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region of the head; the remainder are in the body area of the embryonic region. The segmentation of the paraxial bars commences before their elongation is completed, and the posterior somites are separated off as the paraxial bars are extended by the continued proliferation from the nodal point at the anterior end of the primitive streak.

When they are first defined the somites are solid masses of cells, but in a short time a cavity-the cœlom of the somite or myocœle-is developed in each mass.

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B

FIG. 41.

A. Transverse section of a zygote, showing the constituent parts.
B. Diagram of embryonic area showing parts of neural plate and primitive streak.

Trophoblast of chorion
Mesoderm of chorion

Mesoderm of amnion

Ectoderm of amnion

Neural crest

Roof-plate

Lateral wall of
neural groove

- Floor-plate

A

[blocks in formation]

The apical portion of the hollow mesodermal somite is its scleratogenous segment. The cells of the scleratogenous section of the somite undergo rapid proliferation. Some of the newly formed scleratogenous cells invade the myocole; others migrate towards the notochord; finally, the scleratogenous cells separate from the remainder of the somite, and as they increase in number they migrate along the sides of

[graphic]

Amnion cavity
Paraxial

mesoderm

Embryonic cœlom

Notochord

FIG. 42.

A. Diagram of a transverse section of a zygote, showing the formation of a neural groove in the embryonic area. B. Diagram of a surface view of the embryonic area of the same zygote.

the notochord and the neural tube, which has been formed in the meantime from the neural groove, and join with their fellows of the opposite side, and with their cephalic and caudal neighbours. In this way is formed, around the neural tube and the notochord, a continuous sheath of mesoderm, the membranous vertebral column, from which are differentiated, in later stages, the vertebral column and its ligaments, and the membranes of the brain and the spinal medulla.

After the separation of the scleratogenous segments of the mesodermal somites, the remainders of the somites, each of which consists of a flat plate with incurved dorsal and ventral margins, constitute the muscle plates from which the striped muscle fibres are derived.

In the opinion of some observers the outermost portion of each of the above-described plates is developed into subcutaneous connective tissue cells; consequently it is spoken of as the cutis lamella. According to this view the muscle cells are formed from the innermost cells and the incurved margins of the plates.

The Intermediate Cell Tracts.-The intermediate cell tracts are the rudiments of the internal organs of the genital system and the temporary and permanent urinary system, with the exception of the urinary bladder and the urethra.

The Lateral Plates. From the cells of the lateral plates are formed the lining endothelial cells of the great serous cavities of the body-the pleuræ, the pericardium, and the peritoneum; the majority of the connective tissues, with the exception of those of the vertebral column and the head, the greater part or all the mesoderm of the limbs, and, probably, the unstriped muscle fibres of the walls of the alimentary canal and the blood-vessels.

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FIG. 43.

A. Diagram of a transverse section of a zygote, in which the neural tube has formed but has not separated from the surface ectoderm.

B. Diagram of embryonic area of same zygote. Compare with surface view of embryo in Fig. 38.

The Cephalic Mesoderm.-It has already been noted that the mesoderm of the head becomes segmented only in the region of the caudal part of the hind-brain, where four cephalic mesodermal somites are formed on each side. From the scleratogenous portions of these somites are developed the occipital part of the skull and the corresponding portions of the membranes of the brain, and from their muscle plates the intrinsic muscles of the tongue.

The unsegmented part of the cephalic mesoderm gives rise to the remaining muscles and connective tissues of the head region.

Early Stages of the Development of the Nervous System.-No definite trace of the nervous system is present until the primitive streak has formed and the embryonic area has passed from a circular to an elongated form. Then an area of thickened ectoderm, the neural plate, appears in the anterior part of the embryonic area. It commences a short distance posterior to the anterior end of the area, and its posterior extremity embraces the anterior end of the primitive streak. Its lateral margins fade into the surrounding ectoderm, and, in the earliest stages, cannot be definitely defined; but, as the elongation of the plate continues coincidently with the elongation of the embryonic area, the lateral margins of the plate are elevated as the mesoderm beneath them thickens, and so they become distinct.

As the lateral margins of the neural plate are raised the plate is necessarily folded longitudinally, and the sulcus so formed is called the neural groove. Each side. wall of the neural groove, formed by the corresponding half of the neural plate, is a neural fold. At a very early period the neural folds unite anteriorly to form the anterior boundary of the neural groove, and, somewhat later, they unite posteriorly, caudal to the neurenteric canal and across the anterior end of the primitive streak. After the lateral boundaries and the anterior and posterior extremities of the neural groove are defined, the lateral margins of the neural folds converge until they meet and fuse in the median plane, and the neural groove is thus converted into the neural tube, which possesses a floor or ventral wall, formed by the central part of

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FIG. 44.-DIAGRAMS illustrating the formation of (1) the rudiments of the primitive ganglion from the neural crest. (2) The differentiation of different parts of the primitive ganglion into permanent ganglion root, sympathetic ganglion, and masses of chromaffin cells. (3) The formation of the anterior and posterior nerve-roots. (4) The differentiation of the walls of the neural tube into ependymal matter and peripheral layers.

The cells of the primitive ganglion which form the primitive sheaths of the nerves are not shown in the diagrams.

the original neural plate and called the basal plate or floor-plate; a dorsal wall or roof-plate, and two lateral walls formed by the lateral parts of the neural plate.

The fusion of the lateral margins of the neural plate to form the roof-plate of the neural tube commences in the cervical region, and from there extends cranialwards and caudalwards, therefore the last parts of the roof-plate which are formed are its anterior and its posterior extremities; consequently, for a time, the neural canal, which is the cavity of the tube, opens on the surface at its anterior and posterior ends; the anterior opening being called the anterior neuropore, whilst the open part at the posterior end is termed the posterior neuropore (Fig. 43). Eventually, however, about the third week of embryonic

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.1

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

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.

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.

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 tube.

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 retina, 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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