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At the cephalic end of the embryonic area the medial margins of the mesodermal sheets fuse together across the median plane, forming a transverse bar of mesodermal cells which may be called the pericardial mesoderm (Fig. 48), because the pericardial sac, which envelops the heart, is afterwards developed from it. The area in which this mesoderm lies may be named the pericardial region of the embryonic area (Fig. 48).

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Between the bar of pericardial mesoderm, the cephalic end of the neural groove, and the medial margins of the mesodermal plates lies a small segment of the embryonic area from which the primary mesoderm entirely disappears, leaving the ectoderm and entoderm in contact. This is the bucco-pharyngeal area. afterwards becomes the bucco-pharyngeal membrane (Figs. 50, 55), which separates the primitive mouth or stomatodæum from the cephalic end of the primitive entodermal alimentary canal. As already stated, the bucco-pharyngeal membrane disappears during the third week, when the stomatodæum and the primitive alimentary canal become continuous with each other.

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Between the buccopharyngeal area and the cephalic end of the primitive streak the medial margins of the mesodermal plates are separated from one another by the notochord and the neural groove (Fig. 36), and still more caudally they are united with the sides of the streak (Fig. 34).

Cellular trophoblast Chorion

Mesoderm lining
trophoblast

Mesoderm covering
entoderm vesicle

Mesoderm of amnion

After the permanent mesodermal plates are definitely established a series of clefts appear in their peripheral margins. The clefts, on each side, soon fuse together to form the bilateral rudiments of the embryonic cœlom (Fig. 36).

The septum of cells at the lateral border of the embryonic area on each side, which, for a time, separates the embryonic from the extra-embryonic coelom, soon disappears, and the cœlom then forms a continuous cavity (Fig. 37).

The embryonic colom also extends medially, but the medial extension ceases whilst the cavity is still at some distance from the median plane, except at the cephalic end of the embryonic area, where the two lateral halves of the embryonic cœlom become continuous with one another through the interior of the pericardial mesodermal bar (Figs. 49, 55).

Mesodermal somites
(paraxial mesoderm)

Ectoderm of amnion

FIG. 38. SCHEMA OF DORSAL ASPECT OF EMBRYO, showing partial
closure of neural groove.
Portions of the chorion and amnion have been removed.
The neural folds have fused, except in the cephalic and caudal regions,
both the cephalic and the caudal ends of the embryo have been bent
ventrically and thirteen mesodermal somites have been formed.

As the embryonic coelom is forming and extending, a longitudinal constriction appears in each lateral half of the mesoderm, a short distance from its medial border. This constriction separates each plate into three parts: (1) a medial bar, the paraxial mesoderm, which lies at the side of the neural groove and the notochord (Fig. 37); (2) the constricted portion, which is called the intermediate cell tract; and (3) the part lateral to the constriction, which is called the lateral plate (Fig. 37).

The embryonic coelom is confined, as a rule, in the human subject, to the lateral plate, which it divides into a superficial layer, next the ectoderm, the somatic mesoderm, and a deeper layer, next the entoderm, the splanchnic mesoderm.

The medial borders of the somatic and splanchnic mesoderm are continuous

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The lateral border of the with one another round the medial border of the cœlom. 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

Mesoderm of chorion

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Trophoblast cellular layer
Plasmodial trophoblast
Neural tube

[graphic]

Mesodermal somite

Notochord

Intermediate cell tract
Amnion cavity

Amnion
Somatic mesoderm

Colom

Splanchnic mesoderm

Primitive gut.

Extra-embryonic coelom
Wall of yolk-sac

Cavity of yolk-sac

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

Chorion

Scleratogenous mesoderm

Muscle plates

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Amnion

Amnion cavity

Neural tube

Spinal ganglion

Sympathetic ganglion

Aorta

Intra-embryonic cœlom
Extra-embryonic cœlom

Gut

Yolk-sac

Colom

Umbilicus

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

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

Mesoderm of entoderm vesicle

Entoderm

Cavity of entoderm

vesicle

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

Entoderm
Mesoderm of
entoderm vesicle
Cavity of entoderm

vesicle

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

A

Amnion cavity

Paraxial mesoderm

-Lateral plate mesoderm

Notochord

Mesoderm of chorion Trophoblast of chorion

[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.

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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

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