334 A - w - BELLAMY. 



cavity which becomes very large. At the time of death, the 

 egg resembles in some respects an amphioxus blastula (Fig. n). 

 Superficial indications of bilaterality are obscure. 



Where inhibition is more severe, e.g., in w/io,ooo HgCl2 

 (Experiment IV 40), eggs exposed to the solution for 48 hours 

 from a two-cell stage, stop development in late cleavage stages. 

 Gastrulation never occurs. In all of the eggs, the downward 

 migration of materials from the apical pole is completely in- 

 hibited and the walls of the very small segmentation cavity 

 lying near the center of the egg, are 2/5 to 4/9 the diameter of 

 the egg in thickness (Fig. 12). 



The partial or complete obliteration of the segmentation cavity 

 may occur in an entirely different way under less severe inhibiting 

 conditions. Eggs in late segmentation stages, exposed 15 min- 

 utes to w/io,ooo HgClo (Experiment IV 56) then returned to 

 water, show after three days, the segmentation cavity completely 

 filled with small yolk cells that have proliferated from the floor 

 of the cavity. A faint line indicates the walls of the now ob- 

 literated cavity. The walls of what was the segmentation cavity 

 are much thinner than when the eggs had been exposed 48 hours 

 to m/io,ooo HgClo from the beginning of cleavage. 



Likewise in m/io LiCl, where development proceeds abnormally 

 to the time of hatching, the proliferation of yolk cells from the 

 margin of the floor of the segmentation cavity may be so extensive 

 as to fill it completely. 



All of these different modifications appear to be simply dif- 

 ferent expressions of the differential inhibition which the eggs 

 have suffered. The animal pole cells being most susceptible, are 

 most affected by the adverse conditions, the yolk cells least. 



Those small cells around the margins of the floor of the seg- 

 mentation cavity, which under certain conditions of inhibition, 

 continue division and finally fill the blastocoele completely, 

 appear to be the ones destined to give rise to mesoderm (Morgan, 

 1906, p. 129; Kellicott, 1913, p. 107; King, 1902, Fig. 4). It 

 is significant here that in the sea urchin Child (1916, p. 91) 

 found the mesenchyme cells which arise from the basal pole of 

 the egg, to be less susceptible than other parts of the egg, and 

 that under conditions of differential inhibition, these cells tended 



