number of quadrangular blocks of mesoblast, known as the protovertebra; the number of these corresponds to the number of vertebræ of the animal.

These protovertebræ 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 vertebræ.

From these inner portions processes of mesoblast grow upward over the medullary canal to meet with processes from the protovertebræ of the opposite side. Mesoblastic tissue also grows

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-vertebrææ. W.d. Wolffian duct. . Pleuroperitoneal cavity. S.o. Somatopleure. S.. Splanchnopleure. c.v. Vessels. a o Aorta. B. Mesoblast. C. Hypoblast. op. Line of union of opaque and pellucid areas. w. Spheres of the white yolk.

a.o.

do

S.P

FIG. 283.

SP.

inward between the medullary canal and the notochord, and between the notochord and subjacent hypoblast.

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

The portions of the protovertebræ above the medullary canal form the arches of the vertebræ ; from those surrounding the notochord the bodies of the vertebræ 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 protovertebræ 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 vertebræ do not then correspond to the original protovertebræ, 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 vertebræ, and finally is represented only by a mass of soft cells in the centre of an intervertebral disc.

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

DEVELOPMENT OF THE CENTRAL NERVOUS SYSTEM.

SPINAL CORD.

Soon after the closure of the medullary or neural canal at its anterior or cranial end, it is dilated in this region into three vesicles, known as the first, second, and third cerebral vesicles, from which the brain is developed. The spinal cord is formed from that part of the medullary canal which lies over the chorda

dorsalis. The medullary canal is lined by columnar cells derived from the epiblast, which, shortly after they are shut off from the general epiblast, develop at the sides of the canal, so as to narrow the lumen of the tube by the increase in thickness of its sides. The upper and lower parts of the canal do not become thickened.

The side walls approximate to the centre, decreasing laterally the lumen of the canal, which becomes narrow in the middle with a dilatation above and below. The lateral walls of the canal, thus approximated, unite in the centre, and convert the medullary canal into two separate tubes, a dorsal and a ventral.

The lower or ventral tube of the divided canal becomes the central canal of the spinal cord, and the columnar cells of the epiblast form a lining of ciliated columnar epithelium.

The epiblast at the lower part of the canal becomes converted into the anterior gray columns, in connection with which arise the anterior roots of the spinal nerves; while at the upper part the posterior gray columns are formed in connection with the posterior roots of the spinal nerves and their ganglia.

The white columns are thought by some authors to be derived from the mesoblast surrounding the canal, but by others they are assigned to the epiblast.

The upper or dorsal canal becomes converted into a fissure by

the absorption of its roof, and is thus changed into the posterior fissure of the spinal cord.

The anterior fissure is formed by the down-growth of the anterior columns, which diverge, leaving between them an interval which becomes occupied by the pia mater.

The commissures are not formed between the lateral halves of the cord until later. The gray commissure appears first.

Transverse section of the spinal cord of a chick of seven days. (Foster and Balfour.) ep. Epithelium lining the medullary canal. f. Part of the cavity of the medullary canal which becomes the posterior fissure. spc. Permanent medullary tube or central canal of the spinal cord. agc. Anterior gray commissure. af. Anterior fissure, not yet well formed. c. Tissue filling in the upper part of the posterior fissure. pc. Cells forming the posterior gray matter. pcw. Posterior white column. ct. Mesoblast surrounding the spinal cord. lcw. Lateral white column. acw. Anterior white column. ac. Cells forming the anterior gray matter.

THE BRAIN.

Anterior Cerebral Vesicle.-As already mentioned, the brain is formed from the primitive neural canal, the anterior part of which dilates into three little swellings called the anterior, middle and posterior cerebral vesicles. From the anterior, or first cerebral