REARRANGEMENT OF CELLS DURING GASTRULATION 



1A 



121 



PRESUMPTIVE 

 EPIDERMIS 



PRESUMPTIVE 

 NEURAL PLATE 



PRESUMPTIVE 

 NOTOCHORD 



Fig. 63. A continuation of the processes initiated in Figure 62. Region 1, 

 formerly on the surface of the early gastrula, is now located internally and forms 

 the roof of the archenteron. B is relatively closer to C. The lip of the blastopore, 

 now composed of region 2, is encroaching upon V . D is migrating toward E, 

 while F approaches G. 



indentation, D, the dorsal lip of the blastopore. The rearrangements of cells 

 are shown by arrows. Briefly, region B moves to region C; region D travels to 

 E; and region F to G. The remaining three diagrams show these movements 

 in steps. In Figure 63, stage 11, the original dorsal lip of the blastopore, 

 region 1, is now located at the anterior end of the primitive digestive cavity 

 or archenteron. Regions 2, 3, and 4 will follow region 1 into the roof of the 

 archenteron. This turning in brings B closer to C. While these movements are 

 taking place the lip of the blastopore closes over the surface of the yolk, 

 approaching and finally overgrowing V . Thus V becomes internally located 

 in the floor of the archenteron. The blastocoel becomes smaller, and F 

 approximates G, while D migrates toward E. During this phase of gastrula- 

 tion the presumptive neural plate becomes stretched and thinner. Compare 

 the distance between A and B in Figure 62 with the distance between those 



