PHYLU]\[ CHORDATA 



33 



i-ndodcnn, the whcilc oiiibiyo beeuinos cnrvud, witli the con- 

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

 endoderni, the two layers Cdniing t<> lie in chjsc contact and the 

 segnientation-ea\ity thus l)econiinL;- 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 



cci 



;. 733. — Early stages in the dcvcloiimcnt of Clavellina. A, flattened blastula ; B, early 

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

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

 discernible ; I>, sirailar view of a later larva in which the medullary canal has begun to be 

 closed in posterioi-ly. OL 'p. blastopore ; ect. ectodemi ; c,i<l, cndodcrm ; uieO. can. medullary 

 canal ; nerv. cells destined to give rise to the nerve-cord ; nev r. neuropore ; nolo, notocliord ; 

 seri. cav. segmentation cavity, (A and B from Korschelt and Ueider, after .Sceliger; C and D 

 after Van Bencden and Julin.) 



dorsal side. The blastopore gradually becomes constricted 

 (Fig. 733, B) — the closure taking j^lace 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 fiom the rest by their more cubical 



VOL. II D 



