Reproduction 255 



rapidly deepens (Fig. 211B-B'), not by continued invagination, but by 

 active growth of the upper (for later events prove it to be dorsal) lip 

 of the groove — that is, the lip resulting from the infolding of gerni- 

 ring material. This growth process serves to build out the dorsai iip 

 of the original invagination so that the fold is caused to extend farther 

 and farther downward over the yolk-cells. Meanwhile the groove, 

 originally a short crescent, as seen on the surface of the blastula, 

 lengthens laterally or in the direction of the curve of the crescent 

 (Fig. 21 IB) until it describes a semicircle and, continuing, finally 

 completes a circle. As the groove progressively lengthens, the newly 

 arisen region of its outer fold, continuous with the "dorsal lip" of the 

 initial region of the groove, grows centripetally over the surface of the 

 yolk-cells. Therefore the radius of the curve described by the groove 

 is ever decreasing. The groove is obviously deepest at the region where 

 it began to form and shallower in the successively newer parts of it. 

 Having completed the circle, the centripetal growth of the outer fold 

 of the groove continues until the original vegetal hemisphere is com- 

 pletely covered except for a small aperture, through which bulges a 

 mass of yolk-cells, the so-called yolk plug (Fig. 211C-C')- 



The result of the processes just described is the formation of a new 

 cavity in the embryo. This cavity is bounded externally by the two 

 layers of the overgrowing fold and internally by the yolk-cells. It 

 potentially opens to the exterior but its actual opening is partly blocked 

 by the yolk plug. If no process other than those already mentioned 

 were involved, the cavity would be exceedingly thin. It is, in fact, 

 greatly enlarged by another process. During the progress of the over- 

 growth of the vegetal hemisphere, the large yolk-cells become exten- 

 sively rearranged. They move into the blastocoele, finally practically 

 obliterating it. They carry out this movement in such a way that 

 the space left vacant by them is added to the cavity formed by in- 

 vagination and overgrowth. 



Figure 211C represents a median section of a frog embryo at the 

 close of gastrulation. The embryo is two-layered throughout. The 

 outer layer, ectoderm, is uniformly thin. The inner layer, endoderm, 

 is very thin over approximately the dorsal half of the embryo but thick 

 in the ventral region, where the greater part of the original mass of 

 yolk-cells persists. The endoderm surrounds a capacious cavity, the 

 archenteron, whose external opening, the blastopore, is occupied 

 by the yolk plug. The blastopore marks the posterior end of the em- 

 bryo. The greater part of the original yolk is now in the endoderm. 



The difference between gastrulation in Amphioxus and in the 

 amphibian is essentially this: in Amphioxus the vegetal hemisphere 

 (prospective endoderm) of the blastula actively moves into the interior 



