Chapter IX (1)

THE DEVELOPMENT OF THE ORGANS IN THE HEAD.

_The Development of the Brain._

_General History._ In stage G the brain presents a very simple constitution (Pl. 8, fig. G), and is in fact little more than a dilated termination to the cerebro-spinal axis. Its length is nearly one-third that of the whole body, being proportionately very much greater than in the adult.

It is divided by very slight constrictions into three lobes, the posterior of which is considerably the largest. These are known as the fore-brain, the mid-brain, and the hind-brain. The anterior part of the brain is bent slightly downwards about an axis passing through the mid-brain. The walls of the brain, composed of several rows of elongated columnar cells, have a fairly uniform thickness, and even the roof of the hind-brain is as thick as any other part. Towards the end of stage G the section of the hind-brain becomes somewhat triangular with the apex of the triangle directed downwards.

In Pristiurus during stage H no very important changes take place in the constitution of the brain. In Scyllium, however, indications appear in the hind-brain of its future division into a cerebellum and medulla oblongata. The cavity of the anterior part dilates and becomes rounded, while that of the posterior part assumes in section an hour-glass shape, owing to an increase in the thickness of the lateral parts of the walls. At the same time the place of the original thick roof is taken by a very thin layer, which is formed not so much through a change in the character and arrangements of the cells composing the roof, as by a divarication of the two sides of the hind-brain, and the simultaneous introduction of a fresh structure in the form of a thin sheet of cells connecting dorsally the diverging lateral halves of this part of the brain. By stage I, the hind-brain in Pristiurus also acquires an hour-glass shaped section, but the roof has hardly begun to thin out (Pl. 15, figs. 4_a_ and 4_b_).

During stages I and K the cranial flexure becomes more and more pronounced, and causes the mid-brain definitely to form the termination of the long axis of the embryo (Pl. 15, figs. 1, 2, etc.), and before the close of stage K a thin coating of white matter has appeared on the exterior of the whole brain, but no other histological changes of interest have occurred.

During stage L an apparent rectification of the cranial flexure commences, and is completed by stage Q. The changes involved in this process may be advantageously studied by comparing the longitudinal sections of the brain during stages L, P, and Q, represented in Pl. 16, figs. 1_a_, 5 and 7_a_.

It will be seen, first of all, that so far from the flexure of the brain itself being diminished, it is increased, and in P (fig. 5) the angle in the floor of the mid-brain becomes very acute indeed; in other words, the anterior part of the brain has been bent upon the posterior through nearly two right angles, and the infundibulum, or primitive front end of the brain, now points nearly directly backwards. At the same time the cerebral hemispheres have grown directly forwards, and if figures 1_a_ and 5 in Pl. 16 be compared it will be seen that in the older brain of the two the cerebral hemispheres have assumed a position which might be looked on as the result of their having been pushed dorsalwards and forwards against the mid-brain, and having in the process pressed in and nearly obliterated the original thalamencephalon. The thalamencephalon in fig. 1_a_, belonging to stage L, is relatively large, but in fig. 5, belonging to stage P, it only occupies a very small space between the front wall of the mid-brain and the hind wall of the cerebral hemispheres. It is therefore in part by the change in position of the cerebral hemispheres that the angle between the trabeculæ and parachordals becomes increased, _i.e._ their flexure _diminished_, while at the same time the flexure of the brain itself is _increased_. More important perhaps in the apparent rectification of the cranial flexure than any of the previously mentioned points, is the appearance of a bend in the hind-brain which tends to correct the original cranial flexure. The gradual growth of this fresh flexure can be studied in the longitudinal sections which have been represented. It is at its maximum in stage Q. This short preliminary sketch of the development of the brain as a whole will serve as an introduction to the history of the individual divisions of the brain.

_Fore-brain._ In its earliest condition the fore-brain forms a single vesicle without a trace of separate divisions, but buds off very early the optic vesicles, whose history is described with that of the eye (Pl. 15, fig. 3, _op.v_). Between stages I and K the posterior part of the fore-brain sends outwards a papilliform process towards the exterior, which forms the rudiment of the pineal gland (Pl. 15, fig. 1, _pn_). Immediately in front of the rudiment a constriction appears, causing a division of the fore-brain into a large anterior and a small posterior portion. This constriction is shallow at first, but towards the close of stage K becomes much deeper (Pl. 15, fig. 2 and fig. 16_a_), leaving however the two cavities of the two divisions of the fore-brain united ventrally by a somewhat wide canal.

The posterior of the two divisions of the fore-brain forms the thalamencephalon. Its anterior wall adjoining the cerebral rudiment becomes excessively thin (Pl. 15, fig. 11); and its base till the close of stage K is in close contact with the mouth involution, and presents but a very inconspicuous prominence which marks the eventual position of the infundibulum (Pl. 15, figs. 9_a_, 12, 16_a_, _in_). The anterior and larger division of the fore-brain forms the rudiment of the cerebral hemispheres and olfactory lobes. Up to stage K this rudiment remains perfectly simple, and exhibits no signs, either externally or internally, of a longitudinal constriction into two lobes. From the canal uniting the two divisions of the fore-brain (which eventually forms part of the thalamencephalon) there spring the hollow optic nerves. A slight ventral constriction separating the cerebral rudiment from that part of the brain where these are attached appears even before the close of stage K (Pl. 15, fig. 11, _op.n_).

During stage L the infundibulum becomes much produced, and forms a wide sack in contact with the pituitary body, and its cavity communicates with that of the third ventricle by an elongated slit-like aperture. This may be seen by comparing Pl. 16, figs. 1_a_ and 1_c_. In fig. 1_c_ taken along the middle line, there is present a long opening into the infundibulum (_in_), which is shewn to be very narrow by being no longer present in fig. 1_a_ representing a section slightly to one side of the middle line. During the same stage the pineal gland grows into a sack-like body, springing from the roof of the thalamencephalon, fig. 1_b_, _pn_. This latter (the thalamencephalon) is now dorsally separated from the cerebral rudiment by a deep constriction, and also ventrally by a less well marked constriction. At its side also a deep constriction is being formed in it, immediately behind the pineal gland. The cerebral rudiment is still quite unpaired and exhibits no sign of becoming constricted into two lobes.

During the next two stages the changes in the fore-brain are of no great importance, and I pass at once to stage O. The infundibulum is now nearly in the same condition as during stage L, though (as is well shewn in the figure of a longitudinal section of the next stage) it points more directly backwards than before. The remaining parts of the thalamencephalon have however undergone considerable changes. The more important of these are illustrated by a section of stage O, Pl. 16, fig. 3, transverse to the long axis of the embryo, and therefore, owing to the cranial flexure, cutting the thalamencephalon longitudinally and horizontally; and for stage P in a longitudinal and vertical section through the brain (Pl. 16, fig. 5). In the first place the roof of the thalamencephalon has become very much shortened by the approximation of the cerebral rudiment to the mid-brain. The pineal sack has also become greatly elongated, and its somewhat dilated extremity is situated between the cerebral rudiment and the external skin. It opens into the hind end of the third ventricle, and its posterior wall is continuous with the front wall of the mid-brain. The sides of the thalamencephalon have become much thickened, and form distinct optic thalami (_op._) united by a very well marked posterior commissure (_pc._). The anterior wall of the thalamencephalon as well as its roof are very thin. The optic nerves have become by stage O quite solid except at their roots, into which the ventricles of the fore-brain are for a short distance prolonged. This solidification is arrived at, so far as I have determined, without the intervention of a fold. The nerves are fibrous, and a commencement of the chiasma is certainly present. From the chiasma there appears to pass out on each side a band of fibres, which runs near the outer surface of the brain to the base of the optic lobes (mid-brain), and here the fibres of the two sides again cross.

By stage O important changes are perceptible in the cerebral rudiment. In the first place there has appeared a slight fold at its anterior extremity (Pl. 16, fig. 3, _x_), destined to form a vertical septum dividing it into two hemispheres, and secondly, lateral outgrowths (vide Pl. 16, fig. 2, _ol.l_), to form the olfactory lobes. Its thin posterior wall presents on each side a fold which projects into the central cavity. From the peripheral end of each olfactory lobe a nerve similar in its histological constitution to any other cranial nerve makes its appearance (Pl. 16, fig. 2); this divides into a number of branches, one of which passes into the connective tissue between the two layers of epithelium in each Schneiderian fold. On the root of this nerve there is a large development of ganglionic cells. I have not definitely observed its origin, but have no reason to doubt that it is a direct outgrowth from the olfactory lobe, exactly similar _in its mode of development_ to any other nerve of the body.

The cerebral rudiment undergoes great changes during stage P. In addition to a great increase in the thickness of its walls, the fold which appeared in the last stage has grown backwards, and now divides it in front into two lobes, the rudiments of the cerebral hemispheres. The greater and posterior section is still however quite undivided, and the cavities of the lobes (lateral ventricles) though separated in front are still quite continuous behind. At the same time, the olfactory lobes, each containing a prolongation of the ventricle, have become much more pronounced (vide Pl. 16, figs. 4_a_ and 4_c_, _ol.l_). The root of the olfactory nerve is now very thick, and the ganglion cells it contains are directly prolonged into the ganglionic portion of the olfactory bulb; in consequence of which it becomes rather difficult to fix on the exact line of demarcation between the bulb and the nerve.

Stage Q is the latest period in which I have investigated the development of the brain. Its structure is represented for this stage in general view in Pl. 16, figs. 6_a_, 6_b_, 6_c_, in longitudinal section in Pl. 16, figs. 7_a_, 7_b_, and in transverse section Pl. 16, figs. 8_a-d_. The transverse sections are taken from a somewhat older embryo than the longitudinal. In the thalamencephalon there is no fresh point of great importance to be noticed. The pineal gland remains as before, and has become, if anything, longer than it was, and extends further forwards over the summit of the cerebrum. It is situated, as might be expected, in the connective tissue within the cranial cavity (fig. 8_a_, _pn_), and does not extend outside the skull, as it appears to do, according to Götte's investigations, in Amphibians. Götte[269] compares the pineal gland with the long persisting pore which leads into the cavity of the brain in the embryo of Amphioxus, and we might add the Ascidians, and calls it "ein Umbildungsprodukt einer letzten Verbindung des Hirns mit der Oberhaut." This suggestion appears to me a very good one, though no facts have come under my notice which confirm it. The sacci vasculosi are perhaps indicated at this stage in the two lateral divisions of the trilobed ventricle of the infundibulum (fig. 8_c_).

Footnote 269: _Ent. d. Unke_, p. 304.

The lateral ventricles (fig. 8_a_) are now quite separated by a median partition, and a slight external constriction marks the lobes of the two hemispheres; these, however, are still united by nervous structures for the greater part of their extent. The olfactory lobes are formed of a distinct bulb and stalk (fig. 8_a_, _ol.l_), and contain, as before, prolongations of the lateral ventricles. The so-called optic chiasma is very distinct (fig. 8_b_, _op.n_), but the fibres from the optic nerves appear to me simply to cross and not to intermingle.

_The mid-brain._ The mid-brain is at first fairly marked off from both the fore and hind brains, but less conspicuously from the latter than from the former. Its roof becomes progressively thinner and its sides thicker up to stage P, its cavity remaining quite simple. The thinness of the roof gives it, in isolated brains of stage P, a bilobed appearance (vide Pl. 16, fig. 4_b_, _mb_, in which the distinctness of this character is by no means exaggerated): During stage Q it becomes really bilobed through the formation in its roof of a shallow median furrow (Pl. 16, fig. 8_b_). Its cavity exhibits at the same time the indication of a division into a central and two lateral parts.

_The hind-brain._ The hind-brain has at first a fairly uniform structure, but by the close of stage I, the anterior part becomes distinguished from the remainder by the fact, that its roof does not become thin as does that of the posterior part. This anterior, and _at first very insignificant portion_, forms the rudiment of the cerebellum. Its cavity is quite simple and is continued uninterruptedly into that of the remainder of the hind-brain. The cerebellum assumes in the course of development a greater and greater prominence, and eventually at the close of stage Q overlaps both the optic lobes in front and the medulla behind (Pl. 16, fig. 7_a_). It exhibits in surface-views of the hardened brain of stages P and Q the appearance of a median constriction, and the portion of the ventricle contained in it is prolonged into two lateral outgrowths (Pl. 16, figs. 8_c_ and 8_d_, _cb_).

The posterior section of the hind-brain which forms the medulla undergoes changes of a somewhat complicated character. In the first place its roof becomes in front very much extended and thinned out. At the raphe, where the two lateral halves of the brain originally united, a separation, as it were, takes place, and the two sides of the brain become pushed apart, remaining united by only a very thin layer of nervous matter (Pl. 15, fig. 6, _iv.v._). As a result of this peculiar growth in the brain, the roots of the nerves of the two sides which were originally in contact at the dorsal summit of the brain become carried away from one another, and appear to rise at the sides of the brain (Pl. 15, figs. 6 and 7). Other changes also take place in the walls of the brain. Each lateral wall presents two projections towards the interior (Pl. 15, fig. 5_a_). The ventral of these vanish, and the dorsal approximate so as nearly to divide the cavity of the hind-brain, or fourth ventricle, into a large dorsal and a small ventral channel (Pl. 15, fig. 6), and this latter becomes completely obliterated in the later stages. The dorsal pair, while approximating, also become more prominent, and stretch into the dorsal moiety of the fourth ventricle (Pl. 15, fig. 6). They are still very prominent at stage Q (Pl. 16, fig. 8_d_, _ft_), and correspond in position with the fasciculi teretes of human anatomy. Part of the root of the seventh nerve originates from them. They project freely in front into the cavity of the fourth ventricle (Pl. 16, fig. 7_a_, _ft_).

By stage Q restiform tracts are indistinctly marked off from the remainder of the brain, and are anteriorly continued into the cerebellum, of which they form the peduncles. Near their junction with the cerebellum they form prominent bodies (Pl. 16, fig. 7_a_, _rt_), which are regarded by Miklucho-Maclay[270] as representing the true cerebellum.

Footnote 270: _Das Gehirn d. Selachier_, Leipzig, 1870.

By stage O the medulla presents posteriorly, projecting into its cavity, a series of lobes which correspond with the main roots (not the branches) of the vagus and glosso-pharyngeal nerves (Pl. 17, fig. 5). There appear to me to be present seven or eight projections: their number cannot however be quite certainly determined. The first of them belongs to the root of the glosso-pharyngeal, the next one is interposed between the glosso-pharyngeal and the first root of the vagus, and is without any corresponding nerve-root. The next five correspond to the five main roots of the vagus. For each projection to which a nerve pertains there is a special nucleus of nervous matter, from which the root springs. These nuclei do not stain like the remainder of the walls of the medulla, and stand out accordingly very conspicuously in stained sections.

The coating of white matter which appeared at the end of stage K, on the exterior of each lateral half of the hind-brain, extends from a point just dorsal to the attachment of the nerve-roots to the ventral edge of the medulla, and is specially connected with the tissue of the upper of the two already described projections into the fourth ventricle.

A rudiment of the tela vasculosa makes its appearance during stage Q, and is represented by the folds in the wall of the fourth ventricle in my figure of that stage (Pl. 16, fig. 7_a_, _tv_).

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The development of the brain in Elasmobranchii has already been worked out by Professor Huxley, and a brief but in many respects very complete account of it is given in his recent paper on Ceratodus[271]. He says, pp. 30 and 31, "The development of the cerebral hemispheres in Plagiostome Fishes differs from the process by which they arise in the higher Vertebrata. In a very early stage, when the first and second visceral clefts of the embryo Scyllium are provided with only a few short branchial filaments, the anterior cerebral vesicle is already distinctly divided into the thalamencephalon (from which the large infundibulum proceeds below, and the small tubular peduncle of the pineal gland above, while the optic nerve leaves its sides) and a large single oval vesicle of the hemispheres. On the ventral face of the integument covering these are two oval depressions, the rudimentary olfactory sacs.

Footnote 271: _Proceedings of the Zoological Society_, 1876,
Pt. 1. pp. 30 and 31.

"As development proceeds the vesicle of the hemispheres becomes
divided by the ingrowth of a median longitudinal septum, and the
olfactory lobes grow out from the posterior lateral regions of each
ventricle thus formed, and eventually rise on to the dorsal faces of
the hemispheres, instead of, as in most Vertebrata, remaining on their
ventral sides. I may remark, that I cannot accept the views of
Miklucho-Maclay, whose proposal to alter the nomenclature of the parts
of the Elasmobranch's brain, appears to me to be based upon a
misinterpretation of the facts of development."

The last sentence of the paragraph brings me to the one part on which it is necessary to say a few words, viz. the views of Miklucho-Maclay. His views have not received any general acceptance, but the facts narrated in the preceding pages shew, beyond a doubt, that he has 'misinterpreted' the facts of development, and that the ordinary view of the homology of the parts is the correct one. A comparison of the figures I have given of the embryo brain with similar figures of the brain of higher Vertebrates shews this point conclusively. Miklucho-Maclay has been misled by the large size of the cerebellum, but, as we have seen, this body does not begin to be conspicuous till late in embryonic life. Amongst the features of the embryonic brain of Elasmobranchii, the long persisting unpaired condition of the cerebral hemisphere, upon which so much stress has already been laid by Professor Huxley, appears to me to be one of great importance, and may not improbably be regarded as a real ancestral feature. Some observations have recently been published by Professor B. G. Wilder[272] upon this point, and upon the homologies and development of the olfactory lobes. Fairly good figures are given to illustrate the development of the cerebral hemispheres, but the conclusions arrived at are in part opposed to my own results. Professor Wilder says: "The true hemispheres are the lateral masses, more or less completely fused in the middle line, and sometimes developing at the plane of union a bundle of longitudinal commissural fibres. The hemispheres retain their typical condition as anterior protrusions of the anterior vesicle; but they lie mesiad of the olfactory lobes, _and in Mustelus at least seem to be formed after them_." The italics are my own. From what has been said above, it is clear that the statement italicised, for Scyllium at least, completely reverses the order of development. Still more divergent from my conclusions are Professor Wilder's statements on the olfactory lobes. He says: "The true olfactory lobe, or rhinencephalon, seems, therefore, to embrace only the hollow base of the crus, more or less thickened, and more or less distinguishable from the main mass as a hollow process. The olfactory bulb, with the more or less elongated crus of many Plagiostomes, seems to be developed independently, or in connection with the olfactory sack, as are the general nerves;" and again, "But the young and adult brains since examined shew that the ventricle (_i.e._ the ventricle of the olfactory lobe) ends as a rounded cul-de-sac before reaching the 'lobe.'"

Footnote 272: "Anterior brain-mass with Sharks and Skates,"
_American Journal of Science and Arts_, Vol. XII. 1876.

The majority of the statements contained in the above quotations are not borne out by my observations. Even the few preparations of which I have given figures, appear to me to prove that (1) the olfactory lobes (crura and bulbs) are direct outgrowths from the cerebral rudiment, and develop quite independently of the olfactory sack; (2) that the ventricle of the cerebral rudiment does not stop short at the base of the crus; (3) that from the bulb a nerve grows out which has a centrifugal growth like other nerves of the body, and places the central olfactory lobe in communication with the peripheral olfactory sack. In some other Vertebrates this nerve seems hardly to be developed, but it is easily intelligible, that if in the ordinary course of growth the olfactory sack became approximated to the olfactory lobe, the nerve which grew out from the latter to the sack might become so short as to escape detection.

_Organs of Sense._

_The olfactory organ._ The olfactory pit is the latest formed of the three organs of special sense. It appears during a stage intermediate between _I_ and _K_, as a pair of slight thickenings of the external epiblast, in the normal vertebrate position on the under side of the fore-brain immediately in front of the mouth (Pl. 15, figs. 1 and 2, _ol_).

The epiblast cells which form this thickening are very columnar, but present no special peculiarities. Each thickened patch of skin soon becomes involuted as a shallow pit, which remains in this condition till the close of the stage _K_. The epithelium very early becomes raised into a series of folds (Schneiderian folds). These are bilaterally symmetrical, and diverge like the barbs of a feather from a median line (Pl. 15, fig. 14). The nasal pits at the close of stage K are still separated by a considerable interval from the walls of the brain, and no rudiment of an olfactory lobe arises till a later period; but a description of the development of this as an integral part of the brain has already been given, p. 401.

_Eye._ The eye does not present in its early development any very special features of interest. The optic vesicles arise as hollow outgrowths from the base of the fore-brain (Pl. 15, fig. 3, _op.v_), from which they soon become partially constricted, and form vesicles united to the base of the brain by comparatively narrow hollow stalks, the rudiments of the optic nerves. The constriction to which the stalk or optic nerve is due takes place from above and backwards, so that the optic nerves open into the base of the front part of the thalamencephalon (Pl. 15, fig. 13_a_, _op.n_). After the establishment of the optic nerves, there take place the formation of the lens and the pushing in of the anterior wall of the optic vesicle towards the posterior.

The lens arises in the usual vertebrate fashion. The epiblast in front of the optic vesicle becomes very much thickened, and then involuted as a shallow pit, which eventually deepens and narrows. The walls of the pit are soon constricted off as a nearly spherical mass of cells enclosing a very small central cavity, in some cases indeed so small as to be barely recognizable (Pl. 15, fig. 7, _l_). The pushing in of the anterior wall of the optic vesicle towards the posterior takes place in quite the normal manner; but, as has been already noticed by Götte[273] and others, is not a simple mechanical result of the formation of the lens, as is shewn by the fact that the vesicle assumes a flattened form even before the appearance of the lens. The whole exterior of the optic cup becomes invested by mesoblast, but _no mesoblastic cells grow in between the lens and the adjoining wall of the optic cup_.

Footnote 273: _Entwicklungsgeschichte d. Unke._

Round the exterior of the lens, and around the exterior and interior of the optic cup, there appear membrane-like structures, similar to those already described round the spinal cord and other organs. These membrane-like structures appear with a varying distinctness, but at the close of stage _K_ stand out with such remarkable clearness as to leave no doubt that they are not artificial products (Pl. 15, fig. 13_a_)[274]. They form the rudiments of the hyaloid membrane and lens capsule. Similar, though less well marked membranes, may often be seen lining the central cavity of the lens and the space between the two walls of the optic cup. The optic cup is at first very shallow, but owing to the rapid growth of the free edge of its walls soon becomes fairly deep. The growth extends to the whole circumference of the walls except the point of entrance of the optic nerve (Pl. 15, fig. 13_a_), where no growth takes place; here accordingly a gap is left in the walls which forms the well-known choroid slit. While this double walled cup is increasing in size, the wall lining the cavity of the cup becomes thick, and the outer wall very thin (fig. 13_a_). No further differentiations arise before the close of stage K.

Footnote 274: The engraver has not been very successful in
rendering these membranes.

The lens is carried outwards with the growth of the optic cup, leaving the cavity of the cup quite empty. It also grows in size, and its central cavity becomes larger. Still later its anterior wall becomes very thin, and its posterior wall thick, and doubly convex (fig. 13_a_). Its changes, however, so exactly correspond to those already known in other Vertebrates, that a detailed description of them would be superfluous.

_No mesoblast passes into the optic cup round its edge_, but a process of mesoblast, accompanied by a blood-vessel, passes into the space between the lens and the wall of the optic cup through the choroid slit (fig. 13_a_, _ch_). This process of tissue is very easily seen, and swells out on entering the optic cup into a mushroom-like expansion. It forms the processus falciformis, and from it is derived the vitreous humour.

About the development of the parts of the eye, subsequently to stage _K_, I shall not say much. The iris appears during stage O, as an ingrowing fold of both layers of the optic cup with a layer of mesoblast on its outer surface, which tends to close over the front of the lens. Both the epiblast layers comprising the iris are somewhat atrophied, and the outer one is strongly pigmented. At stage O the mesoblast first also grows in between the external skin and the lens to form the rudiment of the mesoblastic structures of the eye in front of the lens. The layer, when first formed, is of a great tenuity.

The points in my observations, to which I attach the greatest importance, are the formation of the lens capsule and the hyaloid membrane; with the development of these may be treated also that of the vitreous humour and rudimentary _processus falciformis_. The development of these parts in Elasmobranchii has recently been dealt with by Dr Bergmeister[275], and his observations with reference to the vitreous humour and processus falciformis, the discovery of which in embryo Elasmobranchii is due to him, are very complete. I cannot, however, accept his view that the hyaloid membrane is a mesoblastic product. Through the choroid slit there grows, as has been said, a process of mesoblast, the processus falciformis, which on entering the optic cup dilates, and therefore appears mushroom-shaped in section. At the earliest stage (K) a blood-vessel appeared in connection with it, but no vascular structure came under my notice in the later stages. The structure of this process during stage P is shewn in Pl. 17, fig. 6, _p.fal._; it is there seen to be composed of mesoblast-cells with fibrous prolongations. The cells, as has been noticed by Bergmeister, form a special border round its dilated extremity. This process is formed much earlier than the vitreous humour, which is first seen in stage O. In hardened specimens this latter appears either as a gelatinous mass with a meshwork of fibres or (as shewn in Pl. 17, fig. 6) with elongated fibres proceeding from the end of the processus falciformis. These fibres are probably a product of the hardening reagent, but perhaps represent some preformed structure in the vitreous humour. I have failed to detect in it any cellular elements. It is more or less firmly attached to the hyaloid membrane.

Footnote 275: "Embryologie d. Coloboms," _Sitz. d. k. Akad.
Wien_, Bd. LXXI. 1875.

On each side of the processus falciformis in stage P a slight fold of the optic cup is to be seen, but folds so large as those represented by Bergmeister have never come under my notice, though this may be due to my not having cut sections of such late embryos as he has. The hyaloid membrane appears long before the vitreous humour as a delicate basement membrane round the inner surface of the optic cup (Pl. 15, fig. 13_a_), which is perfectly continuous with a similar membrane round the outer surface. In the course of development the hyaloid membrane becomes thicker than the membrane outside the optic cup, with which however it remains continuous. This is very clear in my sections of stage M. By stage O the membrane outside the cup has ceased to be distinguishable, but the hyaloid membrane may nevertheless be traced to the very edge of the cup round the developing iris; but does not unite with the lens capsule. It can also be traced quite to the junction of the two layers of the optic cup at the side of the choroid slit (Pl. 17, fig. 6, _hy.m_). When the vitreous humour becomes artificially separated from the retina, the hyaloid membrane sometimes remains attached to the former, but at other times retains in preference its attachment to the retina. My observations do not throw any light upon the junction of the hyaloid membrane and lens capsule to form the suspensory ligament, nor have I ever seen (as described by Bergmeister) the hyaloid membrane extending across the free end of the processus falciformis and separating the latter from the vitreous humour. This however probably appears at a period subsequent to the latest one investigated by me. The lens capsule arises at about the same period as the hyaloid membrane, and is a product of the cells of the lens. It can be very distinctly seen in all the stages subsequent to its first formation. The proof of its being a product of the epiblastic lens, and not of the mesoblast, lies mainly in the fact of there being no mesoblast at hand to give rise to it at the time of its formation, vide Pl. 15, fig. 13_a_. If the above observations are correct, it is clear that the hyaloid membrane and lens capsule are respectively products of the retina and lens; so that it becomes necessary to go back to the older views of Kölliker and others in preference to the more modern ones of Lieberkühn and Arnold. It would take me too far from my subject to discuss the arguments used by the later investigators to maintain their view that the hyaloid membrane and lens capsule are mesoblastic products; but it will suffice to say that the continuity of the hyaloid membrane over the pecten in birds is no conclusive argument against its retinal origin, considering the great amount of apparently independent growth which membranes, when once formed, are capable of exhibiting.

Bergmeister's and my own observations on the vitreous humour clearly prove that this is derived from an ingrowth through the choroid slit. On the other hand, the researches of Lieberkühn and Arnold on the Mammalian Eye appear to demonstrate that a layer of mesoblast becomes in Mammalia involuted with the lens, and from this the vitreous humour (including the _membrana capsulo-pupillaris_) is said to be in part formed. Lieberkühn states that in Birds the vitreous humour is formed in a similar fashion. I cannot, however, accept his results on this point. It appears, therefore, that, so far as is known, all groups of Vertebrata, with the exception of Mammalia, conform to the Elasmobranch type. The differences between the types of Mammalia and remaining Vertebrata are, however, not so great as might at first sight appear. They are merely dependent on slight differences in the manner in which the mesoblast enters the optic cup. In the one case it grows in round one specialized part of the edge of the cup, _i.e._ the choroid slit; in the other, round the whole edge, including the choroid slit. Perhaps the mode of formation of the vitreous humour in Mammalia may be correlated with the early closing of the choroid slit.

_Auditory Organ._ With reference to the development of the organ of hearing I have very little to say. Opposite the interval between the seventh and the glosso-pharyngeal nerves the external epiblast becomes thickened, and eventually involuted as a vesicle which remains however in communication with the exterior by a narrow duct. Towards the close of stage K the auditory sack presents three protuberances--one pointing forwards, a second backwards, and a third outwards. These are respectively the rudiments of the anterior and posterior vertical and external horizontal semicircular canals. These rudiments are easily visible from the exterior (Pl. 15, fig. 2).

* * * * *

As has been already pointed out, the epiblast of Elasmobranchii during the early periods of development exhibits no division into an epidermic and a nervous layer, and in accordance with its primitive undifferentiated condition, those portions of the organs of sense which are at this time directly derived from the external integument are formed indiscriminately from the whole, and not from an inner or so-called nervous part of it only. In the Amphibians the auditory sack and lens are derived from the nervous division of the epiblast only, while the same division of the layer plays the major part in forming the olfactory organ. It is also stated that in Birds and Mammals the part of the epiblast corresponding to the nervous layer is alone concerned in the formation of the lens, though this does not appear to be the case with the olfactory or auditory organs in these groups of Vertebrates.

_Mouth involution and Pituitary body._

The development of the mouth involution and the pituitary body is closely related to that of the brain, and may conveniently be dealt with here. The epiblast in the angle formed by the cranial flexure becomes involuted as a hollow process situated in close proximity to the base of the brain. This hollow process is the mouth involution, and it is bordered on its posterior surface by the front wall of the alimentary tract, and on its anterior by the base of the fore-brain.

The uppermost end of this does not till near the close of stage K become markedly constricted off from the remainder, but is nevertheless the rudiment of the pituitary body. Pl. 15, figs. 9_a_ and 12, _m_ shew in a most conclusive manner the correctness of the above account, and demonstrate that it is from the mouth involution, and not, as has usually been stated, from the alimentary canal, that the pituitary body is derived.

This fact was mentioned in my preliminary account of Elasmobranch development[276]; and has also been shewn to be the case in Amphibians by Götte[277]; and in Birds by Mihalkowics[278]. The fact is of considerable importance with reference to speculations as to the meaning of this body.

Footnote 276: _Quarterly Journal of Microscopic Science_, Oct.
1874.

Footnote 277: _Entwicklungsgeschichte der Unke._ Götte was the
first to draw attention to this fact. His observations were
then shewn to hold true for Elasmobranchii by myself, and
subsequently for Birds by Mihalkowics.

Footnote 278: _Arch. f. micr. Anat._ Vol. XI.

Plate 15, fig. 7 represents a transverse section through the head during a stage between I and K; but, owing to the cranial flexure, it cuts the fore part of the head longitudinally and horizontally, and passes through both the fore-brain (_fb_) and the hind-brain (_iv.v._). Close to the base of the fore-brain are seen the mouth (_m_), and the pituitary involution from this (_pt._). In contact with the pituitary involution is the blind anterior termination of the throat, which a little way back opens to the exterior by the first visceral cleft (I. _v.c._). This figure alone suffices to demonstrate the correctness of the above account of the pituitary body; but the truth of this is still further confirmed by other figures on the same plate (figs. 9_a_ and 12, _m_); in which the mouth involution is in contact with, but still separated from, the front end of the alimentary tract. By the close of stage K, the septum between the mouth and throat becomes pierced, and the two are placed in communication. This condition is shewn in Pl. 15, fig. 16_a_, and Pl. 16, figs. 1_a_, 1_c_, _pt_ In these figures the pituitary involution has become very partially constricted off from the mouth involution, though still in direct communication with it. In later stages the pituitary involution becomes longer and dilated terminally, while the passage connecting it with the mouth becomes narrower and narrower, and is finally reduced to a solid cord, which in its turn disappears. The remaining vesicle then becomes divided into lobes, and connects itself closely with the infundibulum (Pl. 16, figs. 5 and 6 _pt_). The later stages for Elasmobranchii are fully described by W. Müller in his important memoir on the Comparative Anatomy and development of this organ[279].

Footnote 279: W. Müller, "Ueber Entwicklung and Bau d.
Hypophysis u. d. Processus infundibuli cerebri," _Jenaische
Zeitschrift_, Bd. VI.

_Development of the Cranial Nerves._

The present section deals with the whole development (so far as I have succeeded in elucidating it) of the cranial nerves (excluding the optic and olfactory nerves and the nerves of the eye-muscles) from their first appearance to their attainment of the adult condition. My description commences with the first development of the nerves, to this succeeds a short description of the nerves in the adult Scyllium, and the section is completed by an account of the gradual steps by which the adult condition is attained.

_Early Development of the Cranial Nerves._--Before the close of stage H the more important of the cranial nerves make their appearance. The fifth and the seventh are the first to be formed. The fifth arises by stage G (Pl. 15, fig. 3, V), near the anterior end of the hind-brain, as _an outgrowth from the extreme dorsal summit of the brain, in identically the same way as the dorsal root of a spinal nerve_.

The roots of the two sides sprout out from the summit of the brain, in contact with each other, and grow ventralwards, one on each side of the brain, in close contact with its walls. I have failed to detect more than one root for the two embryonic branches of the fifth (ophthalmic and mandibular), _and no trace of an anterior or ventral root has been met with in any of my sections_.

The seventh nerve is formed nearly simultaneously with or shortly after the fifth, and some little distance behind and independently of it, opposite the anterior end of the thickening of the epiblast to form the auditory involution. It arises precisely like the fifth, from the extreme dorsal summit of the neural axis (Pl. 15, fig. 4_a_, VII). So far as I have been able to determine, the auditory nerve and the seventh proper possess only a single root common to the two. There is no anterior root for the seventh any more than for the fifth.

Behind the auditory involution, at a stage subsequent to that in which the fifth and seventh nerves appear, there arise a series of roots from the dorsal summit of the hind-brain, which form the rudiments of the glosso-pharyngeal and vagus nerves. These roots are formed towards the close of stage H, but are still quite short at the beginning of stage I. Their manner of development resembles that of the previously described cranial nerves. The central ends of the roots of the opposite sides are at first in contact with each other, and there is nothing to distinguish the roots of the glosso-pharyngeal and of the vagus nerves from the dorsal roots of spinal nerves. Like the dorsal roots of the spinal nerves, they appear as a series of ventral prolongations of a continuous outgrowth from the brain, which outgrowth is moreover continuous with that for the spinal nerves[280]. The outgrowth of the vagus and glosso-pharyngeal nerves is not continuous with that of the seventh nerve. This is shewn by Pl. 15, figs. 4_a_ and 4_b_. The outgrowth of the seventh nerve though present in 4_a_ is completely absent in 4_b_ which represents a section just behind 4_a_.

Footnote 280: In the presence of this continuous outgrowth of
the brain from which spring the separate nerve stems of the
vagus, may perhaps be found a reconciliation of the apparently
conflicting statements of Götte and myself with reference to
the vagus nerve. Götte regards the vagus as a single nerve,
from its originating as an undivided rudiment; but it is clear
from my researches that, for Elasmobranchii at least, this
method of arguing will not hold good, since it would lead to
the conclusion that all the spinal nerves were branches of one
single nerve, since they too spring as processes from a
continuous outgrowth from the brain!

Thus, by the end of stage I, there have appeared the rudiments of the 5th, 7th, 8th, 9th and 10th cranial nerves, all of which spring from the hind-brain. These nerves all develop precisely as do the posterior roots of the spinal nerves, and it is a remarkable fact _that hitherto I have failed to find a trace in the brain of a root of any cranial nerve arising from the ventral corner of the brain as do the anterior roots of the spinal nerves_[281].

Footnote 281: The conclusion here arrived at with reference to
the anterior roots, is opposed to the observations of both
Gegenbaur on Hexanchus, _Jenaische Zeitschrift_, Vol. VI., and
of Jackson and Clarke on Echinorhinus, _Journal of Anatomy and
Physiology_, Vol. X. These morphologists identify certain roots
springing from the medulla below and behind the main roots of
the vagus as true anterior roots of this nerve. The existence
of these roots is not open to question, but without asserting
that it is impossible for me to have failed to detect such
roots had they been present in the embryo, I think I may
maintain if these anterior roots are not present in the embryo,
their identification as vagus roots must be abandoned; and they
must be regarded as belonging to spinal nerves. This point is
more fully spoken of at p. 428.

It is admittedly difficult to prove a negative, and it may still turn out that there are anterior roots of the brain similar to those of the spinal cord; in the mean time, however, the balance of evidence is in favour of there being none such. This at first sight appears a somewhat startling conclusion, but a little consideration shews that it is not seriously opposed to the facts which we know. In the first place it has been shewn by myself[282] that in Amphioxus (whose vertebrate nature I cannot doubt) only dorsal nerve-roots are present. Yet the nerves of Amphioxus are clearly mixed motor and sensory nerves, and it appears to me far more probable that Amphioxus represents a phase of development in which the nerves had not acquired two roots, rather than one in which the anterior root has been lost. In other words, the condition of the nerves in Amphioxus appears to me to point to the conclusion _that primitively the cranio-spinal nerves of vertebrates were nerves of mixed function with one root only, and that root a dorsal one; and that the present anterior or ventral root is a secondary acquisition_. This conclusion is further supported by the fact that the posterior roots develop in point of time before the anterior roots. If it be admitted that the vertebrate nerves primitively had only a single root, then the retention of that condition in the brain implies that this became differentiated from the remainder of the nervous system at a very early period before the acquirement of anterior nerve-roots, and that these eventually become developed only in the case of spinal nerves, and not in the case of the already highly modified cranial nerves.

Footnote 282: _Journal of Anatomy and Physiology_, Vol. X.
[This Edition, No. IX.]

* * * * *

_Subsequent Changes of the Nerves._ To simplify my description of the subsequent growth of the cranial nerves, I have inserted a short description of their distribution in the adult. This is taken from a dissection of Scyllium stellare, which like other species has some individualities of its own not found in the other Elasmobranchii. For points not touched on in this description I must refer the reader to the more detailed accounts of my predecessors, amongst whom may specially be mentioned Stannius[283] for Carcharias, Spinax, Raja, Chimæra, &c.; Gegenbaur[284] for Hexanchus; Jackson and Clarke[285] for Echinorhinus.

Footnote 283: _Nervensystem d. Fische_, Rostock, 1849.

Footnote 284: _Jenaische Zeitschrift_, Vol. VI.

Footnote 285: _Journal of Anatomy and Physiology_, Vol. X.

The ordinary nomenclature has been employed for the branches of the fifth and seventh nerves, though embryological data to be adduced in the sequel throw serious doubts upon it. Since I am without observations on the origin of the nerves to the muscles of the eyes, all account of these is omitted.

The fifth nerve arises from the brain by three roots[286]: (1) an
anterior more or less ventral root; (2) a root slightly behind, but
close to the former[287], formed by the coalescence of two distinct
strands, one arising from a dorsal part of the medulla, and a second
and larger from the ventral; (3) a dorsal and posterior root, in its
origin quite distinct and well separated from the other two, and
situated slightly behind the dorsal strand of the second root. This
root a little way from its attachment becomes enclosed for a short
distance in the same sheath as the dorsal part of the second root, and
a slight mixture of fibres seems to occur, but the majority of its
fibres have no connection with those of the second root. The first and
second roots of the fifth appear to me partially to unite, but before
their junction the ramus ophthalmicus profundus is given off from the
first of them.

Footnote 286: My results with reference to these roots accord
exactly, so far as they go, with the more carefully worked out
conclusions of Stannius, _loc. cit._ pp. 29 and 30.

Footnote 287: The root of the seventh nerve cannot properly be
distinguished from this root.

The fifth nerve, according to the usual nomenclature, has three main
divisions. The first of these is the ophthalmic. It is formed by the
coalescence of two entirely independent branches of the fifth, which
unite on leaving the orbit. The dorsalmost of these, or ramus
ophthalmicus superficialis, originates from the third and posterior of
the roots of the fifth, nearly the whole of which appears to enter
into its formation. This root is situated on the dorsal part of the
"lobi trigemini," _at a point posterior to that of the other roots of
the fifth or even of the seventh nerve_. The branch itself enters the
orbit by a separate foramen, and, keeping on the dorsal side of it,
reenters the cartilage at its anterior wall, and is there joined by
the _ramus ophthalmicus profundus_. This latter nerve arises from the
anterior root of the fifth, separately pierces the wall of the orbit,
and takes a course slightly ventral to the superior ophthalmic nerve,
but does not (as is usual with Elasmobranchii) run below the superior
rectus and superior oblique muscles of the eye. The nerve formed by
the coalescence of the superficial and deep ophthalmic branches
courses a short way below the surface, and supplies the mucous canals
of the front of the snout. It is a purely sensory nerve. Strong
grounds will be adduced in the sequel for regarding the _ramus
ophthalmicus superficialis_, though not the _ophthalmicus profundus_,
as in reality a branch of the seventh, and not of the fifth nerve.

The second division of the fifth nerve is the superior maxillary,
which appears to me to arise from both the first and second roots of
the fifth, though mainly from the first. It divides once into two main
branches. The first of these--the buccal nerve of Stannius--after
passing forwards along the base of the orbit takes its course
obliquely across the palatine arch and behind and below the nasal
sack, supplying by the way numerous mucous canals, and dividing at
last into two branches, one of these passing directly forwards on the
ventral surface of the snout, and the second keeping along the front
border of the mouth. The second division of the superior maxillary
nerve (superior maxillary of Stannius), after giving off a small
branch, which passes backwards in company with a branch from the
inferior maxillary nerve to the levator maxillæ superioris, itself
keeps close to the buccal nerve, and eventually divides into numerous
fine twigs to the mucous canals of the skin at the posterior region of
the upper jaw. It anastomoses with the buccal nerve. The inferior
maxillary nerve arises mainly from the second root of the fifth. After
sending a small branch to the levator maxillæ superioris, it passes
outwards along the line separating the musculus adductor mandibulæ
from the musculus levator labii superioris, and after giving branches
to these muscles takes a course forward along the border of the lower
jaw. It appears to be a mixed motor and sensory nerve.

The seventh or facial nerve arises by a root close to, but behind and
below the second root of the fifth, and is intimately fused with this.
It divides almost at once into a small anterior branch and large
posterior.