68 COMPARATIVE ANATOMY 



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. Meanwliile 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. 44B) until it describes a 

 semicircle and, continuing, finally completes a circle. As the groove 

 progressively lengthens, the newly arisen region of its outer fold, con- 

 tinuous with the "dorsal lip" of the initial region of the groove, at once 

 begins to grow 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. 44C,C'). 



If the processes just described can be visualized, it will be seen that 

 their result must be 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 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 44C' 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 embryo. The greater 

 part of the original yolk is now in the endoderm. 



This gastrula is essentially like that of Amphioxus. It is formed by 

 the cooperation of several processes of which the more important are 

 (i) invagination, which plays a minor part in the formation of the defini- 

 tive archenteron; (2) epiboly — that is, overgrowth of the yolk mass by 

 the fold resulting from invagination; (3) rearrangement of the yolk cells 



