Fig. 209. — Development of amphioxus, Branchiostoma lanceolatum. 



A, zygote. S, cleavage stage. C, blastula in .section. Z) to F, gastrula stages. G to J, 

 transverse sections showing origin of neural tube or central nervous system, notochord, and 

 mesoderm. J, section in median plane of future adult (c/. E, F, and L). K, transverse 

 section of a stage (c/. G, H, and I) in which the coelome has appeared. L, section in median 

 plane and with the segmental arrangement of the mesoderm shown (c/. K) . bp, blastopore; 

 c, notochord; ca;, ccelome; e, archenteron; ec, ectoderm; en, endoderm; m, mesoderm; m.s., 

 mesodermal segment; n, neural canal; ne.c, neurenteric canal; np, neuropore; n.<., neural 

 tube; p.b., polar body; sc, segmentation cavity; s.t., tail of spermatozoon. (After 

 Hatschek, with modifications from Cerfontaine.) 



399 



