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life both apertures are closed and, for a time, the neural canal becomes a completely closed cavity.

As the margins of the neural groove rise and converge they carry with them the adjacent ectoderm to which they are attached, and which forms part of the surface covering of the embryo; consequently, when the lateral margins of the folds meet and unite, the tube, which is completed by their fusion, is embedded in the body of the embryo, but, for a time, its dorsal wall is attached to the surface ectoderm by a ridge of cells, formed by the fused lateral margins of the neural plate. This ridge is called the neural crest (Figs. 41-44).

The neural crest is the rudiment of the cerebral and spinal nerve ganglia, the sympathetic ganglia, the chromaffin cells of the chromaffin organs, and the cellular sheaths of the peripheral nerves; whilst the walls of the neural tube become transformed into the various constituent parts of the central nervous system, the brain and spinal medulla, the retina of the eye-balls, and the optic nerves.

The Formation of the Nerve Ganglia, the Chromaffin Tissues, and the Primitive Nerve Sheaths. The primitive ganglia grow as cell buds from the neural crest which, for a time, connects the dorsal wall of the neural tube with the surface ectoderm. In the body region they correspond in number with the spinal nerves and with the primitive segments into which the mesoderm becomes divided, but in the cephalic region their arrangement is more irregular, and some of the ganglia of the cerebral nerves receive additional cell elements from the surface ectoderm.

Simultaneously with the appearance of the cell buds which form the primitive ganglia, the neural crest disappears, and directly after the ganglia are formed they lose their connexion with both the neural tube and the surface ectoderm and become isolated cell clumps. At this period, therefore, the nervous system consists of the neural tube and the primitive ganglia.

After the primitive ganglia have lost their connexion with the neural tube they increase in size by the proliferation of their constituent cells, and they migrate ventrally along the sides of the neural tube, but the migration ceases before the ventral ends of the ganglia reach the level of the ventral wall of the tube. As the migration proceeds clumps of cells are budded off from the ventral ends of the ganglia. These secondary cell buds are the rudiments of the sympathetic ganglion cells and of the chromaffin tissue which is found in the sympathetic nerve plexuses, the medulla of the suprarenal glands, and in the carotid glands. In the first instance the secondary cell buds which form the sympathetic ganglia wander ventrally and medially, from the ventral ends of the primitive ganglia, until they attain the positions afterwards occupied by the ganglia of the sympathetic trunks on the ventro-lateral aspects of the vertebral column. From the primary sympathetic ganglia, buds of cells are given off; these buds wander still further ventrally to become the cells of the ganglia of the cardiac, coeliac, and other great ganglionic nerve plexuses, as well as to form the chromaffin cells of the chromaffin organs.

The exact manner in which the cells of the primitive sheaths of the nerves originate from the primitive ganglia is not known, but it has been shown by Harrison, in the case of the frog, that if the primitive ganglia are destroyed, the primitive sheaths of the nerves are not formed. Presumably, therefore, in the frog the cellular sheaths of the nerves are derived from cells produced by the primitive ganglia, and it may be assumed that they have a similar origin in the human subject.

After the rudiments of the sympathetic system, the chromaffin cells, and the cellular sheaths of the nerves have separated, the remains of the primitive ganglia become the permanent spinal and cerebral nerve ganglia.

In the early stages these ganglia are completely isolated structures which lie along the sides of the neural tube between the lateral walls of the tube medially, and the mesoderm somites laterally.

Some time after the ganglia of the cerebral and spinal nerves become isolated their cells give off processes which become nerve-fibres. These fibres grow out both from the dorsal and the ventral ends of the ganglia, and, together with the ganglia, they form, in the cranial region, certain of the cerebral nerves, and, in the spinal region, the posterior roots of the spinal nerves.

1 It is stated that some of the sympathetic nerve-cells are derived from the ventral parts of the lateral walls of the neural tube, but the evidence on this point is not entirely satisfactory.

The fibres which grow out of the dorsal ends of the ganglia enter the walls of the neural tube, and by their means the ganglia regain connexion with the tute.

The fibres which grow out from the ventral end of each spinal ganglion unite with the fibres of the corresponding anterior nerve-root, which, in the meantime, has grown out from the cells of the ventral part of the lateral wall of the spinal portion of the neural tube, and form with them a spinal nerve-trunk.

The Differentiation of the Neural Tube. Before the neural groove is converted into a closed tube, an expansion of its anterior part indicates the separation of the neural rudiment into cerebral and spinal sections, the dilated portion being the rudiment of the brain and undilated part the rudiment of the spinal medulla.

Whilst the cerebral portion is still unclosed, three secondary dilatations of its walls indicate its separation into three sections, the primitive fore-brain, the mid-brain, and the hind-brain; the primitive fore-brain being the most cephalward or anterior and the hind-brain the most caudal or posterior of the three (Fig. 38).

Shortly after the three segments of the brain are defined, and before it becomes a closed tube, a vesicular evagination forms at the cephalic end of each lateral wall of the primitive fore-brain region. These evaginations are the primary optic vesicles, and they are the rudiments of the optic nerves, the retinæ, and the posterior epithelium of the ciliary body and the iris of the eye-ball.

When the cerebral portions of the neural folds meet and fuse dorsally the cerebral dilatations become the primitive brain vesicles, each vesicle possessing its own cavity and walls, but the cavities of the three vesicles are continuous with one another, and the cavity of the hind-brain vesicle is continuous, caudally, with the central canal of the spinal part of the neural tube.

After the primitive brain vesicles are formed, a diverticulum grows out from the cephalic end of the primitive fore-brain vesicle. This is the rudiment of the secondary fore-brain. Its cephalic end soon divides into two lateral halves, which are the rudiments of the cerebral hemispheres of the adult brain (Fig. 45).

After their formation the cerebral hemispheres expand rapidly in all directions. They soon overlap the primitive fore-brain and mid-brain (Fig. 63), and, eventually, the hind-brain also, and each gives off from the cephalic end of its ventral wall a secondary diverticulum, the olfactory diverticulum, which becomes converted, later, into the olfactory bulb and olfactory tract.

When they first appear the rudiments of the cerebral hemispheres are connected together, across the median plane, by a part of the cephalic end of the wall of the secondary fore-brain dilatation, which is called the lamina terminalis. This primitive connexion between the two cerebral hemispheres persists throughout the whole of life, and it is supplemented, at a later period, by the formation of three secondary commissures, the corpus callosum and the fornix, which grow across the space between the cerebral hemispheres and connect their medial walls together, and the anterior commissure which grows through the lamina terminalis and connects the temporal portions of the two hemispheres.

The Fate of the Walls of the Primitive Brain Vesicles.— The primitive hind-brain, which is also called the rhombencephalon, is separated in the later stages of development into two parts. (1) A caudal portion which is connected with the medulla spinalis, and which becomes the medulla oblongata or myelencephalon of the adult brain. (2) A cephalic portion which is continuous at one end with the medulla oblongata and at the other with the mid-brain. The ventral wall of the cephalic portion of the primitive hind-brain is ultimately converted into the pons, and its dorsal wall differentiates into two parts—a caudal part which becomes the cerebellum ; and a cephalic part which is converted into the anterior medullary velum and the brachia conjunctiva. The brachia conjunctiva connect the cerebellum with the ventral part of the mid-brain. The pons and cerebellum form the metencephalon of the adult, whilst the brachia conjunctiva

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and the anterior medullary velum constitute parts of the isthmus rhombencephali (Figs. 45, 63).

The ventral portion of the primitive mid-brain is converted into the two peduncles of the cerebrum of the adult brain, and the dorsal portion is transformed into four rounded elevations, the colliculi or corpora quadrigemina.

The transformations which take place in the region of the primitive fore-brain or prosencephalon are numerous and complicated; therefore its ventral, lateral, and dorsal walls require separate consideration.

By the expansion of its cephalic (anterior) extremity is formed the secondary fore-brain, which becomes divided, as already explained, into the two secondary vesicles which are the rudiments of the cerebral hemispheres of the completed brain. After the formation of the rudiments of the cerebral hemispheres, which

constitute the telencephalon of the adult, the primi

tive fore-brain and TEGMEA

the undivided stalk of the secondary fore-brain diverticulum become the diencephalon.

The cephalic or anterior end of the diencephalon is closed by the lamina terminalis (see p. 33), in association with which are subse

quently developed FIG. 45.—DIAGRAMS TO ILLUSTRATE THE ALAR AND BASAL LAMINÆ. In both two columns cases the embryonic brain is represented in mesial section (His).

which run dorsoA. The different subdivisions of the brain are marked off from each other by dotted ventrally, the lines, and the dotted line running in the long axis of the neural tube indicates the columns of the separation of the alar from the basal lamina of the lateral wall.

fornix B. Medial section through the brain of a human embryo at the end of the first

(O.T. anmonth. Dotted lines mark off the different regions and also the alar and basal terior pillars), and laminæ from each other.

two transverse H, Buccal part of hypophysis cerebri ; RL, Olfactory lobe ; C.Str, Corpus striatum ; commissures, one

A, Entrance to optic stalk ; 0, Optic recess ; I, Infundibular recess ; T, Tuber of which connects cinereum ; M, Mamillary eminence.

together the two cerebral hemispheres and is called the anterior commissure, whilst the other is the optic chiasma in which the medial fibres of the optic nerves decussate.

From the cephalic or anterior end of the ventral wall of the diencephalon a diverticulum is projected ventrally towards the dorsal wall of the primitive mouth. The ventral end of this diverticulum becomes the posterior lobe of the hypophysis (0.T. pituitary body of the adult, the dorsal end becomes the tuber cinereum, and the intermediate part is the infundibulum which connects the tuber cinereum of the adult brain with the posterior lobe of the hypophysis.

Caudal to the hypophyseal diverticulum a single elevation appears in the ventral wall of the diencephalon. It is the corpus mamillare, which afterwards separates into the paired corpora mamillaria of the adult brain.

Still more caudally the ventral wall of the diencephalon takes part in the formation of the substantia perforata posterior, which lies between the two peduncles of the cerebrum and is partly developed from the cephalic or anterior end of the ventral wall of the primitive mid-brain.

The greater part of the dorsal wall of the diencephalon is ultimately reduced to a single laver of epithelial cells, but near its caudal end a diverticulum is projes

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rudimentary in man as contrasted with many other animals. At a later period two transverse bands of fibres appear in the dorsal wall of the diencephalon, one in front of and the other immediately behind the root of the epiphyseal recess. The anterior band is the dorsal or habenular commissure, and the posterior is the posterior commissure of the adult brain.

These structures, collectively, together with a small diverticulum of the epithelial roof, which appears

Roof-plate anterior to the dorsal commissure, and is called the Spongioblast

Spongioblast supra-pineal recess, constitute the so-called epithalamus.

Each lateral wall of the diencephalon is differentiated into a dorsal and a ventral part. The dorsal part forms a large gray mass called the

016101 thalamus, and on the posterior

Floor-plate

Floor-plate end of the thalamus are developed two rounded eleva

FIG. 46. tions, the medial and the A. Diagram of a transverse section of a spinal medulla which has lateral geniculate bodies, B. Diagram of a transverse section of a spinal medulla showing

not differentiated into groups of cells. which constitute the meta

positions of germinal cells. thalamus of the adult brain.

The ventral or basal portion of the lateral wall of the diencephalon, together with the adjacent part of the ventral wall, forms the hypothalamus of the fully developed brain.

The Fate of the Spinal Portion of the Primitive Neural Tube. --The spinal portion of the neural tube, during the first three months of intra-uterine life, develops equally in its whole extent, but after that period a longer cephalic or anterior (superior in the erect posture) and a shorter caudal portion are recognisable. The cephalic portion undergoes still further development and is converted into Columnar cells of roof-plate

the spinal medulla of the adult,

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Histological Differentiation

of the Walls of the Neural Tube. Spongioblast

- In the earliest stages of its deEpendyma cells

velopment the walls of the neural tube consist of a mass of nucleated protoplasm, more or less distinctly differentiated into cell areas, of columnar form, which extend between and are connected with an internal limiting membrane, bound

ing the neural canal, and an exColumnar cells of floor-plate

ternal limiting membrane, which FIG. 47.-SAOWING ELEMENTS OF CENTRAL NERVOUS SYSTEM.

surrounds the whole tube. At this time the outline of a transverse section of the primitive neural tube is somewhat ovoid. The cavity of the tube is compressed laterally into a dorsoventral cleft, which is bounded by dorsal, ventral, and lateral walls. In the dorsal and ventral walls, called respectively the roof- and floor-plates, the columnar character of the primitive epithelial elements of the medulla spinalis is retained throughout the whole of life, but the peripheral parts of some of the cells are converted into fibrils.

In the lateral walls of the embryonic medulla spinalis some of the cells soos

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with one another round the medial border of the cælom. The lateral border of the
somatic mesoderm is continuous, at the margin of the embryonic area, with the
mesoderm which covers the outer surface of the amnion, and the lateral border
of the splanchnic layer is continuous with the mesoderm on the wall of the
extra-embryonic or yolk-sac portion of the entodermal sac.
The Paraxial Mesoderm.—Each paraxial mesodermal bar soon assumes the form

Trophoblast cellular layer Mesoderm of chorion
Plasmodial trophoblast

Neural tube
Mesodermal somite

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Fig. 39.—TRANSVERSE SECTION OF THE ZYGOTE SHOWN IN FIG. 38, showing the differentiation

of the mesoderm.

of a triangular prism with the apex directed ventro-medially, towards the notochord, and the base dorso-laterally, towards the surface ectoderm.

The cephalic portion of each paraxial bar, as far caudalwards as the middle of the hind-brain, remains unsegmented, but the remainder is cut into a number of

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Fig. 40.-SCHEMA OF A TRANSVERSE SECTION OF A ZYGOTE, showing differentiation of mesoderm and

extension of amnion.

segments, the mesodermal somites, by a series of transverse clefts (Fig. 38). The first cleft appears in the region of the hind-brain, and the others are formed successively, each caudal to its predecessor. Only three or four somites lie in the

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