32 



ZOOLOGY 



SECT. 



before backwards, and the opening eventually being reduced to a 

 small pore at the posterior end of the dorsal surface. 



The embryo elongates in the direction of the future long axis. 

 The dorsal surface becomes recognisable by being flatter, while 

 the ventral remains convex. The ectoderm cells bordering the 

 blastopore are distinguished from the rest by their more cubical 

 shape ; these cells, which form the earliest rudiment of the nervous 

 system, become arranged, as the blastopore undergoes contraction, 



eel 



nolo 



trv noto 



nd 



end 



ecf 



ccl 



FIG. 742. Early stages in the development of Clavellina. A, flattened blastula ; B, early 

 gastrula ; C, approximately median optical section of more advanced gastrula in which the 

 blastopore has become greatly reduced and in which the first rudiment of the notochord is 

 discernible ; D, similar view of a later larva in which the medullary canal has begun to be 

 closed in posteriorly, bl. p. blastopore ; ect. ectoderm ; end. endoderm ; med. can. medullary 

 canal ; nerv. cells destined to give rise to the nerve-cord ; neur. neuropore ; noto. notochord ; 

 seg. cao. segmentation cavity. (A and B from Korschelt and Heider, after Seeliger ; C and D 

 after Van Beneden and Julin.) 



in the form of a plate the medullary plate on the dorsal surface. 

 On the surface of this plate appears a groove the medullary groove 

 bounded by right and left medullary folds, which pass into one 

 another behind the blastopore. 



The medullary folds grow upwards and inwards over the medul- 

 lary groove, and unite together (D), the union beginning behind 

 and progressing forwards in such a way as to form a canal, the 

 neuroccele, in the hinder portion of which is the opening of the 



