EFFECT OF RADIUM ON DEVELOPMENT OF CHvETOPTERUS. 59 



nucleus develops normally and fuses with the sperm nucleus. 

 But during cleavage the egg chromosomes do not move together 

 with the sperm chromosomes but lag behind in the future cleavage 

 plane. Under the influence of longer radiations the egg is so 

 affected that its nucleus is unable to move from its position under 

 the polar bodies, but develops there more or less abnormally, and 

 plays no part in the cleavage of the egg. In both cases the 

 cleavage is regular and haploid. A prolonged radiation so affects 

 the egg that the polar bodies may be formed abnormally or not 

 at all; in such instances the sperm also fails to develop, though it 

 can move up to the position of the polar spindle. Polyspermy 

 becomes more and more frequent as the duration of exposure in- 

 creases. This leads to abnormal cleavage into three or more cells. 



DISCUSSION. 



The facts presented in the foregoing pages show that with in- 

 creasing periods of exposure the number of diploid cleavages is 

 lessened and the number of haploid cleavages, in which only the 

 sperm nucleus takes part, is greatly augmented. Actual counts 

 of sections of dividing eggs give a clear picture of the proportion 

 which obtains between these two types. These counts are 

 represented graphically in the accompanying text figure. The 

 ordinates represent the percentage of eggs which cleave, and the 

 abscissae, the duration of the exposure in minutes. Polyspermic 

 eggs were disregarded in making these counts because they be- 

 long to a different category of phenomena. The dotted line 

 represents the percentage of eggs which divided regularly. 



It is seen that brief exposures do not prevent eggs from di- 

 viding, but that exposures of 30 to 35 minutes bring about a 

 marked decrease in the numbers which cleave. This relation 

 was noticed in the living eggs, and the curve plotted from the 

 latter data corresponds closely to that derived from a study of the 

 sections. After this critical period the number of eggs which 

 cleave regularly increases to about 70 per cent, and then, with 

 prolonged exposures, gradually falls off until, after an exposure of 

 130 minutes, practically no eggs divide. 



The other curves show why this sudden drop at the critical 



