatus capensis, x 4 ; viewed from the dorsal surface. (From a
drawing by Miss Balfour. )
 
PLATE 47.
 
Fig. 2. A left leg of Peripatus capensis, viewed from the ventral surface ; x 30.
(From a drawing by Miss Balfour.)
 
P'ig. 3. A right leg of Peripatus capensis, viewed from the front side. (From a
drawing by Miss Balfour.)
 
Fig. 4. .The last left (i7th) leg of a male Peripatus capensis, viewed from the
ventral side to shew the papilla at the apex of which the accessory gland of the male,
or enlarged crural gland, opens to the exterior. (From a drawing by Miss Balfour.)
Prof. Balfour left a rough drawing (not reproduced) shewing the papilla, to which is
appended the following note. " Figure shewing the accessory genital gland of male,
which opens on the last pair of legs by a papilla on the ventral side. The papilla has
got a slit-like aperture at its extremity."
 
Fig. 5. Ventral view of head and oral region of Peripatus capensis. (From a
drawing by Miss Balfour.)
 
PLATE 48.
 
Figs. 6 and 7 are from one drawing.
 
Fig. 6. Peripatus capensis dissected so as to shew the alimentary canal, slime
glands, and salivary glands ; x 3. (From a drawing by Miss Balfour.)
 
Fig. 7. The anterior end of Fig. 6 enlarged ; x 6. (From a drawing by Miss
Balfour.) The dissection is viewed from the ventral side, and the lips, L., have been
cut through in the middle line behind and pulled outwards, so as to expose the jaws,
/., which have been turned outwards, and the tongue, T. , bearing a median row of
chitinous teeth, which branches behind into two. The junction of the salivary ducts,
j. d., and the opening of the median duct so formed into the buccal cavity is also
shewn. The muscular pharynx, extending back into the space between the ist and
2nd pairs of legs, is followed by a short tubular oesophagus. The latter opens into
the large stomach with plicated walls, extending almost to the hind end of the animal.
The stomach at its point of junction with the rectum presents an S-shaped ventrodorsal curve.
 
 
A. Anus. at. Antenna. F. i, K. 2. First and second feet. /. Jaws. L. Lips.
ae. OZsophagus. or. p. Oral papilla, ph. Pharynx. R. Rectum, s. d. Salivary
duct. s. g. Salivary gland, si. d. Slime reservoir, si. g. Portion of tubules of slime
gland, st. Stomach. T. Tongue in roof of mouth.
 
Fig. 8. Peripatus capensis, X4; male. (From a drawing by Miss Balfour.)
Dissected so as to shew the nervous system, slime glands, ducts of the latter passing
into the oral papilla, accessory glands opening on the last pair of legs (enlarged crural
glands), and segmental organs, viewed from dorsal surface. The first three pairs of
segmental organs consist only of the vesicle and duct leading to the exterior. The
fourth and fifth pairs are larger than the succeeding, and open externally to the crural
glands. The ventral nerve-cords unite behind dorsal to the rectum.
 
A. Anus. a. g. Accessory generative gland, or enlarged crural gland of the iyth
leg. at. Antenna, c. g. Supra-oesophageal ganglia with eyes. co. Commissures
between the ventral nerve-cords, d. n. Large median nerve to dorsal integument from
hinder part of brain. F. i, i, &c. Feet. g. o. Generative orifice, <x. (Esophagus.
KS. co. QEsophageal commissures, or. p. Oral papilla, p.d.c. Posterior dorsal commissure between the ventral nerve-cords, ph. Pharynx, p. n. Nerves to feet, one
pair from each ganglionic enlargement. si. d. Reservoir of slime gland. si. g.
Tubules of slime gland. s. o. i, 2, 3, &c. Segmental organs. v. c. Ventral nervecords, "v. g. Imperfect ganglia of ventral cords.
 
Figs. 9 and 10. Left jaw of Peripatus capensis (male), shewing reserve jaws.
(From a drawing by Miss Balfour.)
 
Fig. 9. Inner jaw.
Fig. 10. Outer jaw.
 
PLATE 49.
 
Figs, ii 16. A series of six transverse sections through the head of Peripatus
capensis.
 
Fig. n. The section is taken immediately behind the junction of the supracesophageal ganglia, c. g., and passes through the buccal cavity, M., and jaws, o.j.
and i.j.
 
Fig. 12. The section is taken through the hinder part of the buccal cavity at the
level of the opening of the mouth into the pharynx and behind the jaws. The cuticular rod-like continuation (le.) of the inner jaw lying in a backwardly directed pit of
the buccal cavity is shewn; on the right hand side the section passes through the
opening of this pit.
 
Fig. 1 3. The section passes through the front part of the pharynx, and shews the
opening into the latter of the median backward diverticulum of the mouth (M 1 ),
which receives the salivary ducts. It also shews the commencement of the ventral
nerve-cords, and the backwardly projecting lobes of the brain.
 
Fig. 14. The section passes through the anterior part of the pharynx at the level
of the second commissure (co. 2), between the ventral nerve-trunks, and shews the
mass of cells developed on this commissure, which is in contact with the epithelium of
the backward continuation of the buccal cavity (M 1 ).
 
 
 
QI 8 EXPLANATION OF PLATES.
 
Fig. 15. Section through the point of junction of the salivary ducts with the
median oral diverticulum.
 
Fig. 1 6. Section behind the pharynx through the oesophagus.
 
b. c. Body-cavity. C. Cutis. c. b. c. Central compartment of body-cavity, c. g.
Supra-oesophageal ganglia, c. m. Layer of circular muscles, co. Commissure between
ventral nerve-cords. co. i. Second commissure between the ventral nerve-cords.
co 1 . i. Mass of cells developed on second commissure (probably sensory), c. s. d.
Common duct for the two salivary glands, d. /. m. Dorsal longitudinal muscles of
pharynx, d. o. Muscles serving to dilate the opening of the pharynx. Ep. Epidermis, e. n. Nerve passing outwards from ventral nerve-cord. H. Heart, i.j. Inner
jaw. j. p. Jaw papillae. L. Lips of buccal cavity. /. b. c. Lateral compartment of
body-cavity, le. Rod-like cuticular continuation of inner jaw, lying in a pit of the
buccal cavity. /. m. Bands of longitudinal muscles. M. Buccal cavity. M 1 . Median
backward continuation of buccal cavity, m. 1. Muscles of jaw lever, m. s. Muscular
sheets passing from side walls of pharynx to dorsal body wall. ce. CEsophagus.
ces. co. CEsophageal commissures. o.j. Outer jaw. ph. Pharynx, s. d. Salivary
duct. s. g. Salivary gland, si. d. Reservoir of slime gland, sy. Sympathetic nerves
running in muscles of tongue or pharynx, sy 1 . Origin of sympathetic nerves to
pharynx. T. Tongue, v. c. Ventral nerve-cords.
 
Figs. 17, 1 8. Two longitudinal horizontal sections through the head of Peripatus
capensis. Fig. 17 is the most ventral. They are both taken ventral to the cerebral
ganglia. In Fig. 17 dorsal tracheal pits are shewn with tracheae passing off from
them. (Zeiss a a, Hartnack's camera.) C. Cutis. c. s. d. Common salivary duct.
ep. Epidermis, i.j. Inner jaw. M. Buccal cavity. M 1 . Median backward diverticulum of mouth, o.j. Outer jaw. s. d. Salivary ducts. T. Tongue, t. Teeth on
tongue, tr. Tracheae, tr. p. Tracheal pits.
 
 
 
PLATE 50.
 
Fig. 19. "A, B, c, D, E, F, G. Seven transverse sections illustrating the structure
of the- supra- cesophageal ganglia. (Zeiss A, Hartnack's camera.) a. Dorso-lateral
horn of white matter. b. Ventro-lateral horn of white matter, c. Postero-dorsal
lobe of white matter, d. Ventral protuberance of brain, e. Central lobe of white
matter, o.p. Optic ganglion.
 
" A. Section through anterior portions of ganglia close to the origin of the antennary nerve. B. Section a little in front of the point where the two ganglia unite, c.
Section close to anterior junction of two ganglia. D. Section through origin of optic
nerve on the right side. E. Section shewing origin of the optic nerve on the left side.
F. Section through the dorso-median lobe of white matter. G. Section near the termination of the dorsal tongue of ganglion cells."
 
PLATE 51.
 
Fig. 10. Portion of a transverse section through the hinder part of Peripatus
capensis (male). The section passes through a leg, and shews the opening of the
segmental organ (p. s.), and of a crural gland, o.f.g., and the forward continuation of
the enlarged crural gland of the i7th leg (/ g!.). (Zeiss a a, Hartnack's camera.) a-g.
accessory gland of male (modified crural gland of last leg), c. Cutis. cL Claw.
cu. Cuticle, ep. Epidermis, f.gl. Crural gland, h. Cells in lateral compartment of
body cavity, o.f. g. Orifice of accessory foot gland, o. s. Opening of segmental
organ, p. Three spinous pads on ventral surface of foot. pr. Prostate. R. M.
Retractor muscle of claw. s. Vesicle of nephridium. s. c. i. Region No. i of coiled
part of nephridium. si. g. Tubule of slime gland, s. o. t. Terminal portion of nephridium. st. Stomach, st. e. Epithelium of stomach, v. c. Ventral nerve-cord, v. d.
Vas deferens.
 
Fig. 21. "Longitudinal vertical section through the supra-oesophageal ganglion
and oesophageal commissures of Peripatus capensis. (Zeiss a a, Hartnack.)" at. Antenna, e. Central lobe of white matter. /. Part of jaw. s. g. Salivary gland.
 
Fig. 22: drawn by Miss Balfour. Brain and anterior part of the ventral nervecords of Peripatus capensis enlarged and viewed from the ventral surface. The paired
appendages (d) of the ventral surface of the brain are seen, and the pair of sympathetic
nerves (sy 1 ) arising from the ventral surface of the hinder part.
 
From the commencement of the cesophageal commissures (as. co. ) pass off on each
side a pair of nerves to the jaws (/. .).
 
The three anterior commissures between the ventral nerve-cords are placed close
together; immediately behind them the nerve-cords are swollen, to form the ganglionic
enlargements from which pass off to the oral papillce a pair of large nerves on each
side (or. n. )
 
Behind this the cords present a series of enlargements, one pair for each pair of
feet, from which a pair of large nerves pass off on each side to the feet (p. n). at. n.
Antennary nerves, co. Commissures between ventral cords, d. Ventral appendages
of brain. E. Eye. e. n. Nerves passing outwards from ventral cord. F-g- Ganglionic enlargements from which nerves to feet pass off. j. n. Nerves to jaws. or. g.
Ganglionic enlargement from which nerves to oral papillce pass off. or. n. Nerves to
oral papillae, p.c. Posterior lobe of brain, p. n. Nerves to feet. s.y. Sympathetic
nerves.
 
Fig. 23. "Longitudinal horizontal section through the head of Peripatus capensis,
shewing the structure of the brain, the antennary and optic nerves, &c. (Zeiss a a,
Hartnack's camera.)" at. Antenna, at. n. Antennary nerve, cor. Cornea, e.
Central mass of white matter. /. Lens. op. n. Optic nerve, ph. Pharynx, p.p.
Primary papilla covered with secondary papillte and terminating in a long spine, sy.
Pharyngeal sympathetic nerves.
 
Fig. 24. "Eye of Peripatus capensis, as shewn in a longitudinal horizontal section
through the head. The figure is so far diagrammatic that the lens is represented as
filling up the whole space between the rods and the cornea. In the actual section
there is a considerable space between the parts, but this space is probably artificial,
being in part caused by the shrinkage of the lens and in part by the action of the
razor. (Zeiss c, Hartnack's camera.)" (It appears that the ganglionic region of the
eye is covered by a thin capsule, which is omitted in the figure.)
 
cor. Cornea. /. Lens. op. Optic ganglion, op-, n. Optic nerve. //'. r. Pigment.
Re. rods. s. p. Secondary papillae.
 
 
Fig. 25. Longitudinal horizontal section through the dorsal skin, shewing the
peculiar arrangement of the circular muscular fibres. (Zeiss A, Hartnack's camera.)
 
PLATE 52.
 
Fig. 26. Portion of ventral cord of Peripatus capensis enlarged, shewing two
ganglionic enlargements and the origin of the nerves and commissures. (From a
drawing by Miss Balfour.)
 
co. Commissures. E. n. Nerves passing out from ventral cords. F. n. Nerves to
feet. g. co. Commissures between the ventral cords containing ganglion cells, v. g.
Ganglionic enlargements.
 
Fig. 27. Segmental organ from the 5th pair of legs of Peripatus capensis. This
nephridium resembles those of the 4th legs, and differs from all the others in its large
size and in the absence of any dilatation giving rise to a collecting vesicle on its external
portion (enlarged). The terminal portion has the same histological characters as in
the case of the hinder segmental organs. (From a drawing by Miss Balfour. )
 
Fig. 28. Segmental organ or nephridium from the 9th pair of legs of Peripatus
capensis^ shewing the external opening, the vesicle, the coiled portion and the
terminal portion with internal opening (enlarged). (From a drawing by Miss
Balfour.)
 
o. s. External opening of segmental organ, p.f. Internal opening of nephridium
into the body-cavity (lateral compartment). s. Vesicle of segmental organ, j 1 .
Portion of segmental organ of 4th and 5th legs, corresponding to vesicle of the other
nephridia. s. c. i. First or external portion of coiled tube of nephridium, lined by
columnar epithelium with small nuclei ; the cells project for very different distances,
giving the inner boundary of this region a ragged appearance, s. c. 2. Region No. 2
of coiled tube of nephridium, lined by small closely-packed columnar cells, s. c. 3.
Region No. 3 of coiled tube of segmental organ, lined by large flat cells with
large disc-shaped nuclei, s. c. 4. Region No. 4 of coiled tube of nephridium ; this
region is very short and lined by small columnar cells, s. o. t. Terminal portion of
nephridium.
 
Fig. 29. " Portion of nephridium of the hindermost leg of Peripatus capensis, seen
in longitudinal and vertical section. The figure is given to shew the peritoneal funnel
of the nephridium. Portions of the collecting sack (s.) and other parts are also represented. (Zeiss B, Hartnack's camera.)"
 
p.f. Peritoneal funnel, s. Vesicle, s.c.i, s.c.i, s.c.$. Portions of coiled tube.
 
Fig. 30. " Section through a tracheal pit and diverging bundles of tracheal tubes"
taken transversely to the long axis of the body. (Zeiss E, oc. 2.) (From a rough
drawing by Prof. Balfour.)
 
tr. Tracheae, shewing rudimentary spiral fibre, tr. c. Cells resembling those
lining the tracheal pits, which occur at intervals along the course of the trachere.
tr. s. Tracheal stigma, tr. p. Tracheal pit.
 
Fig- 31. "Sense organs and nerves attached from antenna of Peripatus capensis
(Zeiss, immersion 2, oc. 2.)" (From a rough drawing by Prof. Balfour.) The figure
shews the arrangement of the epidermis cells round the base of the spine. The spine
is seen to be continuous with the inner layer of the cuticle.
 
 
 
EXPLANATION OF PLATE 53. 92 1
 
Fig. 32. Section through the skin of Peripatus capensis ; it shews the secondary
papillae covered with minute spinous tubercles and the relation of the epidermis to
them. (The cuticle in the process of cutting has been torn away from the subjacent
cells.) The cells of the epidermis are provided with large oval nuclei, and there is a
deposit of pigment in the outer ends of the cells. The granules in the protoplasm of
the inner ends of the cells are arranged in lines, so as to give a streaked appearance.
(Zeiss E, oc. 2.) (From a rough drawing by Prof. Balfour.)
 
c. Dermis. cu. Cuticle, ep. c. Epidermis cells, pi. Deposit of pigment in outer
ends of epidermis cells, s.p. Secondary papillae.
 
Fig. 33. Female generative organs of Peripatus capensis, x 5. (From a rough
drawing by Prof. Balfour.) The following note was appended to this drawing:
"Ovary rather to dorsal side, lying in a central compartment of body-cavity and
attached to one of the longitudinal septa, dividing this from the lateral compartment
between the penultimate pair of legs and that next in front. The oviducts cross
before opening to the exterior, the right oviduct passing under the rectum and the
left over it. They meet by opening into a common vestibule, which in its turn opens
below the anus. On each side of it are a pair of short papillae (aborted feet ?)."
 
F. 16, 17. Last two pairs of legs. od. Oviduct, ov. Ovary, ut. Uterus, v. c.
Nerve-cord.
 
 
 
PLATE 53.
 
 
 
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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.

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This historic 1885 book edited by Foster and Sedgwick is the first of Francis Balfour's collected works published in four editions. Francis (Frank) Maitland Balfour, known as F. M. Balfour, (November 10, 1851 - July 19, 1882) was a British biologist who co-authored embryology textbooks.



Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.
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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)


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Vol I. Separate Memoirs (1885)

XX. On the Development of the Skeleton of the Paired Fins of Elasmobranchii, considered in Relation to its Bearings on the Nature of the Limbs of the Vertebrata

(With Plate 33.)

SOME years ago the study of the development of the soft parts of the fins in several Elasmobranch types, more especially in Torpedo, led me to the conclusion that the vertebrate limbs were remnants of two continuous lateral fins 2 . More or less similar views (which I was not at that time acquainted with) had been previously held by Maclise, Humphrey, and other anatomists ; these views had not, however, met with much acceptance, and diverge in very important points from those put forward by me. Shortly after the appearance of my paper, J. Thacker published two interesting memoirs comparing the skeletal parts of the paired and unpaired fins 3 .

In these memoirs Thacker arrives at conclusions as to the nature of the fins in the main similar to mine, but on entirely independent grounds. He attempts to shew that the structure of the skeleton of the paired fins is essentially the same as that of the unpaired fins, and in this comparison lays special stress on the very simple skeleton of the pelvic fin in the cartilaginous Ganoids, more especially in Acipenscr and Polyodon. He points out that the skeleton of the pelvic fin of Polyodon consists essentially of a series of nearly isolated rays, which have a strikingly similar arrangement to that of the rays of the skeleton in

1 From the Proceedings of the Zoological Society of London, 1881.

2 "Monograph on the Development of Elasmobranch Fishes," pp. 319, 320.

3 J. K. Thacker, " Median and Paired Fins ; a Contribution to the History of the Vertebrate Limbs," Trans, of the Connecticut Acad. Vol. in. 1877. "Ventral Fins of Ganoids," Trans, of the Connecticut Acad. Vol. iv. 1877.


SKELETON OF THE PAIRED FINS OF ELASMOBRANCHS. 715

many unpaired fins. He sums up his views in the following way 1 :

"As the dorsal and anal fins were specializations of the median folds of Amphioxus, so the paired fins were specializations of the two lateral folds which are supplementary to the median in completing the circuit of the body. These lateral folds, then, are the homologues of Wolffian ridges, in embryos of higher forms. Here, as in the median fins, there were formed chondroid and finally cartilaginous rods. These became at least twice segmented. The orad ones, with more or less concrescence proximally, were prolonged inwards. The cartilages spreading met in the middle line ; and a later extension of the cartilages dorsad completed the limb-girdle.

" The limbs of the Protognathostomi consisted of a series of parallel articulated cartilaginous rays. They may have coalesced somewhat proximally and orad. In the ventral pair they had extended themselves mesiad until they had nearly or quite met and formed the hip-girdle ; they had not here extended themselves dorsad. In the pectoral limb the same state of things prevailed, but was carried a step further, namely, by the dorsal extension of the cartilage constituting the scapular portion, thus more nearly forming a ring or girdle."

The most important point in Thacker's theories which I cannot accept is the derivation of the folds, of which the paired fins of the Vertebrata are supposed to be specializations, from the lateral folds of Amphioxus ; and Thacker himself recognizes that this part of his theory stands on quite a different footing to the remainder.

Not long after the publication of Thacker's paper, an important memoir was published by Mivart in the Transactions of this Society 2 . The object of the researches recorded in this paper was, as Mivart explains, to test how far the hard parts of the limbs and of the azygos fins may have arisen through centripetal chondrifications or calcifications, and so be genetically exoskeletal 3 .

1 Loc. cit. p. 298.

2 St George Mivart, "On the Fins of Elasmobranchii," Zoological Trans. Vol. X.

3 Mivart used the term exoskeletal in an unusual and (as it appears to me) inconvenient manner. The term is usually applied to dermal skeletal structures ; but the

46 2


716 DEVELOPMENT OF THE SKELETON

Mivart's investigations and the majority of his views were independent of Thacker's memoir ; but he acknowledges that he has derived from Thacker the view that pelvic and pectoral girdles, as well as the skeleton of the limbs, may have arisen independently of the axial skeleton.

The descriptive part of Mivart's paper contains an account of the structure of a great variety of interesting and undescribed types of paired and unpaired fins, mainly of Elasmobranchii. The following is the summary given by Mivart of the conclusions at which he has arrived ' :

" i. Two continuous lateral longitudinal folds were developed, similar to dorsal and ventral median longitudinal folds.

" 2. Separate narrow solid supports (radials), in longitudinal series, and with their long axes directed more or less outwards at right angles with the long axis of the body, were developed in varying extents in all these four longitudinal folds.

" 3. The longitudinal folds became interrupted varidusly, but so as to form two prominences on each side, i.e. the primitive paired limbs.

" 4. Each anterior paired limb increased in size more rapidly than the posterior limb.

" 5. The bases of the cartilaginous supports coalesced as was needed, according to the respective practical needs of the different separate portions of the longitudinal folds, i.e. the respective needs of the several fins.

"6. Occasionally the dorsal radials coalesced (as in Notidanus, &c.) and sought centripetally (Pristis, &c.) adherence to the skeletal axis.

" 7. The radials of the hinder paired limb did so more constantly, and ultimately prolonged themselves inwards by mesiad growth from their coalesced base, till the piscine pelvic structure arose, as, e.g., in Squatina.

" 8. The pectoral radials with increasing development also coalesced proximally, and thence prolonging themselves inwards to seek a point cTappui, shot dorsad and ventrad to obtain a firm support, and at the same time to avoid the visceral cavity.

skeleton of the limbs, with which we are here concerned, is undoubtedly not of this nature.

1 Loc. cit. p. 480.


OF THE PAIRED FINS OF ELASMOBRANCHS. 717


Thus they came to abut dorsally against the axial skeleton, and to meet ventrally together in the middle line below.

" 9. The lateral fins, as they were applied to support the body on the ground, became elongated, segmented, and narrowed, so that probably the line of the propterygium, or possibly that of the mesopterygium, became the cheiropterygial axis.

" 10. The distal end of the incipient cheiropterygium either preserved and enlarged preexisting cartilages or developed fresh ones to serve fresh needs, and so grew into the developed cheiropterygium ; but there is not yet enough evidence to determine what was the precise course of this transformation.

" II. The pelvic limb acquired a solid connection with the axial skeleton (a pelvic girdle) through its need of a point cVappui as a locomotive organ on land..

" 12. The pelvic limb became also elongated ; and when its function was quite similar to that of the pectoral limb, its structure became also quite similar (e.g. Ichthyosaurus, Plesiosaurus, CJielydra, &c.) ; but for the ordinary quadrupedal mode of progression it became segmented and inflected in a way generally parallel with, but (from its mode of use) in part inversely to, the inflections of the pectoral limb."

Giinther 1 has propounded a theory on the primitive character of the fins, which, on the whole, fits in with the view that the paired fins are structures of the same nature as the unpaired fins. The interest of Giinther's views on the nature of the skeleton of the fins more especially depends upon the fact that he attempts to evolve the fin of Ceratodus from the typical Selachian type of pectoral fin. His own statement on this subject is as follows z :

" On further inquiry into the more distant relations of the Ceratodus-\\mb t we may perhaps be justified in recognizing in it a modification of the typical form of the Selachian pectoral fin. Leaving aside the usual treble division of the carpal cartilage (which, indeed, is sometimes simple), we find that this shovellike carpal forms the base for a great number of phalanges, which are arranged in more or less regular transverse rows (zones) and in longitudinal rows (series). The number of phalanges of

1 " Description of Ceratodus,'" Phil. Trans. 1871. ' 2 Loc. cit. p. 534.


7l8 DEVELOPMENT OF THE SKELETON '

the zones and series varies according to the species and the form of the fin ; in Cestracion philippi the greater number of phalanges is found in the proximal zones and middle series, all the phalanges decreasing in size from the base of the fin towards the margins. In a Selachian with a long, pointed, scythe-shaped pectoral fin, like that of Ceratodus, we may, from analogy, presume that the arrangement of the cartilages might be somewhat like that shewn in the accompanying diagram, which I have divided into nine zones and fifteen series.

" When we now detach the outermost phalanx from each side of the first horizontal zone, and with it the other phalanges of the same series, when we allow the remaining phalanges of this zone to coalesce into one piece (as, in nature, we find coalesced the carpals of Ceratodus and many phalanges in Selachian fins), and when we repeat this same process with the following zones and outer series, we arrive at an arrangement identical with what we actually find in Ceratodus"

While the researches of Thacker and Mivart are strongly confirmatory of the view at which I had arrived with reference to the nature of the paired fins, other hypotheses as to the nature of the skeleton of the fins have been enunciated, both before and after the publication of my memoir, which are either directly or indirectly opposed to my view.

Huxley in his memoir on Ceratodus, which throws light on so many important morphological problems, has dealt with the nature of paired fins 1 .

He holds, in accordance with a view previously adopted by Gegenbaur, that the limb of Ceratodus "presents us with the nearest known approximation to the fundamental form of vertebrate limb or archipterygium," and is of opinion that in a still more archaic fish than Ceratodtis the skeleton of the fin " would be made up of homologous segments, which might be termed pteromeres, each of which would consist of a mesomere with a preaxial and a postaxial paramere." He considers that the pectoral fins of Elasmobranchii, more especially the fin of Notidamts, which he holds to be the most primitive form of Elasmobranch fin, " results in the simplest possible manner from the

1 T. H. Huxley, " On Ceratodus Fosteri, with some Observations on the Classification of Fishes," Proc. Zool. Soc. 1876.


OF THE PAIRED FINS OF ELASMOBRANCHS. 719

shortening of the axis of such a fin-skeleton as that of Ceratodus, and the coalescence of some of its elements." Huxley does not enter into the question of the origin of the skeleton of the pelvic fin of Elasmobranchii.

It will be seen that Huxley's idea of the primitive structure of the archipterygium is not easily reconcilable with the view that the paired fins are parts of a once continuous lateral fin, in that the skeleton of such a lateral fin, if it has existed, must necessarily have consisted of a series of parallel rays.

Gegenbaur 1 has done more than any other living anatomist to elucidate the nature of the fins ; and his views on this subject have undergone considerable changes in the course of his investigations. After Gunther had worked out the structure of the fin of Ceratodus, Gegenbaur suggested that it constituted the most primitive persisting type of fin, and has moreover formed a theory as to the origin of the fins founded on this view, to the effect that the fins, together with their respective girdles, are to be derived from visceral arches with their rays.

His views on this subject are clearly explained in the subjoined passages quoted from the English translation of his Elements of Comparative Anatomy, pp. 473 and 477.

"The skeleton of the free appendage is attached to the extremity of the girdle. When simplest, this is made up of cartilaginous rods (rays), which differ in their size, segmentation, and relation to one another. One of these rays is larger than the rest, and has a number of other rays attached to its sides. I have given the name of archipterygium to the ground-form of the skeleton which extends from the limb-bearing girdle into the free appendage. The primary ray is the stem of this archipterygium, the characters of which enable us to follow out the lines of development of the skeleton of the appendage. Cartilaginous arches beset with the rays form the branchial skeleton. The form of skeleton of the appendages may be compared with

1 C. Gegenbaur, Untersuchungen z. vergleich. Anat. d. Wirbelthiere (Leipzig 1864-5): erstes Heft, "Carpus u. Tarsus;" zweites Heft, " Brustflosse d. Fische." "Ueb. d. Skelet d. Gliedmaassen d. Wirbelthiere im Allgemeinen u. d. Hintergliedmaassen d. Selachier insbesondere," Jenaische Zeitschrift, Vol. v. 1870. " Ueb. d. Archipterygium," Jenaische Zeitschrift, Vol. vn. 1873. " Zur Morphologic d. Gliedmaassen d. Wirbelthiere, " Morphologisches Jahrbuch, Vol. II. 1876.


72O DEVELOPMENT OF THE SKELETON

them ; and we are led to the conclusion that it is possible that they may have been derived from such forms. In the branchial skeleton of the Selachii the cartilaginous bars are beset with simple rays. In many a median one is developed to a greater size. As the surrounding rays become smaller, and approach the larger one, we get an intermediate step towards that arrangement in which the larger median ray carries a few smaller ones. This differentiation of one ray, which is thereby raised to a higher grade, may be connected with the primitive form of the appendicular skeleton ; and as we compare the girdle with a branchial arch, so we may compare the median ray and its secondary investment of rays with the skeleton of the free appendage.

"All the varied forms which the skeleton of the free appendages exhibits may be derived from a ground-form which persists in a few cases only, and which represents the first, and consequently the lowest, stage of the skeleton in the fin the archipterygium. This is made up of a stem which consists of jointed pieces of cartilage, which is articulated to the shouldergirdle and is beset on either side with rays which are likewise jointed. In addition to the rays of the stem there are others which are directly attached to the limb-girdle.

" Ceratodus has a fin-skeleton of this form ; in it there is a stem beset with two rows of rays. But there are no rays in the shoulder-girdle. This biserial investment of rays on the stem of the fin may also undergo various kinds of modifications. Among the Dipnoi, Protopterus retains the medial row of rays only, which have the form of fine rods of cartilage; in the Selachii, on the other hand, the lateral rays are considerably developed. The remains of the medial row are ordinarily quite small, but they are always sufficiently distinct to justify us in supposing that in higher forms the two sets of rays might be better developed. Rays are still attached to the stem and are connected with the shoulder-girdle by means of larger plates. The joints of the rays are sometimes broken up into polygonal plates which may further fuse with one another ; concrescence of this kind may also affect the pieces which form the base of the fin. By regarding the free rays, which are attached to these basal pieces, as belonging to these basal portions, we are able to


OF THE PAIRED FINS OF F.LASMOBRANCHS. 721

divide the entire skeleton of the fin into three segments pro-, meso-, and metapterygium.

"The metapterygium represents the stem of the archipterygium and the rays on it. The propterygium and the mesopterygium are evidently derived from the rays which still remain attached to the shoulder-girdle."

Since the publication of the memoirs of Thacker, Mivart, and myself, a pupil of Gegenbaur's, M. v. Davidoff 1 , has made a series of very valuable observations, in part directed towards' demonstrating the incorrectness of our theoretical views, more especially Thacker's and Mivart's view of the genesis of the skeleton of the limbs. Gegenbaur 2 has also written a short paper in connection with Davidoff's memoir, in support of his own as against our views.

It would not be possible here to give an adequate account of Davidoff's observations on the skeleton, muscular system, and nerves of the pelvic fins. His main argument against the view that the paired fins are the remains of a continuous lateral fin is based on the fact that a variable but often considerable number of the spinal nerves in front of the pelvic fin are united by a longitudinal commissure with the true plexus of the nerves supplying the fin. From this he concludes that the pelvic fin has shifted its position, and that it may once therefore have been situated close behind the visceral arches. Granting, however, that Davidoff's deduction from the character of the pelvic plexus is correct, there is, so far as I see, no reason in the nature of the lateral-fin theory why the pelvic fins should not have shifted ; and, on the other hand, the longitudinal cord connecting some of the ventral roots in front of the pelvic fin may have another explanation. It may, for instance, be a remnant of the time when the pelvic fin had a more elongated form than at present, and accordingly extended further forwards.

In any case our knowledge of the nature and origin of nervous plexuses is far too imperfect to found upon their characters such conclusions as those of Davidoff.

1 M. v. Davidoff, " Beitrage z. vergleich. Anat. d. hinteren Gliedmaassen d. Fische, I.," Morphol. Jahrbuch, Vol. V. 1879.

2 "Zur Gliedmaassenfrage. An die Untersuchungen von Davidoff's angekniipfte Bemerkungen," Morphol. Jahrbuch, Vol. v. 1879.


722 DEVELOPMENT OF THE SKELETON

Gegenbaur, in his paper above quoted, further urges against Thacker and Mivart's views the fact that there is no proof that the fin of Polyodon is a primitive type ; and also suggests that the epithelial line which I have found connecting the embryonic pelvic and pectoral fins in Torpedo may be a rudiment indicating a migration backwards of the pelvic fin.

With reference to the development of the pectoral fin in the Teleostei there are some observations of 'Swirski 1 , which unfortunately do not throw very much light upon the nature of the limb.

'Swirski finds that in the Pike the skeleton of the limb is formed of a plate of cartilage continuous with the pectoral girdle, which soon becomes divided into a proximal and a distal portion. The former is subsequently segmented into five basal rays, and the latter into twelve parts, the number of which subsequently becomes reduced.

The observations which I have to lay before the Society were made with the object of determining how far the development of the skeleton of the limbs throws light on the points on which the anatomists whose opinions have just been quoted are at variance.

They were made, in the first instance, to complete a chapter in my work on comparative embryology ; and, partly owing to the press of other engagements, but still more to the difficulty of procuring material, my observations are confined to the two British species of the genus Scy Ilium, viz. Sc. stellare and Sc. canicula; yet I venture to believe that the results at which I have arrived are not wholly without interest.

Before dealing with the development of the skeleton of the fin, it will be convenient to describe with great brevity the structure of the pectoral and pelvic fins of the adult. The pectoral fins consist of broad plates inserted horizontally on the sides of the body ; so that in each there may be distinguished a dorsal and a ventral surface, and an anterior and a posterior border. Their shape may best be gathered from the woodcut (fig. i) ; and it is to be especially noted that the narrowest part

1 G. 'Swirski, Untersuch. ilb. d. Entivick. d. Schtdtergiirtds u. d. Skelets d. Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1 880.


OF THE PAIRED FINS OF ELASMOBRANCHS.


723


of the fin is the base, where is it attached to the side of the body. The cartilaginous skeleton only occupies a small zone at the base of the fin, the remainder being formed of a fringe supported by radiately arranged horny fibres 1 .

FIG. i.



Pectoral fins and girdle of an adult of Scyllium canicula (natural size,

seen from behind and above).

co. Coracoid. sc. scapula, pp. propterygium. me p. mesopterygium. mp. metapterygium. fn. part of fin supported by horny fibre.

FIG. 2.



JJTL


Right pelvic fin and part of pelvic girdle of an adult female of Scyllium

canmila (natural size).

il. iliac process, pn. pubic process, cut across below, bp. basipterygium. a/, anterior cartilaginous fin-ray articulated to pelvic girdle, fn. part of fin supported by horny fibres.

1 The horny fibres are mesoblastic products; they are formed, in the first instance, as extremely delicate fibrils on the inner side of the membrane separating the epiblast from the mesoblast.


724 DEVELOPMENT OF THE SKELETON

The true skeleton consists of three basal pieces articulating with the pectoral girdle ; on the outer side of which there is a series of more or less segmented cartilaginous fin-rays. Of the basal cartilages one (J>p) is anterior, a second (mep] is placed in the middle, and a third is posterior (mp}. They have been named by Gegenbaur the propterygium, the mesopterygium, and the metapteryginm ; and these names are now generally adopted.

The metapterygium is by far the most important of the three, and in Scyllium canicula supports 12 or 13 rays 1 . It forms a large part of the posterior boundary of the fin, and bears rays only on its anterior border.

The mesopterygium supports 2 or 3 rays, in the basal parts of which the segmentation into distinct rays is imperfect ; and the propterygium supports only a single ray.

The pelvic fins are horizontally placed, like the pectoral fins, but differ from the latter in nearly meeting each other along the median ventral line of the body. They also differ from the pectoral fins in having a relatively much broader base of attachment to the sides of the body. Their cartilaginous skeleton (woodcut, fig. 2) consists of a basal bar, placed parallel to the base of the fin, and articulated in front with the pelvic girdle.

On its outer border it articulates with a series of cartilaginous fin-rays. I shall call the basal bar the basipterygium. The rays which it bears are most of them less segmented than those of the pectoral fin, being only divided into two ; and the posterior ray, which is placed in the free posterior border of the fin, continues the axis of the basipterygium. In the male it is modified in connection with the so-called clasper.

The anterior fin-ray of the pelvic fin, which is broader than the other rays, articulates directly with the pelvic girdle, instead of with the basipterygium. This ray, in the female of Scyllium canicula and in the male of Scyllinm catulus (Gegenbaur), is peculiar in the fact that its distal segment is longitudinally divided into two or more pieces, instead of being single as is the case with the remaining rays. It is probably equivalent to two of the posterior rays.

1 In one example where the metapterygium had 13 rays the mesopterygium had only 2 rays.


OF THE PAIRED FINS OF ELASMOBRANCHS. 725

Development of the paired Fins. The first rudiments of the limbs appear in Scy Ilium, as in other fishes, as slight longitudinal ridge-like thickenings of the epiblast, which closely resemble the first rudiments of the unpaired fins.

These ridges are two in number on each side an anterior immediately behind the last visceral fold, and a posterior on the level of the cloaca. In most Fishes they are in no way connected ; but in some Elasmobranch embryos, more especially in that of Torpedo, they are connected together at their first development by a line of columnar-epiblast cells. This connecting line of columnar epiblast, however, is a very transitory structure. The rudimentary fins soon become more prominent, consisting of a projecting ridge both of epiblast and mesoblast, at the outer edge of which is a fold of epiblast only, which soon reaches considerable dimensions. At a later stage the mesoblast penetrates into this fold, and the fin becomes a simple ridge of mesoblast covered by epiblast. The pectoral fins are at first considerably ahead of the pelvic fins in development.

The direction of the original epithelial line which connected the two fins of each side is nearly, though not quite, longitudinal, sloping somewhat obliquely ventralwards. It thus comes about that the attachment of each pair of limbs is somewhat on a slant, and that the pelvic pair nearly meet each other in the median ventral line shortly behind the anus.

The embryonic muscle-plates, as I have elsewhere shewn, grow into the bases of the fins ; and the cells derived from these ingrowths, which are placed on the dorsal and ventral surfaces in immediate contact with the epiblast, probably give rise to the dorsal and ventral muscular layers of the limb, which are shewn in section in Plate 33, fig. I m, and in Plate 33, fig. 7 m.

The cartilaginous skeleton of the limbs is developed in the indifferent mesoblast cells between the two layers of muscles. Its early development in both the pectoral and the pelvic fins is very similar. When first visible it differs histologically from the adjacent mesoblast simply in the fact of its cells being more concentrated ; while its boundary is not sharply marked.

At this stage it can only be studied by means of sections. It arises simultaneously and continuously with the pectoral and pelvic girdles, and consists, in both fins, of a bar springing at


/26 DEVELOPMENT OF THE SKELETON

right angles from the posterior side of the pectoral or pelvic girdle, and running parallel to the long axis of the body along the base of the fin. The outer side of this bar is continued into a thin plate, which extends into the fin.

The structure of the skeleton of the fin slightly after its first differentiation will be best understood from Plate 33, fig. T, and Plate 33, fig. 7. These figures represent transverse sections through the pelvic and pectoral fins of the same embryo on the same scale. The basal bar is seen at bp, and the plate at this stage (which is considerably later than the first differentiation) already partially segmented into rays at br. Outside the region of the cartilaginous plate is seen the fringe with the horny fibres (h. f.) ; and dorsally and ventrally to the cartilaginous skeleton are seen the already well-differentiated muscles (#2).

The pectoral fin is shewn in horizontal section in Plate 33, fig. 6, at a somewhat earlier stage than that to which the transverse sections belong. The pectoral girdle (p. g^) is cut transversely, and is seen to be perfectly continuous with the basal bar (vp) of the fin. A similar continuity between the basal bar of the pelvic fin and the pelvic girdle is shewn in Plate 33, fig. 2, at a somewhat later stage. The plate continuous with the basal bar of the fin is at first, to a considerable extent in the pectoral, and to some extent in the pelvic fin, a continuous lamina, which subsequently segments into rays. In the parts of the plate which eventually form distinct rays, however, almost from the first the cells are more concentrated than in those parts which will form the tissue between the rays ; and I am not inclined to lay any stress whatever upon the fact of the cartilaginous fin-rays being primitively part of a continuous lamina, but regard it as a secondary phenomenon, dependent on the mode of conversion of embryonic mesoblast cells into cartilage. In all cases the separation into distinct rays is to a large extent completed before the tissue of which the plates are formed is sufficiently differentiated to be called cartilage by an histologist.

The general position of the fins in relation to the body, and their relative sizes, may be gathered from Plate 33, figs. 4 and 5 which represent transverse sections of the same embryo as that from which the transverse sections shewing the fin on a larger scale were taken.


OF THE PAIRED FINS OF ELASMOBRANCHS. 727

During the first stage of its development the skeleton of both fins may thus be described as consisting of a longitudinal bar running along the base of the fin, and giving off at right angles series of rays which pass into the fin. The longitudinal bar may be called the basipterygium ; and it is continuous in front with the pectoral or pelvic girdle, as the case may be.

The further development of the primitive skeleton is different in the case of the two fins.

The Pelvic Fin. The changes in the pelvic fin are comparatively slight. Plate 33, fig. 2, is a. representation of the fin and its skeleton in a female of Scyllium stellare shortly after the primitive tissue is converted into cartilage, but while it is still so soft as to require the very greatest care in dissection. The fin itself forms a simple projection of the side of the body. The skeleton consists of a basipterygium (bp}, continuous in front with the pelvic girdle. To the outer side of the basipterygium a series of cartilaginous fin-rays are attached the posterior ray forming a direct prolongation of the basipterygium, while the anterior ray is united rather with the pelvic girdle than with the basipterygium. All the cartilaginous fin-rays except the first are completely continuous with the basipterygium, their structure in section being hardly different from that shewn in Plate 33, fig. i.

The external form of the fin does not change very greatly in the course of the further development ; but the hinder part of the attached border is, to some extent, separated off from the wall of the body, and becomes the posterior border of the adult fin. With the exception of a certain amount of segmentation in the rays, the character of the skeleton remains almost as in the embryo. The changes which take place are illustrated by Plate 33, fig. 3, shewing the fin of a young male of Scyllium stellare. The basipterygium has become somewhat thicker, but is still continuous in front with the pelvic girdle, and otherwise retains its earlier characters. The cartilaginous fin-rays have now become segmented off from it and from the pelvic girdle, the posterior end of the basipterygial bar being segmented off as the terminal ray.

The anterior ray is directly articulated with the pelvic girdle, and the remaining rays continue articulated with the basipterygium. Some of the latter are partially segmented.


728 DEVELOPMENT OF THE SKELETON

As may be gathered by comparing the figure of the fin at the stage just described with that of the adult fin (woodcut, fig. 2), the remaining changes are very slight. The most important is the segmentation of the basipterygial bar from the pelvic girdle.

The pelvic fin thus retains in all essential points its primitive structure.

The Pectoral Fin. The earliest stage of the pectoral fin differs, as I have shewn, from that of the pelvic fin only in minor points (PL 33, fig. 6). Therq is the same longitudinal or basipterygial bar (bp], to which the fin-rays are attached, which is continuous in front with the pectoral girdle (p g). The changes which take place in the course of the further development, however, are very much more considerable in the case of the pectoral than in that of the pelvic fin.

The most important change in the external form of the firi is caused by a reduction in the length of its attachment to the body. At first (PL 33, fig. 6), the base of the fin is as long as the greatest breadth of the fin; but it gradually becomes shortened by being constricted off from the body at its hinder end. In connection with this process the posterior end of the basipterygial bar is gradually rotated outwards, its anterior end remaining attached to the pectoral girdle. In this way this bar comes to form the posterior border of the skeleton of the fin (PL 33, figs. 8 and 9), constituting the metapterygium (mp\ It becomes eventually segmented off from the pectoral girdle, simply articulating with its hinder edge.

The plate of cartilage, which is continued outwards from the basipterygium, or, as we may now call it, the metapterygium, into the fin, is not nearly so completely divided up into fin-rays as the homologous part of the pelvic fin; and this is especially the case with the basal part of the plate. This basal part becomes, in fact, at first only divided into two parts (PL 33, fig. 8) a small anterior part at the front end (me. /), and a larger posterior along the base of the metapterygium (mp) ; and these two parts are not completely segmented from each other. The anterior part directly joins the pectoral girdle at its base, resembling in this respect the anterior fin-ray of the pelvic girdle. It constitutes the (at this stage undivided) rudiment of the meso


OF THE PAIRED FINS OF ELASMOBRANCHS. 729

pterygium and propterygium of Gegenbaur. It bears in my specimen of this age four fin-rays at its extremity, the anterior not being well marked. The remaining fin-rays are prolongations outwards of the edge of the plate continuous with the metapterygium. These rays are at the stage figured more or less transversely segmented; but at their outer edge they are united together by a nearly continuous rim of cartilage. The spaces between the fin-rays are relatively considerably larger than in the adult.

The further changes jn the cartilages of the pectoral limb are, morphologically speaking, not important, and are easily understood by reference to PL 33, fig. 9 (representing the skeleton of the limb of a nearly ripe embryo). The front end of the anterior basal cartilage becomes segmented off as a propterygium (//), bearing a single fin-ray, leaving the remainder of the cartilage as a mesopterygium (mes). The remainder of the now considerably segmented fin-rays are borne by the metapterygium.

General Conclusions. From the above observations, conclusions of a positive kind may be drawn as to the primitive structure of the skeleton ; and the observations have also, it appears to me, important bearings on the theories of my predecessors in this line of investigation.

The most obvious of the positive conclusions is to the effect that the embryonic skeleton of the paired fins consists of a series of parallel rays similar to those of the unpaired fins. These rays support the soft parts of the fins, which have the form of a longitudinal ridge ; and they are continuous at their base with a longitudinal bar. This bar, from its position at the base of the fin, can clearly never have been a median axis with the rays on both sides. It becomes the basipterygium in the pelvic fin, which retains its embryonic structure much more completely than the pectoral fin; and the metapterygium in the pectoral fin. The metapterygium of the pectoral fin is thus clearly homologous with the basipterygium of the pelvic fin, as originally supposed by Gegenbaur, and as has since been maintained by Mivart. The propterygium and mesopterygium are obviously relatively unimportant parts of the skeleton as compared with the metapterygium.

B. 47


730 DEVELOPMENT OF THE SKELETON

My observations on the development of the skeleton of the fins certainly do not of themselves demonstrate that the paired fins are remnants of a once continuous lateral fin ; but they support this view in that they shew the primitive skeleton of the fins to have exactly the character which might have been anticipated if the paired fins had originated from a continuous lateral fin. The longitudinal bar of the paired fins is believed by both Thacker and Mivart to be due to the coalescence of the bases of the primitively independent rays of which they believe the fin to have been originally composed. This view is probable enough in itself, and is rendered more so by the fact, pointed out by Mivart, that a longitudinal bar supporting the cartilaginous rays of unpaired fins is occasionally formed ; but there is no trace in the embryo Scylliums of the bar in question being formed by the coalescence of rays, though the fact of its being perfectly continuous with the bases of the fin-rays is somewhat in favour of such coalescence.

Thacker and Mivart both hold that the pectoral and pelvic girdles are developed by ventral and dorsal growths of the anterior end of the longitudinal bar supporting the fin-rays.

There is, so far as I see, no theoretical objection to be taken to this view ; and the fact of the pectoral and pelvic girdles originating continuously and long remaining united with the longitudinal bars of their respective fins is in favour of it rather than the reverse. The same may be said of the fact that the first part of each girdle to be formed is that in the neighbourhood of the longitudinal bar (basipterygium) of the fin, the dorsal and ventral prolongations being subsequent growths.

On the whole my observations do not throw much light on the theories of Thacker and Mivart as to the genesis of the skeleton of the paired fin ; but, so far as they bear on the subject, they are distinctly favourable to those theories.

The main results of my observations appear to me to be decidedly adverse to the views recently put forward on the structure of the fin by Gegenbaur and Huxley, both of whom, as stated above, consider the primitive type of fin to be most nearly retained in Ceratodus, and to consist of a central multisegmented axis with numerous lateral rays.


OF THE PAIRED FINS OF ELASMOBRANCHS. 731

Gegenbaur derives the Elasmobranch pectoral fin from a form which he calls the archipterygium, nearly like that of Ceratodiis, with a median axis and two rows of rays but holds that in addition to the rays attached to the median axis, which are alone found in Ceratodus, there were other rays directly articulated to the shoulder-girdle. He considers that in the Elasmobranch fin the majority of the lateral rays on the posterior (or median according to his view of the position of the limb) side have become aborted, and that the central axis is represented by the metapterygium ; while the pro- and mesopterygium and their rays are, he believes, derived from those rays of the archipterygium which originally articulated directly with the shoulder-girdle.

This view appears to me to be absolutely negatived by the facts of development of the pectoral fin in Scyllium not so much because the pectoral fin in this form is necessarily to be regarded as primitive, but because what Gegenbaur holds to be the primitive axis of the biserial fin is demonstrated to be really the base, and it is only in the adult that it is conceivable that a second set of lateral rays could have existed on the posterior side of the metapterygium. If Gegenbaur's view were correct, we should expect to find in the embryo, if anywhere, traces of the second set of lateral rays ; but the fact is that, as may easily be seen by an inspection of figs. 6 and 7, such a second set of lateral rays could not possibly have existed in a type of fin like that found in the embryo. With this view of Gegenbaur's it appears to me that the theory held by this anatomist to the effect that the limbs are modified gill-arches also falls, in that his method of deriving the limbs from gill-arches ceases to be admissible, while it is not easy to see how a limb, formed on the type of the embryonic limb of Elasmobranchs, could be derived from a gill-arch with its branchial rays.

Gegenbaur's older view, that the Elasmobranch fin retains a primitive uniserial type, appears to me to be nearer the truth than his more recent view on this subject ; though I hold the ' fundamental point established by the development of these parts in Scyllimn to be that the posterior border of the adult Elasmobranch pectoral fin is the primitive base-line, i.e. line of attachment of the fin to the side of the body.

472


732 DEVELOPMENT OF FINS OF ELASMOBRANCHS.

Huxley holds that the mesopterygium is the proximal piece of the axial skeleton of the limb of Ceratodus, and derives the Elasmobranch fin from that of Ceratodus by the shortening of its axis and the coalescence of some of its elements. The entirely secondary character of the mesopterygium, and its total absence in the young embryo Scyllium, appear to me as conclusive against Huxley's view as the character of the embryonic fin is against that of Gegenbaur ; and I should be much more inclined to hold that the fin of Ceratodus has been derived from a fin like that of the Elasmobranchs by a series of steps similar to those which Huxley supposes to have led to the establishment of the Elasmobranch fin, but in exactly the reverse order.

There is one statement of Davidoff's which I cannot allow to pass without challenge. In comparing the skeletons of the paired and unpaired fins he is anxious to prove that the former are independent of the axial skeleton in their origin and that the latter have been segmented from the axial skeleton, and thus to shew that an homology between the two is impossible. In support of his view he states 1 that he has satisfied himself, from embryos of Acanthias and Scyllium, that the rays of the unpaired fins are undoubtedly products of the segmentation of tJie dorsal and ventral spinous processes.

This statement is wholly unintelligible to me. From my examination of the development of the first dorsal and the anal fins of Scyllium I find that their rays develop at a considerable distance from, and quite independently of, the neural and haemal arches, and that they are at an early stage of development distinctly in a more advanced state of histological differentiation than the neural and haemal arches of the same region. I have also found exactly the same in the embryos of Lepidosteus.

I have, in fact, no doubt that the skeleton of both the paired and the unpaired fins of Elasmobranchs and Lepidosteus is in its development independent of the axial skeleton. The phylogenetic mode of origin of the skeleton both of the paired and of the unpaired fins cannot, however, be made out without further


investigation.


1 Loc. til. p. 514.


EXPLANATION OF PLATE 33. 733


EXPLANATION OF PLATE 33.

Fig. i. Transverse section through the pelvic fin of an embryo of Scy Ilium belonging to stage P 1 , magnified 50 diameters, bp. basipterygium. br. fin ray. m. muscle, hf. horny fibres supporting the peripheral part of the fin.

Fig. 2. Pelvic fin of a very young female embryo of Scyllium stellare, magnified 1 6 diameters, bp. basipterygium. pu. pubic process of pelvic girdle (cut across below), il. iliac process of pelvic girdle, fa. foramen.

Fig. 3. Pelvic fin of a young male embryo of Scyllium stellare, magnified 16 diameters, bp. basipterygium. mo. process of basipterygium continued into clasper. il. iliac process of pelvic girdle, pu. pubic section of pelvic girdle.

Fig. 4. Transverse section through the ventral part of the trunk of an embryo Scyllium of stage P, in the region of the pectoral fins, to shew how the fins are attached to the body, magnified 18 diameters, br. cartilaginous fin-ray, bp. basipterygium. m. muscle of fin. mp. muscle-plate.

Fig. 5. Transverse section through the ventral part of the trunk of an embryo Scyllium of stage P, in the region of the pelvic fin, on the same scale as fig. 4. bp. basipterygium. br. cartilaginous fin-rays, m. muscle of the fins. mp. muscleplate.

Fig. 6. Pectoral fin of an embryo of Scyllium canicula, of a stage between O and P, in longitudinal and horizontal section (the skeleton of the fin was still in the condition of embryonic cartilage), magnified 36 diameters, bp. basipterygium (eventual metapterygium). fr. cartilaginous fin-rays, p g. pectoral girdle in transverse section. fo. foramen in pectoral girdle, pe. epithelium of peritoneal cavity.

Fig. 7. Transverse section through the pectoral fin of a Scyllium embryo of stage P, magnified 50 diameters, bp. basipterygium. br. cartilaginous fin-ray, m. muscle. hf. horny fibres.

Fig. 8. Pectoral fin of an embryo of Scyllium stellare, magnified 16 diameters. mp. metapterygium (basipterygium of earlier stage), me.p. rudiment of future proand mesopterygium. sc. cut surface of a scapular process, cr. coracoid process. fr. foramen, hf. horny fibres.

Fig. 9. Skeleton of the pectoral fin and part of pectoral girdle of a nearly ripe embryo of Scyllium stellare, magnified 10 diameters, mp. metapterygium. mes. mesopterygium. pp. propterygium. cr. coracoid process.

1 I employ here the same letters to indicate the stages as in my "Monograph on Elasmobranch Fishes."

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