46 



VERTEBRATE ZOOLOGY 



late gastrula shows a distinct bilaterality, a dorsal and ventral aspect, 

 and anterior and posterior lips to the blastopore. The gastrula is a 



ch 



rd 



FIG. 19. Optical section of young embryos of Amphioxus. The cilia are 

 omitted. A. Two-somite stage, approximately at the time of hatching, showing 

 relation of neuropore and neurenteric canal. B. Nine-somite stage, showing 

 origin of anterior gut diverticula. C. Fifteen-somite stage. End of the embryonic 

 period, ap, anterior process of first somite; c, neurenteric canal; ch, notochord; 

 ego, external opening of club-shaped gland; co, ccelomic cavity of somite; cv, 

 cerebral vesicle; g, gut cavity; gs, rudiment of first gill slit; i, intestine; I, left 

 anterior gut diverticulum; m, mouth; mes, unsegmented mesoderm; n, nerve 

 cord; p, pigment spot (eye spot); rd, right anterior gut diverticulum; si, s 2 , first 

 and second mesodermal somites; spc, splanchnocoel (body cavity). (From 

 Kellicott's " Outlines of Chordate Development " [Henry Holt and Company] 

 after Hatschek.) 



ciliated embryo that moves about within its vitelline membrane very 

 much like that of an echinoderm or annelid. After about eight hours 



