REPRODUCTION 47 



plane determined by the first cleavage. The equator and a zone extending 

 superficially somewhat into the vegetal hemisj)her(' are marked by espe- 

 cially rapid cell-proliferation. It is in this ])articularly active region, 

 the germ-ring, that the groove appears. Figure 40.1' represents a section 

 in the median plane of an embryo at this stage. The groove (/) is the 

 result of an invagination which occurs near where the upper thin wall 

 and lower thick wall of the blastula join. The outer layer bounding the 

 invagination consists of smaller cells which have moved inward from the 

 superficial germ-ring region; the deeper wall of the invagination consists 

 of yolk cells. The groove, initiated as a slight invagination, rapidly 

 deepens (Fig. 40B-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 germ-ring material. This 

 growth process serves to build out the dorsal lip 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. 40-B) until it describes a semicircle and, con- 

 tinuing, 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 

 completely covered except for a small aperture through which bulges 

 a mass of yolk cells, the so-called yolk plug (Fig. 40C-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, 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 overgrowth of the vegetal 

 hemisphere, the large yolk cells become extensively 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 invagination and overgrowth. 



Figure 40C' 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 



