BLASTULATION 



95 



Progressive stages in blastulation. 



the gray crescent side of the cleaving egg. It will remain in this position, 

 beneath the animal hemisphere, until it is later displaced by the de- 

 velopment of other cavities. The size of the blastocoel increases with 

 the formation of smaller and smaller surrounding cells. The late blas- 

 tula of the frog, by virtue of this blastocoel, has a volume displace- 

 ment about 20 per cent greater than that of the fertilized egg. 



Following the 32-cell stage the egg loses all semblance of rhythmic 

 or synchronized cleavage and there is developed a gradient of cleavage 

 with the most active region being at the animal pole and the least 

 active being at the vegetal pole. In addition to this there are some 

 characteristic changes in the bias tula of the frog embryo. First, the 

 smaller pigmented animal pole cells tend to spread their activity in a 

 downward direction toward the vegetal pole, by a sort of contagion, 

 so that there is an actual migration of pigment-covered cells toward 

 the vegetal pole. This, and the horizontal cleavages, result in increas- 

 ing the cell layers but thinning of the roof of the blastocoel. Second, 

 the horizontal cleavages in the very small animal pole cells give the 

 blastula a double or multi-layered roof. The single outer layer of cells 

 contains most of the superficial pigment and is now recognized as the 

 epidermal layer which will give rise to epithelium, either of the integu- 

 ment or lining the nervous system. The inner tiers of cells of the 

 blastular roof are less pigmented and are known collectively as the 

 nervous layer because they will give rise largely to the neuroblasts of 

 the nervous system. 



