The earliest evidence of the differentiation of the blastoderm consists in the appearance of the primitive streak which forms the first sign of the embryo. This is a line which appears near what is to be the tail end of the embryo, and runs forward. This primitive line or streak is due to the thickening of the mesoblast, and it becomes converted into a groove by a depression appearing in its centre, forming the primitive groove. This extends in a forward direction, but never reaches the head fold of the embryo, which, in the chick, appears a few hours after the formation of the primitive groove. In front of the primitive groove, and stretching backward to overlap it at the side, arise two folds of the epiblast, called the laminae dorsales or the medullary folds.

These are elevations of the epiblast, -beneath which the meso-blast is thickened. They arise in front, where they are joined immediately behind the head fold, while posteriorly they diverge, and passing on either side of the primitive groove, gradually become lost. Between the two folds is a furrow lined by epiblast, which is called the medullary groove.

View of the area pellucida of a chick of eighteen hours, seen from above.

Fig. 277. View of the area pellucida of a chick of eighteen hours, seen from above. {Foster and Balfour).

A. Medullary folds.

inc. Medullary groove.

pr. Primitive streak and groove.

The medullary folds growing upward, turn in toward one another, and eventually coalesce at their line of meeting, convert-ing the medullary groove into a channel - the medullary canal; this union of the folds takes place from before backward.

Transverse section of the embryo of a chick at the end of the first day.

Fig. 278. Transverse section of the embryo of a chick at the end of the first day. (Kolliker).

sp. Mesoblast. dd. Hypoblast, m. Medullary plate, h. Epiblast. Pv. Medullary groove. Rf. Medullary fold. ch. Chorda dorsalis. uwp. Protovertebral plate, uwh. Division of mesoblast.

The medullary canal thus formed lies in the axis of the embryo on the uncleft mesoblast; it is covered in superficially by several layers of epiblastic cells, which also line its walls. The canal is the earliest representative of the nervous centres, and eventually becomes the brain and spinal cord. The front part of the canal, when completely closed in, becomes dilated into a bulb, thus forming the earliest indication of the brain. The hind part of the medullary groove remains unclosed considerably later than the fore part. It gradually becomes converted into a canal at the tail end, and as it extends backward obliterates the primitive streak and groove, which are lost, and take no permanent part in the formation of the embryo.

Transverse section of an embryo of a chick at the latter end of the second day.

Fig. 279. Transverse section of an embryo of a chick at the latter end of the second day.

(Kolliker).

rw. Medullary fold. rf. Medullary groove, h. Epiblast. ao. Aorta, dd. Hypoblast. p. Pleuroperitoneal cavity. sp. External plate of mesoblast dividing, uwp. Protovertebral plate.

Beneath the medullary canal the cells of the mesoblast are altered to form a rod-shaped cellular body, which following the line of the canal lies in the axis of the embryo; this is the chorda dorsalis or notochord.

Supporting the medullary canal on either side of the chorda dorsalis are masses of mesoblast, somewhat quadrangular in section, which are termed the vertebral plates; continuous with these externally are other thinner masses of mesoblast called the lateral plates.

Transverse section through the embryo of a chick on the second day, where the medullary canal is closed.

Fig. 280. Transverse section through the embryo of a chick on the second day, where the medullary canal is closed. (Kolliker).

mr. Medullary canal, h. Epihlast. uwh. Cavity of protovertebra uw. ung. Wolffian duct. mp. Mesoblast dividing, hpl. Somatopleure. df. Splanchnopleure. sp. Pleuro-peritoneal cavity. dd. Hypoblast, ch. Notochord.

The lateral plates become divided into an upper part or somatopleure, which is in close relationship to the epiblast, and a lower part, the splanchnopleure, which is next to the hypoblast; the space between these being the pleuroperitoneal cavity. The vertebral plates become separated from the lateral plates by a longitudinal partition, so that on either side of the neural canal is a mass of undivided mesoblast extending laterally toward the divided mesoblast.

In each vertebral plate there appear transverse vertical interruptions at definite intervals, which split the plate up into a number of quadrangular blocks of mesoblast, known as the protonertebrce; the number of these corresponds to the number of vertebrae of the animal.

These protovertebrae become subdivided by transverse fissures into external parts, the muscle plates, which form eventually the dorsal and other muscles, and internal parts which become the permanent vertebrae.

Embryo chick at the end of the second day, seen from below.

Fig. 281. Embryo chick at the end of the second day, seen from below. (Kolliker).

Vh. Forebrain.

Ab. Optic vesicles.

Ch. Notochord.

H. Heart.

om. Omphalo-mesenteric veins.

Vd. Lower opening of foregut.

Division of the vertebral column of a chick. (Kolliker after Remak).

Fig. 282. Division of the vertebral column of a chick. (Kolliker after Remak).

1. Notochord.

2. Points of separation of the original protovertebrae.

3. Points of division of the permanent vertebrae.

4. Arches of the vertebrae.

5. Spinal ganglia.

c. Body of first cervical vertebra.

d. One of the lower vertebrae.

From these inner portions processes of mesoblast grow upward over the medullary canal to meet with processes from the protovertebrae of the opposite side. Mesoblastic tissue also grows inward between the medullary canal and the notochord, and between the notochord and subjacent hypoblast.

Transverse section through the dorsal region of an embryo chick of forty five hours.

Fig.283. Transverse section through the dorsal region of an embryo chick of forty-five hours.

{Foster and Balfour).

A. Epiblast. M.c. Medullary canal. P.v. Proto-vertebrse. W.d. Wolffian duct. p.p. Pleuroperitoneal cavity. S.o. Somatopleure. S.p. Splanchnopleure. c.v. Vessels. a.o Aorta. B. Mesoblast. C. Hypoblast, c.p. Line of union of opaque and pellucid areas, w. Spheres of the white yolk.

These projections beneath the notochord meet with others from a mass of the mesoblast, lying between the protovertebrae and the cleft mesoblast, and known as the intermediate cell mass.

The portions of the protovertebrae above the medullary canal form the arches of the vertebrae; from those surrounding the notochord the bodies of the vertebrae are developed.

The outer part of each protovertebra divides into an anterior or pre-axial part, from which arises the ganglion of a spinal nerve, and into a posterior or post-axial part.

After this the original lines of separation between the protovertebrae disappear, and the spinal column is fused into a cartilaginous mass. New segmentation now appears in the centre of each original protovertebra, midway between the primary divisions. Thus the vertebral column is divided into a number of component parts, each of which is destined to become a permanent vertebra.

The vertebrae do not then correspond to the original protovertebrae, but rather to the posterior half of that which lay in front of the primary division joined to the anterior half of the one behind. The ganglia of the spinal nerves, by this arrangement, instead of belonging to the front, become joined to the posterior part of the vertebra to which they belong.

The notochord atrophies with ossification of the vertebrae, and finally is represented only by a mass of soft cells in the centre of an intervertebral disc.

In connection with the vertebrae in the dorsal region, processes grow horizontally, these are the rudiments of the ribs.