J. R. GREGG 241 



to the innermost tip of the archenteron and the edge of the dorsal 

 lip (Fig. 5). It is easily seen that elongation of the archenteron 

 will be accompanied by increases in the value of this angle, which 

 Moore calls the angle of invagination. 



Figure 5 shows that the value of the invagination-angle of an R. 

 pipiens embryo increases sharply throughout the period during 

 which the embryo is gastrulating; for a hybrid, this value in- 

 creases almost as rapidly at first, but thereafter remains nearly 

 constant. Thus, in hybrid embryos, formation of the archenteron 



180» " 



ca il+o» ' 



20 



M3 60 80 



HOURS DEVELOPMENT iSfC 



100 



Fig. 6. The angle of epiboly in developing Rana pipiens and hybrid 

 embryos. (Reconstructed from data of Moore, 1946.) 



gets under way, but is brought to a halt when it is only one- 

 quarter complete. 



Epiboly. As an amphibian embryo gastrulates, its stock of 

 internal tissues is increased by draughts upon that of its external 

 endoderm and mesoderm. The inward transfer is made possible 

 in several ways, but at least partly by epiboly, a thoroughgoing 

 meridional extension of presumptive ectoderm and mesoderm 

 (Fig. 4). Mesodermal tissues are turned inward over the blas- 

 toporal lip; ectodermal ones will eventually constitute the entire 

 external surface. 



