DEVELOPMENT OF THE LANCELET 



577 



to the dorsal surface of the adult, and the blastopore 

 now comes to be situated, as in the frog-embryo (Fig. 64 

 H, K), at the posterior end of the dorsal surface. A 



i- mk 



FIG. 153. Four stages in the development of the notochord, nervous system, and 



mesoderm of A mphioxu s. 



ak. ectoderm ; ch notochord ; dh. cavity of archenteron ; hb. ridge of ectoderm 

 growing over medullary plate ; ik. endoderm ; //z. coelome ; mk. coelomic pouch ; 

 w/k 1 . parietal, and ink'*, visceral layer of mesoderm ; mp. medullary plate ; n. 

 medullary cord; ns. mesodermal segment (' ' protovertebra "). (From Korschelt 

 and Heider's Embryology, after Hatschek.) 



medullary plate and groove (Fig. 153, mp) are then formed, 

 the central nervous system being developed in a manner 

 essentially similar to that already described in the 



PR ACT. ZOOL. P 



