556 THE EMBRYOLOGY OF THE FROG 



the frog blastula is not made up of a single layer of cells, but of a double 

 layer of animal cells. This double layer is the ectoderm. The floor of 

 the blastocoele is made up of the large vegetal cells. 



As the pigmented animal micromeres divide more rapidly from now 

 on, they naturally must grow toward the equator. This causes a thin- 

 ning of the roof, but a thickening of the walls of the segmentation cavity. 

 The equator of the blastula seems to be the region in which the cells 

 multiply most rapidly, and this equatorial region is called the germ-ring 

 or growth zone (Fig. 324). 



At the same time the germ-ring begins its rapid multiplication of 

 cells, the gray crescent extends downward rather rapidly. This region 

 is to become the posterior or caudal side of the embryo. The germ-ring 

 from now on continues extending beyond the equator into the vegetal 

 region until it lies approximately half way between the equator and veg- 

 etal pole. This growing of the germ-ring pushes the yolk more and 

 more within the overgrowing animal cells, as already mentioned. The 

 yolk thus being pushed within, naturally forces the floor of the segmenta- 

 tion cavity into a convex arch. 



This is considered the end of blastulation in the frog's egg. 



THE FORMATION OF THE GASTRULA 



We have at this point, then: most of the yolk withdrawn within 

 the overgrowing animal cells, two layers of which form the outer cover- 

 ing of the blastula at the animal pole; a segmentation cavity with its 

 floor convexly arched, and a definite antero-posterior differentiation, the 

 posterior side being marked by the gray crescent. 



The development of the gastrula begins just beneath the posterior 

 lip of the blastopore by a groove which forms directly between the ani- 

 mal cells and the yolk cells (Fig. 323, A, gc). The groove itself is lined 

 by both kinds of cells on its opposite faces. We know from the develop- 

 ment which takes place later that this groove is the real beginning of 

 invagination. The groove itself becomes the archenteron or primitive 

 intestinal tract (Fig. 323, C, E, F, G, a). The upper lip of this groove is 

 the rim of the blastopore (Fig. 323, b). The animal cells become the 

 ectoderm and the yolk cells become entoderm. 



As the yolk is pushed within the blastula it causes a narrow groove 

 to form in the region of the blastocoele, which separates the rising floor 

 from the remaining yolk. This groove finally becomes a definite narrow 

 slit, which splits off the ectoderm and entoderm at the point of invagina- 

 tion. The original groove is known as the gastrular groove, and the 

 splitting off into ectoderm and entoderm is called gastrular cleavage 

 (Fig. 323, A, gc). This gastrular cleavage extends from the dorsal lip 

 of the blastopore entirely around the gastrula to the opposite side from 

 where invagination takes place. 



In the invagination area, a definite tongue of ectodermal cells pushes 



