230 THE GASTBULA OF AMPHIOXUS. 



from the neural canal into the alimentary tract (fig. ICO C and E). 



Fi«. 169. Embryos of Amfhioxus. (After Kowalevsky.) 

 The parts in black with white Hnes are epiblastic ; the shaded parts are hypoblastic. 



A. Gastrula stage in optical section. 



B. Slightly later stage after the neural plate up has become differentiated, seen as 

 a transparent object from the dorsal side. 



C. Lateral view of a slightly older larva in optical section. 



D. Dorsal view of an older larva with the neural canal completely closed except 

 for a small pore {no) in front. 



E. Older larva seen as a transparent object from the side. 



hi. blastopore (which becomes in D the neurenteric canal); 7ie. neurenteric canal; 

 up. neural or medullary plate ; no. anterior opening of neural canal; ch. notochord; 

 «o', so", first and second mesoblastic somites. 



At a later period this canal closes, and the neural and alimentary 

 canals become separated. 



Such is the simple history of the layers in Amphioxus. In the 

 simplest types of Ascidians the series of phenomena is almost the 

 same, but the blastopore assumes a more definitely dorsal position. 



Here also the blastopore lies at the hinder end of the medullary 

 groove, and on the closure of the groove becomes converted into a 

 neurenteric passage. 



In the true Vertebrates the types which most approach Amphioxus 

 are the Amphibia, Acipenser and Petromyzon. We may take the 

 first of these as typical (thougli Petromyzon is perhaps still more so) 

 and fig. 170 A B C D represents four diagrammatic longitudinal 

 vertical sections through a form belonging to this group (Bombinator). 

 The food-yolk is here concentrated in what I shall call the lower pole 

 of the egg, which becomes the ventral aspect of the future embryo. 

 The part of the egg containing the stored-up food-yolk is, as has already 

 been explained in the chapter on segmentation (Vol. I. pp. 77 and 78), 

 to be regarded as equivalent to part ot those eggs which do not contain 

 food-yolk; a fact which requires to be borne in mind in any attempt 



