THE DEVELOPMENT OF AMPHIOXUS 



17 



termed a ccelogastrula to indicate the fact that the archenteron 

 is an actual widely open cavity. 



While cell multiplication continues in all parts of the embryo 

 during gastrulation, immediately after gastrulation begins, a 

 center of very rapid cell division appears just anterior to the 

 invaginating region on the dorsal side, and extending laterally 

 around the margin of the blastopore; on the ventral side of 



d 



C 



FIG. 7. Diagrams illustrating the relations between the adult axes and the 

 axes of the egg and early stages. After Cerfontaine. A. Fertilized egg. B. 

 Fully formed blastula. C. Gastrulation begun. D. Fully formed gastrula. 

 Note posterior elongation, a-p, Antero-posterior axis of adult; d, v, dorsal and 

 ventral surfaces of adult; x-y, chief egg axis (x, animal pole, y, vegetal pole). 



the blastopore this region is less clearly marked, although 

 distinguishable. This band of rapidly proliferating cells is 

 to be identified as the germ ring, an embryonic region of greater 

 importance in the other Chordates. The increase of cells at 

 this point is so rapid that it disturbs, for a time, the simple 



a, Animal pole; ar, archenteron; b, blastoporal opening; ch, rudiment of noto- 

 chord; dl, dorsal lip of blastopore; ec, ectoderm; en, endoderm; gr, germ ring; 

 nc, neuenteric canal; nf, neural fold; np, neural plate; s, blastoccel or segmentation 

 cavity; v, vegetal pole; rl, ventral lip of blastopore; //, second polar body. 



