CEPHALOCHORDA. 



mentation cavity becomes gradually obliterated, and the embryo 

 first assumes a cup-shaped form with a wide blastopore, but soon 

 becomes elongated, while the communication of the archenteron, 

 or cavity of invagination, with the exterior is reduced to a: small 

 blastopore (fig. 2 A), placed at the pole of the long axis which 

 the subsequent development shews to be the hinder end oj the 



FIG. i. 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 ; no, anterior opening of neural canal ; ch. notochord ; 

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



embryo. The blastopore is often known in other Chordata as 

 the anus of Rusconi. Before the invagination is completed the 

 larva throws off the egg-membrane, and commences to lead a 

 free existence. 



Up to this stage the larva, although it has acquired a 

 cylindrical elongated form, has only the structure of a simple 

 two-layered gastrula ; but the changes which next take place 



I 2 



