subsequent changes as follows:--“The gelatinous investing mass, which, at first, seems only to constitute a band to the right and to the left of the notochord forms around it, in the further course of development, a sheath, which ends in a point posteriorly. Anteriorly, it sends out two processes which underlie the lateral parts of the skull, but very soon coalesce for a longer or shorter distance. Posteriorly, the sheath projects but little beyond the notochord; but, anteriorly, for a considerable distance, as far as the infundibulum. It sends upwards two plates, which embrace the future central parts of the nervous system laterally, probably throughout their entire length.” That is to say, in the _Vertebrata_ the first step is the marking out on the blastoderm of an integrated structure, within which segments subsequently appear. When these do appear, they are for some time limited to the middle region of the spinal axis; and no more then than ever after, do they implicate the general mass of the body in their transverse divisions. On the contrary, before vertebral segmentation has made much progress, the rudiments of the vascular system are laid down in a manner showing no trace of any primordial correspondence of its parts with the divisions of the axis. Equally at variance with the belief that the vertebrate animal is essentially a series of homologous parts, is the heterogeneity which exists among these parts on their first appearance. Though in the head of an adult articulate animal there is little sign of divisibility into segments like those of the body; yet such segments, with their appropriate ganglia and appendages, are easily identifiable in the articulate embryo. But in the _Vertebrata_ this antithesis is reversed. At the time when segmentation has become decided in the dorsal region of the spine, there is no trace of segments in the parts which are to form the skull--nothing whatever to suggest that the skull is being formed out of divisions homologous with vertebræ.[29] And minute observation no more discloses any such homology than does general appearance. “Remak,” says Prof. Huxley, “has more fully proved than any other observer, the segmentation into ‘urwirbel,’ or proto-vertebræ, which is characteristic of the vertebral column, stops at the occipital margin of the skull--the base of which, before ossification, presents no trace of that segmentation which occurs throughout the vertebral column.” [Illustration: Fig. 191.]