28 



ZOOLOGY 



SECT. 



which at the same time changes its position until it becomes 

 placed at what is destined to be 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 blasto- 

 pore become distinguished from the rest by their more cubical 



eel 



eel 



ecf 



FIG. 688. 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 ; rued. can. medullary 

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

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

 after Van Beneden and Julin.) 



shape ; these cells, which form the earliest rudiment of the ner- 

 vous system, become arranged in the form of a plate the medul- 

 lary 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 blasto- 

 pore. At the same time a number of small cells of the inner layer 

 in the neighbourhood of the blastopore form a ring round that 

 opening, and then extend forwards in the form of a plate below the 

 medullary plate. The middle portion of this subsequently forms 



