120 MORPHOLOGICAL DEVELOPMENT. 



Vertebrata the first step is the marking out on the blastoderm 

 of an integrated structure, within which segments subse- 

 quently 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 pro- 

 gress, 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 

 vertebrae.* And minute observation no more discloses any 

 such homology than does general appearance. " Eemak," 

 says Prof. Huxley, " has more fully proved than any other 

 observer, the segmentation into ' urwirbel,' or proto-vertebrae, 

 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." 



Consider next the evidence supplied by comparative mor- 

 phology. In preceding sections ( 206, 208,) it has been 



* Though it is allcjred that at a later stage the posterior part of the skull 

 is formed by fusion of divisions which are assumed to represent vertebrae, yet 

 it is admitted that the anterior part of the skull never shows any signs of 

 such division. Moreover in both parts the bones show no trace of primitive 

 segmentation. 



