Illustration: Figure 247Fig. 247. Longitudinal section through the brain of a young Pristiurus embryo.cer.commencement of the cerebral hemisphere;pn.pineal gland;In.infundibulum;pt.ingrowth from mouth to form the pituitary body;mb.mid-brain;cb.cerebellum;ch.notochord;al.alimentary tract;Iaa.artery of mandibular arch.
Fig. 247. Longitudinal section through the brain of a young Pristiurus embryo.cer.commencement of the cerebral hemisphere;pn.pineal gland;In.infundibulum;pt.ingrowth from mouth to form the pituitary body;mb.mid-brain;cb.cerebellum;ch.notochord;al.alimentary tract;Iaa.artery of mandibular arch.
The most striking peculiarity with reference to the general development of the brain is a curvature which appears in its axis, known as the cranial flexure. The flexure takes place through the mid-brain, and causes the fore-brain to be gradually bent downwards so that the axis of its floor forms, first, a right angle with that of the hinder part of the brain, and subsequently, as a rule, an acute angle.
During these changes the brain, in most Amniota at any rate, becomes in the first instance retort-shaped, the cerebral vesicle forming the swollen part of the retort, but subsequently the retort-shape is lost owing to the great development of the vesicle of the mid-brain, which forms the termination of the long axis of the embryo.Figs.29, 76,and 118, are representative figures of embryos of various vertebrate forms at a period when the mid-brain forms the termination of the long axis of the body.
It is generally stated that the cranial flexure is at its maximum at the stage represented in these figures, and there can be no doubt that viewed from the exterior the cranial flexure ceases to be so marked a feature, and finally disappears as the embryo gradually grows older; but though the mid-brain ceases to form the termination of the long axis of the embryo, the flexure of the brain becomes in many forms absolutely more marked; while in other forms, though stated to diminish, it does not entirely vanish.
Illustration: Figure 248Fig. 248. Longitudinal section through the brain of Scyllium canicula at an advanced stage of development.cer.cerebral hemisphere;pn.pineal gland;op.th.optic thalamus, connected with its fellow by a commissure (the middle commissure). In front of it is seen a fold of the roof of the fore-brain, which is connected with the choroid plexus of the third ventricle;op.optic chiasma;pt.pituitary body;in.infundibulum;cb.cerebellum;au.v.passage leading from the auditory vesicle to the exterior;mel.medulla oblongata;c.in.internal carotid artery.
Fig. 248. Longitudinal section through the brain of Scyllium canicula at an advanced stage of development.cer.cerebral hemisphere;pn.pineal gland;op.th.optic thalamus, connected with its fellow by a commissure (the middle commissure). In front of it is seen a fold of the roof of the fore-brain, which is connected with the choroid plexus of the third ventricle;op.optic chiasma;pt.pituitary body;in.infundibulum;cb.cerebellum;au.v.passage leading from the auditory vesicle to the exterior;mel.medulla oblongata;c.in.internal carotid artery.
The general nature of the changes which take place will perhaps best be understood by a comparison offigs.247and248representing longitudinal sections at two stages through the brain of an embryo Elasmobranch. The actual cranial flexure,i.e.flexure of the floor of the brain, is obviously greater in the older of the two brains, though viewed from the exterior the axis of this brain appears to be quite straight. In the younger stage,fig. 247, the mid-brain (mb) forms the end of the long axis of the body, while in the older one the cerebral hemispheres (cer) have grown very greatly, especially forwards and dorsalwards. They have thus come to lie in front of the mid-brain, and to form the end of the long axis of the body, and have at the same time compressed the originally large thalamencephalon against the mid-brain. The same general features may be seen infig. 250representing a longitudinal section of the brain of an embryo fowl, andfig. 255representing a longitudinal section of the brain of a Mammal.
The infundibulum or perhaps rather the point of origin of the optic nerves is to be regarded as the anterior termination of the axis of the base of the brain.
The cranial flexure is least marked in Cyclostomata (fig. 253), Teleostei, Ganoidei, and Amphibia, while it is very pronounced in Elasmobranchii, Reptilia, Aves, and Mammalia. In Teleostei, and still more in Cyclostomata, it permanently remains slight, owing to the small development of the cerebral hemispheres.
In addition to the cranial flexures, two other flexures make their appearance in the base of the brain. A posterior at the junction of the brain and spinal cord, and an anterior at the boundary between the cerebellum and medulla oblongata, just at the point where the pons Varolii is formed in Mammalia. The anterior of these is the most marked and constant; it is shewn infig. 250. It arises considerably later than the main cranial flexure, and since it is turned the opposite way it assists to a considerable extent in causing the apparent straightening of the cranial axis.
Histogenetic changes[161]. The walls of the brain are at first very thin and, like those of the spinal cord, are formed of a number of ranges of spindle-shaped cells. The processes of each of these cells are stated to be continued through the whole thickness of the wall. In the floor of the hind- and mid-brain a superficial layer of delicate nerve-fibres is formed at an early period. This layer appears in the first instance on the floor and sides of the hind-brain, and very slightly, if at all, later on the floor and the sides of the mid-brain. The cells internal to the nerve-fibres become differentiated into an innermost epithelial layer lining the cavities of the ventricles, and an outer layer of grey matter.
The similarity of the primitive arrangement and histological character of the parts of the brain behind the cerebral hemispheres to that of the spinal cord is very conclusively shewn by the examination of any good series of sections. In both brain and spinal cord the white matter forms a cap on the ventral and lateral parts considerably before it extends to the dorsal surface. In the medulla the white matter does not eventually extend to the roof owing to the peculiar degeneration which that part undergoes.
In the case of the fore-brain the earliest histological changes, except possibly in Mammals, take place on the same general plan as those of the remainder of the central nervous system[162]; but though the general plan is the same, yet the early histological distinction between the fore-brain, and the mid- and hind-brain is more marked than the distinction between the latter and the spinal cord.
On the floor and sides of the thalamencephalon, and apparently the whole of the hemispheres of the lower types, there is formed, somewhat later than in the remainder of the brain, a very delicate layer of white matter. The inner part of the wall, which still remains comparatively thin, is not at first clearly divided into an epithelial and nervous layer. This distinction soon however becomes more or less apparent, though it is not so marked as in most other parts of the brain; and it appears that in the subsequent growth the greater part of the original epithelial layer becomes converted into nervous tissue.
In Mammals the same plan of differentiation would seem to be followed, though somewhat less obviously than in the lower types. The walls of the hemispheres become first divided (Kölliker) into a superficial thinner layer of rounded elements, and a deeper and thicker epithelial layer, and between these the fibres of the crura cerebri soon interpose themselves. At a slightly later period a thin superficial layer of white matter, homologous with that of the remainder of the brain, becomes established.
The inner layer, together with the fibres from the crura cerebri, gives rise to the major part of the white matter of the hemispheres and to the epithelium lining the lateral ventricles.
The outer layer of rounded cells becomes divided into (1) a superficial part with comparatively few cells, which, together with its coating of white matter, forms the cortical part of the grey matter, and (2) a deeper layer with numerous cells which forms the main mass of the grey matter of the hemispheres.
The development of the several parts of the brain will now be described.
The hind-brain. The hind-brain is at first an elongated, funnel-shaped tube, the walls of which are of a nearly uniform thickness, though the roof and floor are somewhat thinner than the sides. It forms a direct continuation of the spinal cord, into which it passes without any sharp line of demarcation. The ventricle it contains is known as the fourth ventricle.
The sides become in the chick marked by a series of transverse constrictions, dividing it into lobes, which are somewhat indefinite in number. The first of these remains permanent, and its roof gives rise to the cerebellum. It is uncertain whether the other constrictions have any morphological significance. More or less similar constrictions are present in Teleostei. In Elasmobranchii the medulla presents on its inner face at a late period a series of lobes corresponding with the roots of the vagus and glossopharyngeal nerves, and it is possible that the earlier constrictions may potentially correspond to so many nerve-roots.
Illustration: Figure 249Fig. 249. Section through the hind-brain of a Chick at the end of the third day of incubation.IV.Fourth ventricle. The section shews the very thin roof and thicker sides of the ventricle.Ch.Notochord;CV.Anterior cardinal vein;CC.Involuted auditory vesicle;CCpoints to the end which will form the cochlear canal;RL. Recessus labyrinthi (remains of passage connecting the vesicle with the exterior);hy.Hypoblast lining the alimentary canal;AO.,AOA.Aorta, and aortic arch.
Fig. 249. Section through the hind-brain of a Chick at the end of the third day of incubation.IV.Fourth ventricle. The section shews the very thin roof and thicker sides of the ventricle.Ch.Notochord;CV.Anterior cardinal vein;CC.Involuted auditory vesicle;CCpoints to the end which will form the cochlear canal;RL. Recessus labyrinthi (remains of passage connecting the vesicle with the exterior);hy.Hypoblast lining the alimentary canal;AO.,AOA.Aorta, and aortic arch.
Throughout the Vertebrata an anterior lobe of the hind-brain becomes very early marked off, so that the primitive hind-brain becomes divided into two regions which may beconveniently spoken of as the cerebellum (figs.247and248,cb) and medulla oblongata. The floor of these regions is quite continuous and is also prolonged without any break into the floor of the mid-brain.
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, consisting of a single row of flattened cells (fig. 249). 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 arise at the sides of the brain.
The thin roof of the fourth ventricle is triangular, or, in Mammalia, somewhat rhomboidal in shape. The apex of the triangle is directed backwards.
At a later period the blood-vessels of the pia mater form a rich plexus over the anterior part of the thin roof of the medulla, which becomes at the same time somewhat folded. The whole structure is known as the tela vasculosa, or choroid plexus of the fourth ventricle (fig. 250,chd4). The floor of the whole hind-brain becomes thickened, and there very soon appears on its outer surface a layer of non-medullated nerve-fibres, similar to those which first appear on the spinal cord. They are continuous with a similar layer of fibres on the floor of the mid-brain, where they constitute the crura cerebri. On the ventral floor of the medulla is a shallow continuation of the anterior fissure of the spinal cord.
In Elasmobranchii and many Teleostei the restiform tracts are well developed, and are anteriorly continued into the cerebellum, of which they form the peduncles. Near their junction with the cerebellum they form prominent bodies, which are regarded by Miklucho-Maclay as representing the true cerebellum of Elasmobranchii.
In Elasmobranchii a dorsal pair of ridges projects into the cavity of the fourth ventricle, corresponding apparently with the fasciculi teretes of the Mammalia.
In Mammalia there develop, subsequently to the longitudinal fibresalready spoken of, first the olivary bodies of the ventral side of the medulla, and at a still later period the pyramids. The fasciculi teretes in the cavity of the fourth ventricle are developed shortly before the pyramids.
When the hind-brain becomes divided into two regions the roof of the anterior part does not become thinned out like that of the posterior, but on the contrary, becomes somewhat thickened and forms a band-like structure roofing over the anterior part of the fourth ventricle (fig. 247andfig. 253,cb).
This is a rudiment of the cerebellum, and in all Craniate Vertebrates it at first presents this simple structure and insignificant size. In Cyclostomata, Amphibia and many Reptilia this condition is permanent. In Elasmobranchii, on the other hand, the cerebellum assumes in the course of development a greater and greater prominence (fig. 248,cb), and eventually overlaps both the optic lobes in front and the medulla behind. In the later embryonic stages it exhibits in surface-views the appearance of a median constriction, and the portion of the ventricle contained in it is prolonged into two lateral outgrowths.
Miklucho-Maclay, from his observations on the brains of adult Elasmobranchii, was led to regard what is here called the cerebellum as identical with the mid-brain, and the true mid-brain as part of the thalamencephalon. Miklucho-Maclay was no doubt 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.
The mid-brain and thalamencephalon (according to the ordinary interpretations) have in the embryo of Elasmobranchs exactly the same relations as in the embryos of other Vertebrates; so that the embryological evidence appears to me to be conclusive against Miklucho-Maclay’s view.
In Birds the cerebellum attains a very considerable development (fig. 250,cbl), consisting of a folded central lobe with an arbor vitæ, into which the fourth ventricle is prolonged. There are two small lateral lobes, apparently equivalent to the flocculi. Anteriorly the cerebellum is connected with the roof of the mid-brain by a delicate membrane, the velum medullæ anterius, or valve of Vieussens (fig. 250,vma). The pons Varolii of Mammalia is represented by a small number of transverse fibres on the floor of the hind-brain immediately below the cerebellum.
In Mammalia the cerebellum attains a still greater development.The median lobe or vermiform process is first developed. In the higher Mammalia the lateral parts forming the hemispheres of the cerebellum become formed as swellings at the sides at a considerably later period, and are hardly developed in the Monotremata and Marsupialia.
Illustration: Figure 250Fig. 250. Longitudinal section through the brain of a Chick of ten days.(After Mihalkovics.)hms.cerebral hemispheres;alf.olfactory lobe;alf1. olfactory nerve;ggt.corpus striatum;oma.anterior commissure;chd3. choroid plexus of the third ventricle;pin.pineal gland;cmp.posterior commissure;trm.lamina terminalis;chm.optic chiasma;inf.infundibulum;hph.pituitary body;bgm.commissure of Sylvius (roof of iter a tertio ad quartum ventriculum);vma.velum medullæ anterius (valve of Vieussens);cbl.cerebellum;chd4. choroid plexus of the fourth ventricle;obl4. roof of fourth ventricle;obl.medulla oblongata;pns.commissural part of medulla;inv.sheath of brain;bls.basilar artery;crts.internal carotid.
Fig. 250. Longitudinal section through the brain of a Chick of ten days.(After Mihalkovics.)hms.cerebral hemispheres;alf.olfactory lobe;alf1. olfactory nerve;ggt.corpus striatum;oma.anterior commissure;chd3. choroid plexus of the third ventricle;pin.pineal gland;cmp.posterior commissure;trm.lamina terminalis;chm.optic chiasma;inf.infundibulum;hph.pituitary body;bgm.commissure of Sylvius (roof of iter a tertio ad quartum ventriculum);vma.velum medullæ anterius (valve of Vieussens);cbl.cerebellum;chd4. choroid plexus of the fourth ventricle;obl4. roof of fourth ventricle;obl.medulla oblongata;pns.commissural part of medulla;inv.sheath of brain;bls.basilar artery;crts.internal carotid.
The cerebellum is connected with the roof of the mid-brain in front and with the choroid plexus of the fourth ventricle behind by delicate membranous structures, known as the velum medullæ anterius (valve of Vieussens) and the velum medullæ posterius.
The pons Varolii is formed on the ventral side of the floor of the cerebellar region as a bundle of transverse fibres at about the same time as the olivary bodies.
The mid-brain. The changes undergone by the mid-brain are simpler than those of any other part of the brain. We have already seen that the mid-brain, on the appearance of the cranial flexure, formsan unpaired vesiclewith a vaulted roof and curved floor, at the front end of the long axis of the body (fig. 118,MB). It is at this period in most Vertebrates relatively much larger than in the adult; and it is only in the Teleostei that it more or less retains in the adult its embryonic proportions.
The cavity of the mid-brain, greatly reduced in size in the higher forms, is known as the iter a tertio ad quartum ventriculum, or aqueductus Sylvii.
The roof of the mid-brain is sharply constricted off from the divisions of the brain in front of and behind it, but these constrictions do not extend to the floor.
In some Vertebrates the region of the mid-brain is stated to undergo hardly any further development. In the Axolotl it remains according to Stieda[163]as a simple tube with nearly uniformly thick walls. In the majority of forms it undergoes, however, a more complicated development.
In Elasmobranchs the sides become thickened to form the optic lobes, which are soon separated by a median longitudinal groove. The floor becomes thickened to form the crura cerebri. The primitive simple median cavity becomes imperfectly divided into a median portion below, and two lateral diverticula in the optic lobes.
In Teleostei the changes, resulting in the formation of (1) a pair of longitudinal ridges projecting from the roof into the cavity of the iter, constituting the fornix of Gottsche, and (2) of the two swellings on the floor, forming the tori semicirculares, are more complicated, but have not been satisfactorily worked out. In Bombinator and the Anura generally the changes are of the same nature as those in Elasmobranchii, except that the prolongations of the ventricle into the optic lobes are still further constricted off from the median portion, which forms the true iter.
In Reptilia and Aves the development of the mid-brain takes place on the same type as in Elasmobranchii and the Anura. In Birds the optic lobes are pushed very much aside, and the roof of the iter is greatly thinned out. In Mammalia the sides of the mid-brain give rise to two pairs of prominences—the corpora quadrigemina—instead of the two optic lobes of other Vertebrata. The prominences, which do not contain prolongations of the iter, become first visible on the appearance of an oblique transverse furrow, while the anterior pair alone are separated by a longitudinal furrow. In the later stages of development the longitudinal furrow is continued so as to bisect the posterior pair.
The floor, which is bounded posteriorly by the pons Varolii, becomes the crura cerebri. The corpora geniculata interna also belong to this division of the brain.
Fore-brain. In its earliest condition the fore-brain forms a single vesicle without a trace of separate divisions, but very early it buds off the optic vesicles, whose history is described with that of the eye.
Illustration: Figure 251Fig. 251. Section through the front part of the head of a Lepidosteus embryo on the seventh day after impregnation.al.alimentary tract;fb.thalamencephalon;l.lens of eye;op.v.optic vesicle. The mesoblast is not represented.
Fig. 251. Section through the front part of the head of a Lepidosteus embryo on the seventh day after impregnation.al.alimentary tract;fb.thalamencephalon;l.lens of eye;op.v.optic vesicle. The mesoblast is not represented.
Illustration: Figure 252Fig. 252. Longitudinal section through the brain of a young Pristiurus embryo.cer.commencement of cerebral hemisphere;pn.pineal gland;In.infundibulum;pt.ingrowth of mouth to form the pituitary body;mb.mid-brain;cb.cerebellum;ch.notochord;al.alimentary tract;Iaa.artery of mandibular arch.
Fig. 252. Longitudinal section through the brain of a young Pristiurus embryo.cer.commencement of cerebral hemisphere;pn.pineal gland;In.infundibulum;pt.ingrowth of mouth to form the pituitary body;mb.mid-brain;cb.cerebellum;ch.notochord;al.alimentary tract;Iaa.artery of mandibular arch.
The optic vesicles become gradually constricted off from the fore-brain in a direction obliquely backwards and downwards. They remain, however, attached to it at the anterior extremity of the base of the fore-brain (fig. 251,op.v.). While the above changes are taking place in the optic vesicles the anterior part of the fore-brain becomes prolonged, and at the same time somewhat dilated. At first there is no sharp boundary between the primitive fore-brain and its anterior prolongation, but there shortly appears a constriction which passes from above obliquely forwards and downwards. This constriction is shallow at first, but soon becomes much deeper, leaving however the cavities of the two divisions of the fore-brain united ventrally by a somewhat wide canal (fig. 252).
Of these two divisions the posterior becomes the thalamencephalon, while the anterior and larger division (cer) forms the rudiment of the cerebral hemispheres and olfactory lobes. For a considerable period this rudiment remains perfectly simple, and exhibits no signs, either externally or internally, of a longitudinal constriction dividing it into two lobes.
From the above description it may be concluded that therudiment of the cerebral hemispheres is contained in the original fore-brain. In spite however of their great importance in all the Craniata, it is probable that the hemispheres were either not present as distinct structures, or only imperfectly separated from the thalamencephalon, in the primitive vertebrate stock.
The thalamencephalon. The thalamencephalon varies so slightly in structure throughout the Vertebrate series that a general description will suffice for all the types.
It forms at first a simple vesicle, the walls of which are of a nearly uniform thickness and formed of the usual spindle-shaped cells.
Illustration: Figure 253Fig. 253. Diagrammatic vertical section through the head of a larva of Petromyzon.The larva had been hatched three days, and was 4.8mm.in length. The optic and auditory vesicles are supposed to be seen through the tissues.c.h.cerebral hemisphere;th.optic thalamus;in.infundibulum;pn.pineal gland;mb.mid-brain;cb.cerebellum;md.medulla oblongata;au.v.auditory vesicle;op.optic vesicle;ol.olfactory pit;m.mouth;br.c.branchial pouches;th.thyroid involution;v.ao.ventral aorta;ht.ventricle of heart;ch.notochord.
Fig. 253. Diagrammatic vertical section through the head of a larva of Petromyzon.The larva had been hatched three days, and was 4.8mm.in length. The optic and auditory vesicles are supposed to be seen through the tissues.c.h.cerebral hemisphere;th.optic thalamus;in.infundibulum;pn.pineal gland;mb.mid-brain;cb.cerebellum;md.medulla oblongata;au.v.auditory vesicle;op.optic vesicle;ol.olfactory pit;m.mouth;br.c.branchial pouches;th.thyroid involution;v.ao.ventral aorta;ht.ventricle of heart;ch.notochord.
The cavity it contains is known as the third ventricle. Anteriorly it opens widely into the cerebral rudiment, and posteriorly into the ventricle of the mid-brain. The opening into the cerebral rudiment becomes the foramen of Munro.
For convenience of description I shall divide it into three regions,viz.(1) the floor, (2) the sides, and (3) the roof.
The floor becomes divided into two parts, an anterior part, giving origin to the optic nerves, in which is formed the optic chiasma; and a posterior part, which becomes produced intoan at first inconspicuous prominence—the rudiment of the infundibulum (fig. 252,In). This comes in contact with an involution from the mouth, which gives rise to the pituitary body (fig. 252,pt), the development of which will be dealt with separately.
In the later stages of development the infundibulum becomes gradually prolonged, and forms an elongated diverticulum of the third ventricle, the apex of which is in contact with the pituitary body (figs.252,254,in, andfigs.250and255,inf).
Along the sides of the infundibulum run the commissural fibres connecting the floor of the mid-brain with the cerebrum.
Illustration: Figure 254Fig. 254. Longitudinal section through the brain of Scyllium canicula at an advanced stage of development.cer.cerebral hemisphere;pn.pineal gland;op. th.optic thalamus, connected with its fellow by a commissure (the middle commissure). In front of it is seen a fold of the roof of the fore-brain, which is the choroid plexus of the third ventricle;op.optic chiasma;pt.pituitary body;in.infundibulum;cb.cerebellum;au.v.passage leading from the auditory vesicle to the exterior;mel.medulla oblongata;c.in.internal carotid artery.
Fig. 254. Longitudinal section through the brain of Scyllium canicula at an advanced stage of development.cer.cerebral hemisphere;pn.pineal gland;op. th.optic thalamus, connected with its fellow by a commissure (the middle commissure). In front of it is seen a fold of the roof of the fore-brain, which is the choroid plexus of the third ventricle;op.optic chiasma;pt.pituitary body;in.infundibulum;cb.cerebellum;au.v.passage leading from the auditory vesicle to the exterior;mel.medulla oblongata;c.in.internal carotid artery.
In its later stages the infundibular region presents considerable variations in the different vertebrate types. In Fishes it generally remains very large, and permanently forms a marked diverticulum of the floor of the thalamencephalon. In Elasmobranchii the distal end becomes divided into three lobes—a median and two lateral. The lateral lobes appear to become the sacci vasculosi of the adult.
In Teleostei peculiar bodies known as the lobi inferiores (hypoaria) make their appearance at the sides of the infundibulum. They appear to correspond in position with the tuber cinereum of Mammalia[164]. In Birds, Reptiles, and Amphibia the lower part of the embryonic infundibulum becomes atrophied and reduced to a mere finger-like process—the processus infundibuli.
In Mammalia the posterior part of the primitive infundibulum becomes the corpus albicans, which is double in Man and the higher Apes; the ventral part of the posterior wall forms the tuber cinereum. Laterally, at the junction of the optic thalami and infundibulum, there are placed the fibres of the crura cerebri, which are probably derived from the walls of the infundibulum. A special process grows out from the base of the infundibulum, which undergoes peculiar changes, and becomes intimately united with the pituitary body; in which connection it will be more fully described.
The sides of the thalamencephalon become very early thickened to form the optic thalami, which constitute the most important section of the thalamencephalon. They are separated, in Mammalia at all events, on their inner aspect from the infundibular region by a somewhat S-shaped groove, known as the sulcus of Munro, which ends in the foramen of Munro. They also become in Mammalia secondarily united by a transverse commissure, the grey or middle commissure, which passes across the cavity of the third ventricle. This commissure is probably homologous with, and derived from, a commissural band in the roof of the thalamencephalon, placed immediately in front of the pineal gland which is well developed in Elasmobranchii (fig. 254).
The roof undergoes more complicated changes. It becomes divided, on the appearance of the pineal gland as a small papilliform outgrowth (the development of which is dealt with separately), into two regions—a longer anterior in front of the pineal gland and a shorter posterior. The anterior region becomes at an early period excessively thin, and at a later period, when the roof of the thalamencephalon is shortened by the approach of the cerebral hemispheres to the mid-brain, it becomes (videfigs.250and255,chd3, and254) considerably folded, while at the same time a vascular plexus is formed in the pia mater above it. On the accomplishment of these changes it is known as the tela choroidea of the third ventricle.
In the roof of the third ventricle behind the pineal gland there appear in Elasmobranchii, the Sauropsida and Mammalia transverse commissural fibres, forming a structure known as the posterior commissure, which connects together the two optic thalami.
The most remarkable organ in the roof of the thalamencephalon is the pineal gland, which is developed in most Vertebrates as a simple papilliform outgrowth of the roof, and is at first composed of cells similar to those of the other parts of the central nervous system (figs.250,252,254and255,pnorpin). In the lower Vertebrata it is directed forwards, but in Mammalia, and to some extent in Aves, it is directed backwards.
In Amphibia it is described by Götte (No.296) as being a product of the point where the roof of the brain remains latest attached to the external skin.
The figure which Götte gives to prove this does not appear to me fully to bear out his conclusion; which if true is very important. Although I directed my attention specially to this point, I could find no indication in Elasmobranchii of a process similar to that described by Götte, and his observations have not as yet been confirmed for other Vertebrates. Götte compares the pineal gland to the long-persisting pore which leads into the cavity of the brain in the embryo of Amphioxus, and we might add the Ascidians, and, should his facts be confirmed, the conclusion he draws from them would appear to be well founded.
The later stages in the development of the pineal gland in different Vertebrates have not in all cases been fully worked out[165].
Illustration: Figure 255Fig. 255. Longitudinal vertical section through the anterior part of the brain of an embryo rabbit of four centimetres.(After Mihalkovics.)The section passes through the median line so that the cerebral hemispheres are not cut; their position is however indicated in outline.spt.septum lucidum formed by the coalescence of the inner walls of part of the cerebral hemispheres;cna.anterior commissure;frx.vertical pillars of the fornix;cal.genu of corpus callosum;trm.lamina terminalis;hms.cerebral hemispheres;olf.olfactory lobes;acl.artery of corpus callosum;fmr.position of foramen of Munro;chd3. choroid plexus of third ventricle;pin.pineal gland;cmp.posterior commissure;bgm.lamina uniting the lobes of the mid-brain;chm.optic chiasma;hph.pituitary body;inf.infundibulum;pns. pons Varolii;pde.cerebral peduncles;agd.iter.
Fig. 255. Longitudinal vertical section through the anterior part of the brain of an embryo rabbit of four centimetres.(After Mihalkovics.)The section passes through the median line so that the cerebral hemispheres are not cut; their position is however indicated in outline.spt.septum lucidum formed by the coalescence of the inner walls of part of the cerebral hemispheres;cna.anterior commissure;frx.vertical pillars of the fornix;cal.genu of corpus callosum;trm.lamina terminalis;hms.cerebral hemispheres;olf.olfactory lobes;acl.artery of corpus callosum;fmr.position of foramen of Munro;chd3. choroid plexus of third ventricle;pin.pineal gland;cmp.posterior commissure;bgm.lamina uniting the lobes of the mid-brain;chm.optic chiasma;hph.pituitary body;inf.infundibulum;pns. pons Varolii;pde.cerebral peduncles;agd.iter.
In Elasmobranchii the pineal gland becomes in time very long, and extends far forwards over the roof of the cerebralhemispheres (fig. 254pn). Its distal extremity dilates somewhat, and in the adult the whole organ forms (Ehlers,No.337) an elongated tube, enlarged at its free extremity, and opening at its base into the brain. The enlarged extremity may either be lodged in a cavity in the cartilage of the cranium (Acanthias), or be placed outside the cranium (Raja).
In Petromyzon its form is very different. It arises (fig. 253pn) as a sack-like diverticulum of the thalamencephalon extending at first both backwards and forwards. In the Ammocœte the walls of this sack are deeply infolded.
The embryonic form of the pineal gland in Amphibia is very much like that which remains permanent in Elasmobranchii; the stalk connecting the enlarged terminal portion with the brain soon however becomes solid and very thin except at its proximal extremity. The enlarged portion also becomes solid, and is placed in the adult externally to the skull, where it forms a mass originally described by Stieda as the cerebral gland.
In Birds the primitive outgrowth to form the pineal gland becomes, according to Mihalkovics, deeply indented by vascular connective tissue ingrowths, so that it assumes a dendritic structure (fig. 250pin).
The proximal extremity attached to the roof of the thalamencephalon forms a special section, known as the infra-pineal process. The central lumen of the free part of the gland finally atrophies, but the branches still remain hollow. The infra-pineal process becomes reduced to a narrow stalk, connecting the branched portion of the body with the brain. The branched terminal portion and the stalk obviously correspond with the vesicle and distal part of the stalk of the types already described. In Mammalia the development of the pineal gland is, according to Mihalkovics, generally similar to that of Birds. The original outgrowth becomes branched, but the follicles or lobes to which the branching gives rise eventually become solid (fig. 255pin). An infra-pineal process is developed comparatively late, and is not sharply separated from the roof of the brain.
No satisfactory suggestions have yet been offered as to the nature of the pineal gland, unless the view of Götte be regarded as such. It appears to possess in all forms an epithelial structure, but, except at the base of the stalk (infra-pineal process) inMammalia, in the wall of which there are nerve-fibres, no nervous structures are present in it in the adult state.
The pituitary body. Although the pituitary body is not properly a nervous structure, yet from its intimate connection with the brain it will be convenient to describe its development here. The pituitary body is in fact an organ derived from the epiblast of the stomodæum. This fact has been demonstrated for Mammalia, Aves, Amphibia and Elasmobranchii, and may be accepted as holding good for all the Craniata[166]. The epiblast in the angle formed by the cranial flexure becomes involuted to form the cavity of the mouth. This cavity 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. Its uppermost end does not at first become markedly constricted off from the remainder, but is nevertheless the rudiment of the pituitary body.
Fig. 256represents a transverse section through the head of an Elasmobranch embryo, in which, owing to the cranial flexure, the fore part of the head is cut longitudinally and horizontally, and the section passes through both the fore-brain (fb) and the hind-brain. 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 (al) which a little way back opens to the exterior by the first visceral cleft (1v.c.). This figure alone suffices to demonstrate the correctness of the above account of the pituitary body; but its truth is still further confirmed byfig. 252; in which the mouth involution (pt) is in contact with, but still separated from, the front end of the alimentary tract. Very shortly after the septum between the mouth and throat becomes pierced, and the two are placed in communication, the pituitary involution becomes very partially constricted off from the mouth involution, though still in direct communication with it. In later stages the pituitary involution becomes longerand is 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.
Before the connection between the pituitary vesicle and the mouth is obliterated the cartilaginous cranium becomes developed, and it may then be seen that the infundibulum projects through the pituitary space to come into close juxtaposition with the pituitary body.
After the pituitary vesicle has lost its connection with the mouth it lies just in front of the infundibulum (figs.250and255hphandfig. 254pt); and soon becomes surrounded by vascular mesoblast, which grows in and divides it into a number of branching tubes. In many forms the cavity of the vesicle completely disappears, and the branches become for the most part solid [Cyclostomata and some Mammalia (the rabbit), Elasmobranchii, Teleostei and Amphibia]. In Reptilia, Aves and most Mammalia the lumen of the organ is more or less retained (W. Müller,No.344).