EARLY DEVELOPMENT OF THE FROG 



269 



The process of gastrulation is profoundly modified by the 

 presence of the food-yolk, and instead of a simple invagination 

 of the lower half of the blastula wall to form the hypoblast, we 

 find the small (epiblast) cells, now arranged in several layers, 

 spreading themselves over the yolk-containing cells until they 

 completely enclose the latter except for a small circular area 



yk.c. 



yk.c. u.l.bp. 



V* 



, FIG. 119. Early Stages in the Development of the Frog (partly from 

 Ziegler's models and partly adapted from Marshall.) 



I, the fertilized ovum ; II V, segmentation of the ovum ; VI, blastula ; VI I, modified 

 blastula, with wall composed of more than one layer of cells ; VIII, commencement 

 of gastrulation; IX, the modified gastrula stage (VI IX in vertical section). 



blc., blastocoel or segmentation cavity; bp., blastopore ; ent., enteron; ep., epiblast; 

 hyp., hypoblast ; u.l.bp., upper lip of blastopore ; yk.c., yolk-containing cells. 



which corresponds to the blastopore or aperture of invagination 

 in Amphioxus blocked up by yolk cells (Fig. 119, VIII, IX). 

 Indeed, practically the whole of the primitive digestive cavity or 

 enteron in the frog's gastrula may be regarded as being filled 

 with yolk cells, from which hypoblast will be derived, and it 

 only gradually opens out later on in development as the yolk is 

 used up. Hence the coelenterate stage of the ancestral history 



