DEVELOPMENT OF THE LEOPARD FROG 409 



this blastopore extends laterally and becomes, within a few hours, 

 a complete circle enclosing a "plug" of lighter-colored cells (c/. 

 Fig. 213 G and 214 F). In this manner the developing embryo 

 becomes entirely covered with darker-colored cells except within 

 the circle of the blastopore, and the late gastrula or ijolk-plug 

 stage, as it is sometimes called, is produced. Further constriction 

 at the edges of the blastopore continues this overgrowth, until 

 the yolk-plug disappears and the blastopore becomes a small slit 

 in the median plane at the posterior end of the embiyo. 



During these external changes the invagination of the arch- 

 enteron has extended (cf. Figs. 213 H and 214 F), both by a folding 

 over of the dark-colored cells at the Up of the blastopore and by an 

 ingrowth at the inner end of the cavity. The blastula cavity 

 disappears as the archenteron is thus extended into the mass of 

 the developing embryo. The process, of course, occurs laterally 

 as well as in the median plane shown in the figures, but the lateral 

 extension of the archenteron may be disregarded in the present 

 description. While these changes are in progress, mesoderm cells 

 (Fig. 213 G and H) become differentiated, between ectoderm and 

 endoderm, beginning in the region of the blastopore. 



Finally, the yolk-plug disappears within the blastopore, and 

 the external surface is completely covered with darkly pigmented 

 cells. The archenteron, or enteron as it may now be called, is an 

 extensive cavity in what will be the dorsal, and also in the lateral, 

 regions of the future animal. The blastula cavity has almost 

 disappeared. The mass of yolk-laden cells hes in a position that 

 is ventral to the enteron. The notochord, or primitive skeletal axis, 

 which was foreshadowed in the cytoplasmic differentiation of the 

 zygote (cf. Fig. 213 B to I), is recognizable as the cells of the 

 median dorsal Hning of the enteron. The ectoderm is represented by 

 the several layers of cells covering the outer surface; the endoderm, 

 by the single layer of cells lining the enteron and the yolk-laden cells 

 which are being gradually incorporated into the growing embryo; 

 the mesoderm is extending between the ectodermal and endo- 

 dermal layers. Thus the three germ layers, ectoderm, endoderm, 

 and mesoderm, from which the various parts of the body are 

 later differentiated, are formed during gastrulation. 



In conclusion may be mentioned the changes by which the blasto- 

 pore, and, hence, the vegetative pole, assume the positions which 

 they occupy at the close of gastrulation. The dorsal lip of the 



