122 



EMBRYOLOGY 



Al_l HEAD REGION 



PRESUMPTIVE 

 EURAL PLATE 



PRESUMPTIVE 

 NOTOCHORD 



Fig. 64. A more advanced stage in gastrulation. The archenteron is larger, 

 the blastocoel smaller. V is now overgrown by the lip of the blastopore. Regions 

 1, 2, and 3 have migrated internally to form the roof of the archenteron, while 

 region 4 is still external. Near region C a ventral lip of the blastopore has de- 

 veloped. 



two points in Figure 63. The stretching and resulting thinning continue as 

 gastrulation proceeds (Fig. 64). 



In Figure 64 the turning in at the lip of the blastopore continues, and 

 regions 1, 2, and 3 now form the roof of the archenteron and V is completely 

 covered. B is closer to C; D closer to E; and F closer to G. By this time the lip 

 of the blastopore has extended around the sides and a ventral lip forms, 

 showing some in-turning of cells. 



Finally, in Figure 65, stage 12, the process is complete. B and C are adja- 

 cent. Region 4 is internal. D and E are together, and F and G come together. 

 These cellular movements result in a large archenteron, and the blastocoele 

 is obliterated. The ectoderm has been stretched to cover the entire surface of 

 the gastrula. The former animal pole, A, now becomes the anterior end of 

 the gastrula; the vegetal pole is located in the floor of the archenteron. 



