Talk:Book - A Laboratory Manual and Text-book of Embryology 12

From Embryology


The Morphogenesis of the Central Nervous System

In discussing the histogenesis of the nervous tissue the early development of the neural tube has been described as an infolding of the neural plate (Fig. 78) and a closure of the neural groove (Fig. 304). The groove begins to close along the mid-dorsal line, near the middle of the body, in embryos of 2 mm. and the closure extends both cranially and caudally (Fig. 324). Until the end of the third week there still persists an opening at either end of the neural tube, somewhat dorsad. These openings are the neuropores (Fig. 330). Before the closure of the neuropores, in embryos of 2 to 2.5 mm. the cranial end of the neural tube has enlarged and is constricted at two points to form the three primary brain vesicies, . The caudal two-thirds of the neural tube, which remains smaller in diameter, constitutes the attUigc of llie spitmi cord.

Fig. 324.— Human embryo o( 2.4 a I. ibowinR a partially closed neunl tube and the brain vewcles (after KolUnanD). X 36.

The Spinal Cord

The spinal iKirlion nl the neural tube is at first nearly straight, but is bent with the flexure of the embryo into a curve which is convex dorsally. Its wall gradually thickens during the first month and the diameter of its cavity is di

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ventral portion of the layer, which is thus narrower than the dorsal portion in 10 to 20 mm. embryos (Figs. 325 and 326). Consequently, the ventral portion of the mantle layer is differentiated first. The neural cavity is at first somewhat rhomboidal in transverse section, wider dorsally than ventrally. Its lateral angle forms the sulcus Umilans (Fig. 334) which marks the subdivision of the lateral walls of the neural tube into the dorsal alar plate (sensory) and ventral hasid plate (motor). When the ependjmal layer ceases to contribute new cells to the mantle layer its walls arc approximated dorsally. As a result, in 20 mm. embryos the neural cavity is wider ventrally (Fig. 326). In the next stage, 34 mm., these walls fuse and the dorsal portion ui the neural cavity is obliterated (Fig. 327). In a 65 mm. (C R) fetus the persisting cavity is becoming rounded (Fig. 328). It forms the central canal of the adult spinal cord. The cells lining the central canal are ependymal cells proper. Those in the floor of the canal form the persistent tfoor plate. Their fibers extend ventrad, reaching the surface of the cord in the depression of the ventral median fissure.

Fig. 326. — Transverse section of the spinal cord from a 20 mm human embryo. X 4

Fig. 327.— TimuvciK section of the spiiul cord from a 34 nun. human embryo, showing also the spnal ganglbn and dun mater on tbc left side. X 44.

Fig. 328.- — Transverse section of the spinal cord from a 65 mm. human fetus. X M.

When the right and left walls of the ependymal layer fuse, the ependymal cells of the roof plate no longer radiate, but form a median septum (Fig, 327).

Later, as the marginal layers of either ^de thicken and are approximated, the median septum is extended dorsally. Thus the roof plate is converted into part of the dorsal median sepliim of the adult spinal cord (Fig. 328).

The Mantle Layer, as wc have seen, is contributed to by the proliferating cells of the cpendyiiial layer. A ventro-Iateral thickening first becomes prominent in embryos of 10 to 15 mm. (Fig. 325). This is the ventral (anterior) gray column, or horn, which in later stages is subdivided, forming also a lateral gray column (Fig, 328). It is a derivative of the basal plate. In embryos of 20 mm. a dorsQ-laleral thickening of the mantle layer is seen, the cells of which constitute the dorsal (posterior) gray column, or horn (Figs, 327 and 328); about these cells the Collaterals of the dorsal root fibers end. The cells of the dorsal gray column thus form terminal nuclei for the afferent spinal nerve fibers and they are derivatives of the alar plate of the cord. Dorsal and ventral to the central canal the mantle layer forms the dorsal and ventral gray commissures. In the ventral floor plate nerve fibers cross from both sides of the cord and form the ventral {anterior) white commissure.

The Marginal Layer is composed primarily of a framework of neuroglia and ependymal cell processes. Into this framework grow the axis cylinder processes of nerve cells, so that the thickening of this layer is due to the increasing number of nerve fibers contributed to it by extrinsic ganglion cells and neuroblasts. When their myelin develops, these fibers form the white substance of the spinal cord. The fibers have three sources (Fig. 360): (1) they may arise from the spinal ganglion cells, entering as dorsal root fibers and coursing cranially and caudally in the marginal layer; (2) they may arise from neuroblasts in the mantle layer of the spinal cord (a) as fibers which connect adjacent nuclei of the cord (fasciculi proprii or ground bundles), (b) as fibers which extend cranially to the brain; (3) they may arise from neuroblasts of the brain (a) as descending tracts from the brain stem, (6) as long descending cerebrospinal tracts from the cortex of the cerebrum.

Of these fiber tracts (1) and (2 a) appear during the first month; (2 b) and (3 a) during the third month; (3 b) at the end of the fifth month.

The dorsal root fibers from the spinal ganglion cells, entering the cord dorsolaterally , subdivide the white substance of the marginal layer into a dorsal funiculus and lateral funiculus. The lateral funiculus is marked off by the ventral root fibers from the ventral funiculus (Fig. 327). The ventral root fibers, as we have seen, take their origin from the neuroblasts of the ventral gray column in the mantle layer. They are thus derivatives of the basal plate.

The dorsal funiculus is formed chiefly by the dorsal root fibers of the ganglion cells and is subdivided into two distinct bundles, the fasciculus gracilis, median, and \ht fasciculus cuneatus, lateral in position. The dorsal funiculi are separated only by the dorsal median septum (Fig. 328).

The lateral and ventral funiculi are composed of fasciculi proprii or ground bundles, originating in the spinal cord, of ascending tracts from the cord to the brain, and of the descending fiber tracts from the brain. The fibers of these fasciculi intermingle and the fasciculi are thus without sharp boundaries. The floor plate of ependymal cells lags behind in its development, and, as it is interposed between the thickening right and left walls of the ventral funiculi, these do not meet and the ventral median fissure is produced (cf. Figs. 325 and 328).

The development of myelin in the nerve fibers of the cord begins late in the fourth month of fetal life and is completed between the fiftecnih and tweriieth years (Flechsig, Bechlerew). Myelin appears first in the root fibers of ihc spinal nerves and in those of the ventral commissure, nest in the ground bundles and dor&al funiculi. The cerebrospinal (pyramidal) fasciculi are the last in which myelin is developed: ihey ate myelinated during the first and second years. As myelin appears in the varitms fiber tracts at different periods, this condition has been utilized in tracing the extent and origin of the various fasciculi in 1 the central nervous system.

The Cervical and Lumbar Enlargement at the level of the two nerve | plexuses, suppljing the upper and lower t emitics, the spinal cord enlarges. As the fibers to the muscles of the extremities arise j from nerve cells in the ventral gray column, the number of these cells and the mass of the gray substance ) is increased; since larger numbers of fibers from the ' integument of the limbs also enter the cord at this level, there are likewise present more cells about which sensory fibers terminate. There is formed consequcntjy at the level of the origin of the nerves of the brachial plexus the cervkai etHargement, opposite the origins of the nerves of the lumbo-sacral [^exus the lumbar cnlargemenl (Fig. 329).

At the caudal end of the neural tube in a 110 mm. (C R) fetus an epithelial sac is formed which is adherent to the integument. Cranial to the sac the central canal is obliterated, this part of the neural tube forming the filum lermitmle. The caudal end of the central canal is irregularly expanded and is known as the terminal ventricle. After the third month the vertebral column grows faster than the spinal cord. As the cord is fixed to the brain the vertebra> and the associated roots and ganglia of the spinal nerves shift caudally along the cord. In the ?riiiit t^"" nrlmn nf thn i coccygeal nerves i-^ opposite the first lumbar vertebrr *1

obliquely downward nearly parallel to the spinal a it tube is attached to the coccyx, its caudal portio \ str slender, solid cord known as the filum terminale. uely nerves, with the filum terminale, constitute the ca:

Dig. 329.— Dissection of the brain and cord of a three months' fetus, showing the cervical and lumbar cnlar^ments (after Kultiker in Marshall). Natural size.

The Brain

We have seen that in embryos of 2 to 2.5 mm. the neural tube is nearly straight, but that its cranial end is enlarged to form the aniage of the brain (Fig. 324). The appearance of two constrictions in the wall of the aniage subdivides it into the three primary brain vesicles — the fore-brain or prosencephalon, mid-braiD or mesencephalon, and hind-brain or rhombencephalon.

In embryos of 3.2 mm., estimated age four weeks, three important changes have taken place (Fig. 330) : (t) the neural tube is bent sharply in the mid-brain region (the cephalic flexure) so thai the axis of the fore-brain now forms a right angle with the axis of the hind-brain; (2) the fore-brain shows indication dorsally of a fold, the margo tkalamicus, which subdivides it into the telencephalon and the diencephalon ; (3) the lateral walls of the fore-brain show distinct evaginations, the optic vesicles, which project laterad and caudad. A ventral bulging of the wall of the hind-brain indicates the position of the future porUine flexure.

m. human embryo (after His). Xabout35. /I, Lateral ] the median sagiltul plane.

In embryos of 7 mm. (five weeks) the neuropores have closed (Fig. 331). The cephalic flexure, now more marked, forms an acute angle, and the pontine flexure, just indicated in the previous stage, is now a prominent ventral bend in the ventro-Iateral walls of the hind-brain. This flexure forms the boundary line which subdivides the rhombencephalon into a cranial portion, the metencephalon, and into a caudal portion, the myeicncEphulon. At a third bend the whole brain is flexed ventralty at an angle with the axis of the spinal cord. This bend is the cervical flexure and is the line of demarcation between the brain and spinal cord (cf. Fig. 333 A). The telencephalon and diencephalon are more distinctly subdivided, and the evaginated optic vesicle forms the optic cup attached to the brain wall by a hollow stalk, in which later grows the optic nerve. The walls of the brain show a distinct differentiation in certain regions. This is especially marked in the myelencephalon, which has a thicker ventro-Iateral wall and thinner dorsal wall.

Embryos of 10.2 mm. show the structure oi uie brain at the beginning of the second month (Figs. 34] and 344). The five brain regions are now sharply differentiated externally, but the boundary line between the telencephalon and diencephalon is still indistinct. The telencephalon consists of paired, lateral outgrowths, the aniages of the cerebral hemispheres and rhinencephalon {olfactory brain). In Fig. 359 the external form of the brain is seen with the origins of the cerebral nerves. It will be noted that, with the exception of the first four (the olfactory, optic, oculomotor, and trochlear), the ce " " ficial origin from the myelencephalon.

Reconstructions of the braia of a 7 m sagittal Eection. nembn-o (Hia). -4, Lateral view; 5, in median

The cephalic flexure forms a very acute angle, resu of the fore-brain is nearly parallel to that of the hii lomotor nerve takes its origin from the ventral wal sally there is a constriction, the isthmus, between cephalon, and here the fibers of the trochlear nerve take their superficial origin. The dorsal wall of the myelencephalon is an exceedingly thin ependymal layer which becomes the tela chorioidea. The ventro-lateral walls of this same region, on the other hand, are very thick.

A median sagittal section of a brain at a somewhat later stage shows the cervical, pontine, and cephahc flexures well marked {Fig. 332). The thin dorsolateral roof of the myelencephalon has been removed. The telencephalon is a paired structure. In the figure its right half projects cranial to the primitive median wall of the fore-brain which persists as the lamina terminalis (cf. Fig. 342). The floor of the telencephalon is greatly thickened caudally as the anlage of the corpus striatum. A slight evagination of the ventral wall of the telencephalon just cranial to the corpus striatum marks the anlage of the rhinencephalon. The remaining portion of the telencephalon forms the pallium or cortex of the cerebral hemispheres. The paired cavities of the telencephalon are the lateral (first and second) ventricles, and these communicate through the interventricular foramina (Monroi) with the cavity of the diencephalon, the third ventricle. The cavities of the otfactory lobes communicate during fetal life with the lateral ventricles and were formerly called the first ventricles.

Fig. 332. — Brain of » 13.6 mm. human embryo in median sagittal section (after His in Sobotta). 1, C^tic recess; 2, ridge foimcd by optic chiasma, 3; 4, infundibular tecess.

The crossing of a portion of the optic nerve fibers in the floor of the brain forms the optic chiasma, and this, with the transverse ridge produced by it internally, is taken as the ventral boundary line between the telencephalon and diencephalon (Fig. 332). A dorsal depression separates the latter from the mesencephalon. The lateral wall of the diencephalon is thickened to form the thalamus, the caudal and lateral portion of which constitutes the nietaihalamus. From the metathalamus are derived the geniculate bodies. In the median dorsal wall, near the caudal boundary line of the diencephalon, an outpocketing begins to appear in embryos of five weeks ( Fig. 332). This is the epithalamus, which later gives rise to the pineal body or epiphysis.

The thalamus is marked off from the more ventral portion of the diencephalic wall, termed the hypothalamus, by the obliquely directed sulcus hypothalamicus (Fig. 341). Cranial to the optic chiasma is the optic recess, regarded as belonging to the telencephalon (Fig. 332). Caudal to it is the pouch-like infundibulumy an extension from which during the fourth week forms the posterior lobe of the hypophysis. Caudal to the infundibulum the floor of the diencephalon forms the tuber cincrcum and the mammillary recess; the walls of the latter thicken later and give rise to the mammillary bodies. An oblique transverse section through the telencephalon and hypothalamic portion of the diencephalon (Fig. 343) shows the relation of the optic recess to the optic stalk, the infundibulum, and Rathke's pocket, and the extension of the third ventricle, the proper cavity of the diencephalon, into the telencephalon between the corpora striata.

The mesencephalon in 13.6 mm. embryos (Fig. 332) is distinctly marked off from the metencephalon by the constriction which is termed the isthmus. Dorsolateral thickenings form the corpora quadrigemina. Ventrally, the mesencephalic wall is thickened to form the tegmentum and crura cerebri. In the tegmentum are located the nuclei of origin for the oculomotor and trochlear nerves. The former, as we have seen, takes its superficial origin ventrally, while the trochlear nerve libers bend dorsad, cross at the isthmus, and emerge on the opposite side. As the walls of the mesencephalon thicken, its cavity later is narrowed to a canal, the cerebral aqueduct (of Syhaus).

The walls of the metencephalon arc thickened dorsally and laterally to form the anlage of the cerebellum. Its thickened ventral wall becomes the pons (X'arolii). Its caxity constitutes the cranial portion of the fourth ventricle.

The caudal border of the pons is taken as the ventral boundary line between the metencephalon and myelcficephalon. The myelencephalon forms the medulla oblongata. Its dorsal wall is a thin, non-nervous ependymal layer, which later becomes the posterior medullary velum. From its thickened vcntro-lateral walls the last eight cerebral nerves take their origin. Its cavity forms the greater part of the fourth ventricle which opens caudally into the central canal of the spinal cord, cranially into the cerebral aqueduct. The increase in the flexures of the brajn and the relative growth of its different regions may be seen by comparing the brains of embryos of four, five, and seven weeks (Fig. 333).

Fig. 333. — Brains of human embryos, from reconstructions bj- His: A, 4.2 mm. embryo (X 2( B, 6.9 nun. embiyo (X 16); C, 18.5 mm. embiyo fX 4). 0. Optic vesicle; in. infundibulunii m, ma miliar)' body; pf. pontine flexure; ol, (factory lobe; b, basilar arteo': ^1 Rathke's pouch (Americ Text-Book of Obstetrics).

In the table on page 332 are given the primitive subdivisions of the neural tube and the parts derived from them:



Cerebral cortex Corpora striata

Cnnial portioii of third ventricles



(Pineal body) Thalamus Optic tract

Tuber cinercum Mammillarj- bodies

Third ventricle



Conwra quadrigemina Tegmentum

Aqiueductus cerebri



Cerebellum Pons

Fourth ventricle

The Later Differentiation of the subdivisions of the brain Myelencephalon.— We have seen that the wall of the spinal cord differentiates dorsally and ventrally into roof plate and floor plate, laterally into the alar plate and basal plate. The boundary line between the alar and basal plates is the sulcus limitans (Fig. 334 ^4). The same subdivisions may be recognized in the myelencephalon. It differs from the spinal cord, however, in that the roof plate is broad, thin, and flattened to fonn the ependymal layer (Figs. 334 B and 335). In the alar and basal plates of the myelencephalon the marginal, mantle, and ependymal zones are differentiated as in the spinal cord {Fig. 335). Owing to the fonnation of the pontine flexure at the beginning of the secoad month, the roof plate is broadened, especially in the cranial portion of the myelencephalon, and the alar plates bulge laterally (Figs. 336 and 337 A). The cavity of the myelencephalon is thus widened from side to side and flattened dorso-ventrally. This is most marked cranially where, between the alar plates of the myelencephalon and metencephalon, are formed the laleral recesses of the fourth ventricle (Figs. 337 and 353). Into the ependymal roof of the myelencephalon blood vessels grow, and. invading the lateral recesses, form there the cfutrioid plexus of the fourth \entricle. The plexus consists of small, flngcr-like folds of the cpcndvinal layer and its covering mesenchymal layer. The line of attachment of the ependymal layer to the alar plate is known as the rhombic lip and later becomes the lania and obex of the fourth ventricle B).

Fig. 334.— Transverse sections. X 44. ,-1, ThrouRh the upper cervical region of the spinal cord in a 10 mm. human embryo; B, through the caudal end of the myelencephalon.

Fig. 335.— Transverse sections through the myelencephalon of a 10.2 mm. embryo (His). X 37. A, Through the nuclei of origin of the spinal accessory and hypoglossal nerves; B, through the vagus and hypoglossal nerves.

Fig. 336.— TransvetBC section through the myelencephali robUuhJram <ilar plate {Rudimtnt of accessory olive) ■n of a 22 mm. embryo (His). X 10

Fig. 337.— TJorsal v'tevs of four sURes in the develt^ment of the cerebellmn. A. a( a- 13.6 m bryo (His); 0, of a 24 mm. embryo; C, of a 110 mm. fetus; D, of a. 150 mm. fclus.

In early stages the floor of the myelencephalon is constricted transversely h the so-called rhombic gromes. six in number; the intervals between successive grooves are ticuromeres (cf. Fiijs. 96 and 122). Some have viewed these as evi dential of a former segmentation of the head sii " :li

more probable, however, that they merely stan ion nerves and hence their segmental arrangement eoindary.

The further growth of the myelencephalon i '1) to of neuroblasts, derived from the ependynm] and la opment of nerve fibers from these neuroblasts. ;o the growth into it of fibers from neuroblasts in the spinal cord and in other parts of the brain.

The neuroblasts of the basal plates early give rise chiefly to the efferetU fibers of the cerebral nerves (Fig. 335). They thus constitute motor nuclei of origin of the trigeminal, abducens, facial, glossopharyngeal, vagus complex, and hypoglossal nerves, nuclei corresponding to the ventral and lateral gray columns of the spinal cord. The basal plate likewise produces part of the reticular formation which is derived in part also from the neuroblasts of the alar plate (Fig. 336). The axons partly cross as external and internal arcuate fibers and form a portion of the median longitudinal bundle, a fasciculus corresponding to the ventral ground bundles of the spinal cord. Other axons grow into the marginal zone of the same side and form intersegmental fiber tracts. The reticular formation is thus differentiated into a gray portion, situated in the mantle zone, and into a white portion, located in the marginal zone (Fig. 336). The marginal zone is further added to by the ascending fiber tracts from the spinal cord and the descending p>Tamidal tracts from the brain. As in the cord, the marginal layers of each side remain distinct, being separated by the cells of the floor plate.

The alar plates differentiate later than the basal plates. The afferent fibers of the cerebral nerves first enter the mantle layer of the alar plates, and, coursing upward and downward, form definite tracts (tractus solitarius, descending tract of fifth nerve). To these are added tracts from the spinal cord so that an inner gray and an outer white substance is formed. Soon, however, the cells of the mantle layer proliferate, migrate into the marginal zone, and surround the tracts. These neuroblasts of the alar plate form groups of cells along the terminal tracts of the afferent cerebral nerves (which correspond to the dorsal root fibers of the spinal nerves) and constitute the receptive or terminal nuclei of the fifth, seventh, eighth, ninth, and tenth cerebral nerves. Caudally, the nucleus gracilis and nucleus cuneatus are developed from the alar plates as the terminal nuclei for the afferent fibers which ascend from the dorsal funiculi of the spinal cord. The axons of the neuroblasts forming these receptive nuclei decussate through the reticular formation chiefly as iniemal arcuate fibers and ascend to the thalamus as the median lemniscus.

There are developed from neuroblasts of the alar plate other nuclei, the axons of which connect the brain stem, cerebellum, and fore-brain. Of these the most conspicuous is the inferior olivary nucleus.

The characteristic form of the adult myelencephalon is determined by the further growth of the above-mentioned structures. The nuclei of origin of the cerebral nerves, oerived from the basal plate, produce swellings in the floor of the fourth ventricle which are bounded laterally by the sulcus limitans. The terminal nuclei of the amed and sensory cerebral nerves lie lateral to this sulcus. The enlarged cuneate and gracile nuclei bound the ventricle caudally and laterally as the cutieus and clava. The inferior olivary nuclei produce lateral rounded prominences and ventral to these are the large cerebrospinal tracts or pyramids. The Metencepfaaloa-^Cranial to the lateral recesses of the fourth ventricle the cells of the alar plate proliferate venlrally and form the numerous relatively large nuclei av»o i the cells of these nuclei mostly cross to the opposit e and foi [ ponlis of the cerebellum. Cerebral fibers from the cerebral pec ut the cells of the pontine nuclei. Others pass through the pons as fas so pyramidal tracts.

Fig. 338.— Median sagittal s r medullary vdum tion of tht cerebellum and part of embrj'o; B, from a ISO mm tetus


When the alar plates of the cranial end of the myelencephalon I are bent out laterally the caudal portions of their continuations into the metenrJ cephalic region are carried laterally also. As a result, the alar plate of the meteo*! cephalon takes up a transverse position and forms the anlages of the cerebellum (Fig. 337 .-(). During the second month the paired cerebellar plates thicken a

bulge intti the ventricle (Fig. 338 A). \ear the tt cate the anlages of the vermis, while the remai anlages of the lateral lobes or cerebellar hemispke The cerebellar anlages grow rapidly both lal surfaces are folded transversely. During the tl ward and form on either ^de a convex lateral lobe brachium pontis (Fig. 337 C). In the meantime

id-line paired thickenings indiiJ

fused in the mid-line, produdng a single structure marked by transverse fissures. The rhombic lip gives rise to the flocculus and nodulus. Between the third and fifth months the cortex cerebelli grows more rapidly than the deeper layers of the cerebellum and its principal lobes, folds and fissures are formed (Fig. 337 C, D), The hemispheres derived from the lateral lobes are the last to be differentiated. Their fissures do not appear until the fifth month.

Cranial to the cerebellum the wall of the neural tube remains thin dorsally and constitutes the anterior medullary velum of the adult (Fig. 338 B), Caudally, the ependymal roof of the fourth ventricle becomes the posterior medullary velumThe points of attachment of the vela remain approximately fixed, while the cerebellar cortex grows enormously. As a result, the vela are folded in under the expanding cerebellum (Fig. 338).

The aniages of the cerebellum show at first diflferentiation into the same three layers which are typical for the neural tube. During the second and third months, cells from the ependymal, and perhaps from the mantle layer of the rhombic lip migrate to the surface of the cerebellar cortex and give rise to the molecular and granular layers which are characteristic of the adult cerebellar cortex (SchSfer). The later differentiation of the cortex is only completed at, or after, birth. The cells of the granular layer become unipolar by a process of unilateral growth. The Purkinje cells differentiate later. Their axons and those of entering afferent fibers form the deep medullary layer of the cerebellum.

The cells of the mantle layer may take little part in the development of the cerebellar cortex, but give rise to neuroglia ceUs and fibers and to the internal nuclei. Of these the dentate nucleus may be seen at the end of the third month; later, its cellular layer becomes folded, produdng its characteristic convolutions. The fibers arising from its cells form the greater part of the hrachium conjunctivum. (For a detailed account of the development of the cerebellum see Streeter, in Keibel and Mall, vol. 2).


The basal and alar plates can be recognized in this subdivision of the brain and each differentiates into the three primitive layers (Fig. 339). In the basal plate the neuroblasts give rise to the axons of motor nerves — the oculomotor cranial in position, the trochlear caudal (Fig. 339 B), In addition to these nuclei of origin, the nucleus ruber (red nucleus) is developed in the basal plates ventral and somewhat cranial to the nucleus of the oculomotor nerve. The origin of the cells forming the red nucleus is not definitely known. The alar plates form the paired superior and inferior coUiculi which together constitute the corpora quadrigemina (Figs. 337 B and 349). The plates thicken and neuroblasts migrate to their surfaces, forming stratified ganglionic layers comparable to the cortical layers of the cerebellum and the cerebellar nuclei. With the development of the superior and inferior colliculi the cavity of the mesencephalic region decreases in size and becomes the cerebral aqueduct.

The mantle layer of the basal plate region is enclosed ventraUy and laterally by the fiber tracts which develop in the marginal zone. Ventro-lateraUy api>ear the median and lateral lemnisci and ventrally develop later the descending tracts from the cerebral cortex, which together constitute the peduncles of the cerebrum.

Fig. 339. — Transverse sections through the mesencephalon of a 10.2 mm. embryo (His). A, Through the isthmus and origin of the trochlear nerve; 5, through the nucleus of origin of the oculomotor nerve; D. IV, decussation of oculomotor nerve; 3/./., mantle layer.

The Diencephalon

In the wall of the diencephalon we may recognize laterally the alar and basal plates, dorsally the roof plate, and ventrally the floor plate (Fig. 340). The roof plate expands, folds as seen in the figure, and into the folds extend blood capillaries. The roof plate thus forms the ependymal lining of the tela chorioidea of the third ventricle. The vessels and ingrowing mesenchymal tissue form the chorioid plexus. Cranially, the tela chorioidea roofs over the median portion of the telencephalon and is folded laterally into the hemispheres as the chorioid plexus of the lateral ventricles. Laterally, the roof plate is attached to the alar plates and at their point of union are developed the ganglia habenula.

Fig. 340. — ^Transverse section through the diencephalon of a 13.8 nmi embryo (His). X 29.

The pineal body, or epiphysis, is developed caiidally as an evagination of the roof plate. It appears at the fifth week (Fig. 335) and is well developed by the third month (Fig. 342). Into the thickened wall of the anlage is incorporated a certain amount of mesenchymal tissue and thus the pineal body proper is formed.

The alar plate is greatly thickened and becomes the anlage of the thalamus and metathalamus. The latter, really a part of the thalamus, gives rise to the lateral and median geniculate bodies.

The sukus hypothalamktts (Fig. 341) forms the boundary line between the thalamus (alar plale) and the hypothalamus (basal plate plus the floor plate).

Fig. 341. — Median sagittal section of the fore- and mid-biain regions of a brain from a 10.2 mm. embryo (after HU).

This sulcus thus corresponds to the sulcus limitans of the spinal cord and brain stem. The basal plate is comparatively unimportant in the diencephalic region, as no nuclei of origin for motor nerves are de\'eloped here. In the floor plate the ridge formed by the optic chiasma constitutes the pars optica hypothalamica. The Hypophysis. — The infundibulum develops as a recess caudal to the pars optica hypothalamica (Figs. 342 and 343). .^t its extremity is the sac-like anlage of the posterior lobe of the hypophysis or pituitary My. During the fourth week the infundibular anlage comes into contact with Ralhke's pouch, the epithelial anlage of the anterior lobe of the hypophysis (Fig. 343). The epithelial anlage is at first flattened and soon is detached from its epithelial stalk. Later, it grows laterally and caudally about the anlage of the posterior lobe, and, during the second month, its wall is differentiated into convoluted tubules which obliterate its cavity. The tubules become closed glandular follicles surrounded by a rich network of blood vessels and produce an important internal secretion. Coincident with the differentiation of the anterior lobe the infundibular antage of the posterior lobe loses its cavity, but the walls of the infundibulum persist as its solid, permanent stalk. The lobe enlarges and its cells are differentiated into a diffuse tissue resembling neuroglia. About the two lobes of the hypophysis the surrounding mesenchyme develops a connective tissue capsule.

e section through ihc dienccphninn and leleacephalon of a 10 mm. embryo. X 61.

Caudal to the infundibulum in the floor plate are developed in order the tuber cinereum and the mammiUary recess (Figs. 341, 344 and 346). The lateral walls of the latter thicken and give rise to the paired mammiUary bodies.

The third ventricle lies largely in the diencephalon and is at first relatively broad. Owing to the thickening of its lateral walls it is compressed tmtil it forms a narrow, vertical cleft. In a majority of adults the thalami are approximated, fuse, and form the massa intemiedia or commissura moUis, which is encircled by the cavity of the ventricle.

Fio. 344.— Lateral view of the tore- and nud-brains of a 10.2 mm. embryo (Ka).

The Telencephalon

This is the most highly differentiated division of the brain (Fig. 344). The primitive structures of the neural tube can no longer be recognized, but the telencephalon is regarded as representing greatly expanded alar plates and is, therefore, essentially a paired structure. Each of the paired outgrowths expands cranially, dorsally, and caudally, and eventually overlies the rest of the brain (Figs. 344, 345 and 346). The telencephalon is differentiated into the corpus striatum, rhinencepkaUm, and pallium (primitive cortex of cerebral hemisphere). The median lamina between the hemispheres lags behind in its] development and thus there is formed the great longitudinal fissure between the hemispheres. The lamina is continuous caudally with the roof plate of the diencephalon; cranially it becomes the lamina tcrminaiis, the cranial boundary of the third ventricle (Figs. 332 and 342).

Chmioid Plexus of Ike lateral Ventricles. — It will be remembered that the chorioid ple.xus of the third ventricle develops in the folds of the roof plate of the diencephaton . Similarly the thin, median wall of the pallium at its junction with the wall of the Jon is . ic lateral ventricle, A vascular plexus, continuous with that of the third ventricle, grows into this fold, and projects into the lateral ventricle of either side (Figs. 345 and 347). The fold of the pallia! wall forms the chorioidul fissure and the vascular plexus is the cftortaid plexus of the lateral ventricle. This is a paired structure and h t... of the third ventricle forms a T-shaped figure, the stem of the iverly third ventricle, its curved arms projecting into the lateral vent: ; just a to the interventricular foramen. Later, as the pallium exten the cho plexus of the lateral ventricles and the chorioidal fissures are extei 'ly elongatec into the temporal lobe and inferior horn of the lateral vent] 348).

Fig. 345. — The fore-brain and mid-brain of an embiyo 13,0 nun. long seen Iram Ihe dDCSal siu!feefl The pallium of tbe tetencephaloD is cut away, exposing tile lateral ventridi: (His).

The interventricular foramen (of Monro) is at first a wide opening (Fig. 343), but is later narrowed to a slit, not by constriction but because its boundaries grow more slowly than the rest of the telencephalon (Fig, 347).

Pars aai. olf. hbe past. olf. lobe

Infuttdibulum Oflic stalk Fig. 346. — Lateral view of ihe fore-brain and mid-brain of a 13

Fio. 347. — Transverse section through the fore-brain of a 16 mm. embiyo showing the early development of the cborioid plexus and fissure (His).

The third ventricle extends some distance into the caudal end of the telencephalon and laterally in this region the optic vesicles develop. Into each optic stalk extends the optic recess (Fig. 343).

Fic. 348.— A transverse section through ihc lelenc phalon nf an !v^ mm fttus (after His). Tk^ | Thalamus; Cs, corpus stnatum A/ h[i)pocaiiipaJ tssure /j marginal graj seam; fi, edge of nrhHe substance.

Fig. 349, — Lateral viev of the brain of a S3 mm. fetus. The greater part of t!ie pallium of the right cerebral hemispbere has been removed, leaving only that covering the lenticular nucleus, and exposing the interna! capsule, caudate nucleus and hippocampus (Hia).

The corpus striatum is developed as a thickening in the floor of each cerebral hemisphere. It is already prominent in embryos of six weeks (13.6 mm.) bulging into the lateral ventricle (Figs. 345 and 347). It is in line caudally with the thalamus of the diencephalon and in development is closely connected with it, although the thalamus always forms a separate structure. The corpus striatum elongates as the cerebral hemisphere lengthens, its caudal portion curving around to the Up of the inferior horn of the lateral ventricle and forming the slender tail of the caudate nucleus (Fig. 349). The thickening of the corpus striatum is due to the active proliferation of cells in the ependymal layer which form a prominent mass of mantle layer cells. Nerve fibers to and from the thalamus to the cerebral cortex course through the corpus striatum as lamins which are arranged in the form of a wide V, open laterally, when seen in horizontal sections. This Vshaped tract of white fibers is the internal capsule, the cranial limb of which partly separates the corpus striatum into the caudate and lenticular nuclei (Rg. 350). The caudal limb of the capsule extends between the lenticular nucleus and the thalamus.

Fig. 35o.— (coronal) section through the fore-brain of a 160 nun. fetus (His).

The thalamus and corpus striatum are separated by a deep groove until the end of the third month (Fig. 347). As the structures enlarge, the groove between them disappears and they form one continuous mass (Fig. 350). According to some investigators, there is direct fusion between the two.

The Rhinencephalon or Olfactory Apparatus

This is divided into a basal portion and a pallial portion. The basal portion consists: (1) in a ventral and cranial evagination (pars anterior), formed mesial to the corpus striatum, which is the anlage of the olfactory lobe and stalk (Fig. 346). This receives the olfactory ri!)ers and its cells give rise to olfactory tracts. The tubular stalk connecting the olfactory lobe with the cerebrum loses its lumen. (2) Caudal to the anlage of the olfactory lobe a thickening of the brain wall develops (pars posterior) which extends mesially along the lamina terminalis and laterally becomes continuous with the tij) of the temporal lobe (Fig. 346). This thickening constitutes the anterior perforated space and the parolfactory area of the adult brain (Fig. 356).

The pallial portion of the rhinencephalon is termed the arckipallium because it forms the entire primitive wall of the cerebrum, a condition which is permanent in fishes and amphibia. Later, when the neopallium, or adult cortex, arises, the archipallium forms a median strip of the pallial wall curving along the dorsal edge of the chorioidal fissure from the anterior perforated space around to the tip of the temporal lobe, where it is again connected with the basal portion of the rhinencephalon. The archipallium differentiates into the hippocampus (Figs. 345 and 349), a portion of the gyrus hippocampi, and into the gyrus dentntus. It resembles the rest of the cerebral cortex in the arrangement of its cells. The infolding of the hi])pocami)us produces the hippocam pal fissure.

The Commissures of the Telencephalon

The important commissures are the corpus callosum, fornix, and anterior conunissure. The first is the great transverse commissure of the neopallium, or cerebral cortex, while the fornix and anterior commissure, smaller in size, are connected with the archipallium of the rhinencephalon. The commissures develop in relation to the lamina terminalis, crossing i)artly in its wall and partly in fused adjacent portions of the median pallial walls. Owing to the fusion of the pallial walls dorsal and cranial to it, the lamina terminalis thickens rapidly in stages between 80 and ISO mm. (C R) (Strei'ter). "It [the lamina terminalis] is distended dorsalward and anterolateralward through the growth of the corpus callosum, the shape of which is determined by the expanding pallium.*' Between the curve of the corpus callosum and the fornix the median pallial walls remain thin and membranous, and constitute the septum pcUucidum of the adult. The walls of this septum enclose a cavity, the so-caUed fijlh ventricle, or space of the septum peUucidum (Fig. 351).

The fornix takes its origin early, chiefly from cells in the hippocampus. The fibers course along the chorioidal ade of the hippocampus cranially, passing dorsal to the foramen of Monro (Fig. 351 A). In the cranial portion of the lamina terminalis fibers are given off to, and received from, the basal portion of the rhinencephalon. In this region, fibers crossing the midline form the kippocampal commissure. Other fibers, as the diverging anlerior pillars of the fornix, curve ventrally and end in the mammiUary body of the hypothalamus. The conunissure of the hippocampus, originally cranial in position, is carried caudalward with the caudal extension of the corpus callosum (Fig. 351 B).

Fig. 351.— Two stages id the development of the cerebral commissure. (Based on leconstructkms by HisandStreeter.) .4,Mediaiiviewoftbe right hemisphere of an 83 nun. fetus; fi, the same of a 120 mm. feius.

Fig. 355. — Lateral view of the right cerebral hemisphere from a seven months' fetus [1

frontal lobe. These are not approximated over the insula until after birth, frontal operculum is included between the anterior limbs of the Sylvian tissui the extent of its development, which is variable, determines the form of these li

In fetuses of ax to seven months four other depressions appear which later form important landmarks in the cerebral topography. These are; (I) the central suicus, or fissure of Rolando, which forms the dorso-lateral boundary line between the frontal and parietal lobes (Fig. 355); (2) the parieto-occipital fissure, which, (»n the median wall of the cerebrum, is the line of separation between the ocdpitat and parietal lobes (Fig. 356) ; (3) the calcaritte fissure, which includes between it and the parieto-occipital fissure the cuneus and marks the position of the visual area of the cerebrum; (4) the collateral fissure on the ventral surface of the temporal lobe, which produces the inward bulging on the floor of the posterior horn of the ventricle known as the collateral eminence. The calcarine fissure also affects the internal wall of the ventricle, causing the convexity termed the calcar avis (hippocampus minor).

months' fetus (KoUmann).

Simultaneously with the development of the collateral fissure appear other shallower depressions known as sulci. These have a definite arrangement and with the fissures mark off from each other the various functional areas of the cerebrum. The surface convolutions between the depressions constitute the gyri and lobules of the adult cerebrum.

Histogenesis of the Cerebral Cortex

In the wall of the pallium are differentiated the three primitive zones typical of the neural tube: the ependymal mantle, and marginal layers. During the first two months the cortex remains thin and differentiation is slow. At eight weeks neuroblasts migrate from the ependymal and mantle zones into the marginal zone and give rise to layers of pyramidal and other cells typical of the cerebrmn. The differentiation of these layers is most active during the third and fourth months, but probably continues until after birth '»'-"---,, Amer. ' — *~-' vol. 14, 1912). From the fourth month on, the cerel wall < ing to the development of (1) the

fibers from the thi us and c (2) of endogenous fibers from the

neuroblasts of the x. The r I'hite, inner, medullary layer sur++++rounded by the gray cortex. Myelin n begins shortly before birth (Flechsig), but some fibers may not acquire their sheaths until after the twentieth year. As the cerebral wall increases in thickness the siz - of the lateral ventricle becomes relatively less, its lateral diameter especially being decreased. 1

volving the neural tube may give rise to various monstrous lia. In sfntia bifida, a sac-like protrusion of the cord, or its cleft in the vertebral column.

Anomalies.— Defects ir condil ions. c. g., cyelopta. acra membranes, extends through t