THE MORPHul.OGICAL COMPOSITION OF AXIMA/.S. 107 



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. Pos- 

 teriorly, the sheath projects but little beyond the notochord; 

 but, anteriorly, for a considerable distance, as far as the in- 

 fundibulum. It sends upwards two plates, which embrace 

 the futiu^e central parts of the nervous system laterally, pro- 

 bably throughout their entire length." All this precedes 

 segmentation. Considered under its broadest aspects, the 

 process is directly opj^osed to the process among the Aii- 

 ^idosa. ^Yhereas among the Anmilosa the first step is the 

 resolution of the germ-mass or of the blastoderm into seg- 

 ments, which may or may not afterwards become inte- 

 grated ; 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 ap- 

 pear, 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 di- 

 visions. On the contrary, before segmentation has made 

 much progress the rudiments of the vascular system are laid 

 down in a manner showing not the remotest trace of any 

 primordial correspondence of its parts with the divisions of the 

 axis. No less 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 articu- 

 late embryo. But in the vertebrata this antithesis is exactly 

 reversed. At the time when segmentation has become de- 

 cided in the dorsal region of the spine, there is no trace oi 

 segments in the parts that are to form the skull — nothing 

 whatever to suggest that the skull is being formed out of 

 divisions homologous with vertebrae. And minute observa- 



