164 



EMBRYOLOGICAL TYPES 



the exterior, but finds itself opening into the hind part of the 

 nerve-tube. In this manner the neur-enteric canal is formed. 



nc n - 



r 1 ms 



,-sp. 



-ec. 



Fig. 69. — Amphioxus : transverse sections through young embryos, 

 showing the origin of the notochord, nerve-cord, and mesoderm. 

 A, early stage showing the enterocoelic pouches (ep) still in communica- 

 tion with the gut-cavity (g) ; the roof of the gut is giving rise to the noto- 

 chord (n) ; the nerve-cord (nc) although overgrown by the ectoderm (ec) 

 has not yet formed a tube ; b, blastocoel ; en, endoderm ; m, mesoderm. 



B, later stage showing the enterocoelic pouches nipped off from the gut. 



C, stage showing the extension of the ccelom (c) between ectoderm and endo- 

 derm, the formation of mesodermal somites (ms) ; the notochord is separate 

 from the gut, and the nerve-cord is rolling up. D, late stage, the nerve- 

 cord is a tube, the ccelom is divided into myocoel (ml) dorsally and splanch- 

 nocoel (sp) ventrally, the inner wail of the latter cavity being the splanchno- 

 pleur (sr) and its outer wall the somatopleur (so). The inner wall of the 

 myocoel is modified into a muscle-plate or myotome (my), and ventral to the 

 latter is the sclerocccl (se). 



The notochord rises up from the rest of the roof of the 

 archenteron and forms a solid rod of cells extending all the 



