212 



A. RICHARDS AND DONNELL J. GOOD. 



the maturation and early cleavage divisions. A tabulated form 

 of the observations is shown in Table I. 



TABLE I. 



Radiation at first serves to stimulate the mitotic activity of 

 these eggs. The results are not as clear cut as in Planorbis 

 where entire egg clusters can be observed without in any way 

 disturbing the eggs in their normal environment. Nevertheless, 

 it is clear here from Table I. that acceleration takes place at first 

 in the irradiated eggs and that subsequent retardation follows 

 as in the case of Planorbis where " the control, started at the time 

 of the exposure goes more slowly than the experiment during 

 the first two mitoses, but by the time the t\venty-four-cell stage 

 is reached the exposed eggs are progressing more slowly than it" 

 (Richards, '14). Packard ('16) obtained similar results in 

 Arbacia. 



In a cluster of Planorbis eggs division takes place almost 

 simultaneously in all the eggs. No such degree of uniformity is 

 to be observed in Cumingia, but the data clearly indicate that 

 the acceleration, while it persists longer than in Planorbis, just 

 as surely gives way to a retardation, and that the longer radia- 

 tion causes a greater retardation than the shorter. It would 



