120 



EMBRYOLOGY 



PRESUMPTIVE 

 EPIDERMI 



PRESUMPTIVE 

 EURAL PLATE 



PRESUMPTIVE 

 NOTOCHORD 



Fig. 62. A section through the blastocoel of a very early amphibian gastrula 

 (stage 10). Certain groups of cells are lettered B through G for purposes of 

 identification. The movements of these cells during gastrulation are indicated by 

 arrows. Thus, B moves toward C, D migrates to E, and F approaches G. Regions 

 above the dorsal lip of the blastopore, D, are numbered 1 through 4. Region 1 

 is shown as having already turned under the dorsal lip. V marks the vegetal pole. 

 A marks animal pole. 



size due to the formation of the blastocoel and archenteron, and this size 

 increase has not been shown in Figure 58. Development up to this time is 

 chiefly a matter of cell division and cell migration. There is as yet no visible 

 differentiation of structures, although experiments tell us that some chemical 

 determination has occurred by the end of gastrulation. 



Rearrangements of cells 

 during gastrulation 



Let us now examine the internal changes during gastrulation. In Figure 62 

 we start with a longitudinal section of a very early gastrula, stage 10. A large 

 blastocoel is present and the beginning of gastrulation is shown by a small 



