The material for this study consists of 16 human. embryos from 5 mm (Streeter's horizon XIV) to 22 mm long.
Before the formation of cartilages of the vertebral column there is a definitive vertebrals segments of the paraxial or sclerotomic mesoderm. Such segments are to be seen as the condensations of mesenchym cells which begin cranial inthe occipital region or cranial to the first cervical nerve in Streeter's horizon XIV embryos and follow caudalward. This mesenchymal condensations are not only in the paraxial region reco-gnizable, but also dorsalward lateral to the spinal ganglions and between them, ventrolateral medial to the corresponding somites. Through the less condensed stratum run on both sides the intersegmental arteries from aorta a little nearer to the cranial than to the caudal condensation.
(Fig. 1) In the next horizons XV and XVI becomes the condensation more and more distinct, and in the horizon between XVI and XVII the occipital -condensation is chondrified. (Fig.2, Plate 1, 2) According Sensenig is the precartilage stage the closely packed mass of cells, showing numerous nuclei, little of the cell boundaries, and no interstitial spaces. Such structure have the vertebral processes and ribs, but not the vertebral bodies. The occipital cartilages appear bilateral .also in the cell condensation as in the case of processes of vertebrae, although it seems as if the occipital praecartilage is not so much condensed .as-the vertebral processes or ribs. As the vertebral bodies do not have the precartilage stage, so we can not the beginning of the chondrification of it definitely discern as that of the occipital region. But the chondrification of occipital region is followed, we can think, by that of the first 2 or .3 cervical vertebral bodies which are situated caudal to, the respective condensations. The more the cartilages of the vertebral bodies distinct, the more so the condensations which continue to the vertebral processes. The cartilages of those processes appear separately in these precartilage tissue a little later than the bodies and so there is in the beginning the not chondrifying sclerotomic tissue zones between the chondrified bodies and processes which soon chondrify afterward.
The chondrification of the vertebral bodies begins belateral from the first cervical caudalward, remaining the mesenchymal zones in the median planes, which are not distictly discernible in the bodies of the allmost all thoracal, lumbar, sacral and coccygeal as in the cervical. These median mesenchymal zones of the cervical vertebral bodies soon disappear. Following the chondrification of the vertebral bodies comes next that of the neural processes in the precartilages, which begins not in the cervical, but in the thoracal vertebrae, progressing from the first thoracal caudalward. When all the thoracal neural are chondrified, so also those of the 5th to 7th cervical, not yet those of first 4 cervicals and lumbals. Following the lower cervical comes next the chondrification of those of the upper cervical and lumbal, and then those of the sacral.
