EARLY DEVELOPMENT IN THE FROG. 333 



pletion of gastrulation, although the proportion of eggs showing 

 this type of differential inhibition varied somewhat in experi- 

 ments involving different chemicals. This increase in value of 

 the cleavage ratio appears less frequently in LiCl, MgCl 2 , and 

 in alcohol, and most frequently in KNC and formaldehyde. 

 In w/i,ooo, m/2,ooo KNC, fully 90 per cent, of the eggs are of 

 this type after 24 to 36 hours' exposure from the beginning of 

 segmentation. Differential inhibition under 'the conditions 

 stated, is diagrammatic, especially in KNC, and represents 

 simply a greater susceptibility to the toxic agent, of the cells 

 most affected, viz., the animal pole cells. The significance of 

 this differential susceptibility is a matter of discussion elsewhere 

 (pp. 346-349). 



(b~) Bilaterality. Under the conditions of these experiments, 

 bilaterality is much obscured and may not become evident at all 

 (Fig. 20). But, that bilaterality is at least potentially present 

 is indicated in those cases where gastrulation does begin (always 

 equatorially) the wrinkling and folding, indicating the initiation 

 of gastrulation usually begins at a definite point on the equatorial 

 region and spreads equatorially around the egg. A few cases 

 have been observed in m 5,000 KNC where the equatorial 

 folding occurred almost simultaneously around the entire equator 

 of the egg. 



(c) The Segmentation Cavity. Under severe inhibiting condi- 

 tions, the segmentation cavity shows several characteristic con- 

 sequences of differential inhibition, depending chiefly upon the 

 severity of the conditions and the stage at which the eggs are 

 exposed to them and upon the length of the exposure. 



S'ome of the data are these: In eggs exposed to m/i,ooo or 

 w/5,ooo KNC (Experiments KNC A-H) from the beginning of 

 the first segmentation, development usually ceases in late cleav- 

 age stages with occasional abortive attempts at equatorial gastru- 

 lation. The yolk cells, which are relatively less inhibited, may 

 continue division for a time after it has nearly or quite stopped 

 in the animal hemisphere. In such cases, one finds that the more 

 peripheral yolk cells, forming the floor of the segmentation 

 cavity, proliferate apicalward, forming a thin layer of yolk cells 

 that may partly or completely line the walls of the segmentation 



