PHYLUM CHORDATA 



33 



endoderm, the whole embryo becomes curved, with the con- 

 cavity on the endodermal side, and the ectoderm extends over the 

 endoderm, the two layers coming to lie in close contact and the 

 segmentation-cavity thus becoming obliterated. The concavity 

 deepens until the embryo assumes the form of a saucer-shaped 

 gastrula with an archenteron and a blastopore which is at first 

 a very wide aperture extending along the whole of the future 



B 



e.cf 



\. 733. 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. U. 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. cav. segmentation cavity. (A and B from Korschelt and Heider, after Seeliger ; C and D 

 after Van Beneden and Julin.) 



lorsal side. The blastopore gradually becomes constricted 

 (Fig. 733, B) the closure taking place from 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 blasto- 

 pore are distinguished from the rest by 'their more cubical 



VOL. II D 



