2/6 



THE GASTRULA OF AMPHIOXUS. 



embryo assumes an elongated cylindrical form with the blasto- 

 pore at its hinder extremity (fig. 169 A). The blastopore now 

 passes to the dorsal surface, and by the flattening of this surface 

 a medullary plate is formed extending forwards from the blasto- 



FIG. 169. EMBRYOS OF AMPHIOXUS. (After Kowalevsky.) 



The parts in black with white lines are epiblastic; the shaded parts are hypo- 

 blastic. 



A. Gastrula stage in optical section. 



B. Slightly later stage after the neural plate np 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. 



bl. blastopore (which becomes in D the neurenteric canal) ; ne. neurenteric canal ; 

 np. neural or medullary plate ; wo. anterior opening of neural canal; ch. notochord; 

 so 1 , so 11 , first and second mesoblastic somites. 



pore (fig. 169 B). On the formation of the medullary groove 

 and its conversion into a canal, the blastopore opens into this 

 canal, and gives rise to a neurenteric passage, leading from the 

 neural canal into the alimentary tract (fig. 169 C and E). 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. 



