EARLY DEVELOPMENT OF AMPHIOXUS. 



39 



tion. The circular opening between the archenteron and the exterior is the 

 blastopore. Its margins are its lips which can be differentiated into dorsal, 

 ventral and lateral lips. At these lips the entoderm and ectoderm are 

 continuous. 



Another factor in gastrulation is a process known as epiboly. When 

 invagination is complete, that is, when the macromere pole of the blastula 

 has infolded until the blastoccel is obliterated, the gastrula approximates a 



FIG. 21. Gastrulation in Amphioxus. Cerfontaine, from Kellicott. 



A, blastula with slightly flattened vegetal pole, showing rapid cell division in postero-dorsal 

 region (germ ring); 5, more pronounced flattening of the vegetal pole; C, beginning of 

 invagination in postero-dorsal region; D, further invagination, showing obliteration of the 

 blastocoel and formation of the archenteron as the result of invagination; E, invagination 

 almost complete; F, beginning elongation of gastrula and narrowing of blastopore; G, 

 continued elongation of gastrula and narrowing of blastopore. Observe the mitotic figures 

 in the germ ring in all stages. In D and E the inflection of cells (involution) around the 

 dorsal lip of the blastopore can be appreciated. In F and G the process of epiboly is 

 represented in the backward growth of the lip of the blastopore. a, Animal pole; ar, 

 archenteron; b, blastopore; dl, dorsal lip of blastopore; ec, ectoderm; en, entoderm; gr, 

 germ ring; s, blastocoel; v, vegetal pole; vl, ventral lip of blastopore; //, second polar body. 



hemisphere and the form of the archenteron coincides. Then, along with 

 the rapid cell proliferation in the dorsal part or the germ ring and the forma- 

 tion of the plate of entodermal cells mentioned in the preceding paragraph, 



