NKinOUS SYSTEM 



559 



presented in l-'ig. 64, F, though much modified by the quantity 

 of food-yolk, will be seen to correspond to the gastrula-stage. 

 As in the' frog, the blastopore soon closes, the mouth and 

 anus being subsequently formed from the stomodteum and 

 the proctodseum respectively (p. 204). 



The gastrula becomes elongated, flattened on one side. 



ViC. 142.— Three stages id the formation of the gastriila of Amphioxns. In A, 

 the nuclei of the endoderm have been omitted. (I'Vom Korschelt and Heider, 

 after Hatschel:.) 



and convex on the other. The flattened side corresponds 

 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 

 medullary plate and groove (Fig. 143, mp) are then formed, 

 the central ner%'ous system being developed in a manner 

 essentially similar to that already described in the 

 case of the tadpole (p. 202), except that the central 

 canal of the medullary cord {n) is formed after the 



