in the general stages of cleavage, blastulation, gastrulation, 

 and organogenesis. They are alike particularly in having 

 the mouth and anus as secondary openings, the latter at the 

 site of the blastopore, in having a large and variable number 

 of segments, and in having a part of the body modified into 

 a tail for swimming. 



The general similarity of the early stages of development 

 strengthens the idea that the vertebrates are monophyletic. 

 There are some indications that the agnaths and gnatho- 



stomes are independent lines from the common ancestor. In 

 terms of development the lamprey appears to be interme- 

 diate between the protochordate and gnathostome, being 

 specialized only in having a solid neural keel. The neural 

 tube of the myxinid tends to cancel this variation. In con- 

 trast to the gnathostome, the head somites are retained into 

 the adult stage, suggesting less modification of this region, 

 and thus giving some support to view that the agnaths 

 preceded the gnathostomes. 



216 • EAABRYOGENESIS OF THE CHORDATES 



