Aves. In Birds the cellular tube formed round the notochord is far thicker than in the Reptilia. It is continuous in the regions of the future vertebræ with neural arches, which do not at first nearly enclose the spinal cord.
On about the fifth day, in the case of the chick, it becomes differentiated into vertebral regions opposite the attachments of the neural arches, and intervertebral regions between them; the two sets of regions being only distinguished by their histological characters. Very shortly afterwards each intervertebral region becomes segmented into two parts, which respectively attach themselves to the contiguous vertebral regions. A part of each intervertebral region, immediately adjoining the notochord, does not however undergo this division, and afterwards gives rise to the ligamentum suspensorium.
The notochord during these changes at first remains indifferent, but subsequently, on about the seventh day in the chick, a slight constriction of each vertebral region takes place; so that the vertebræ have temporarily, as they have also in Amphibia, a biconcave form which repeats the permanent condition of most fishes. By the ninth and tenth days, however, this condition has completely disappeared, and in all the intervertebral portions the notochord has become distinctly constricted, and at the same time in each vertebral portion there have also appeared two constrictions of the notochord giving rise to a central and to two terminal enlargements.
On the twelfth day the ossification of the cartilaginous centra commences.
The first vertebra to ossify is the second or third cervical, and the ossification gradually extends to those behind. It does not commence in the arches till somewhat later than in the bodies. For each arch there are two centres of ossification, one on each side.
The notochord persists for the greater part of fœtal life and even into post-fœtal life. The larger vertebral portions are often the first completely to vanish. They would seem in many cases at any rate (Gegenbaur) to be converted into cartilage, and so form an integral part of the permanent vertebræ. Rudiments of the intervertebral portions of the notochord may long be detected in the ligamenta suspensoria.
Fig. 321. Longitudinal section through the vertebral column of an eight weeks’ human embryo in the thoracic region. (From Kölliker.)
v. cartilaginous vertebral body; li. intervertebral ligament; ch. notochord.
Schwarck (No. [420]) states that in both the intervertebral and the vertebral regions, though less conspicuously in the former, the cartilage is divided into two layers, an inner and an outer. He holds that the inner layer corresponds to the cartilaginous notochordal sheath of the lower types, and the outer to the arch tissue. Ossification (Gegenbaur) of the centra appears in a special inner layer of cartilage, which is probably the same as the inner layer of the earlier stage, though this point has not been definitely established.