344 CHARLES PACKARD 



is that prolonged exposure may so injure the chromatin that it 

 is unable to play a part in cell division. If the sperm alone is 

 radiated, it merely acts as a stimulus to induce in the egg par- 

 thenogenetic development. If the egg is radiated, the sperm 

 nucleus alone divides, the egg nucleus taking no part in sub- 

 sequent development. A less severe radiation of either element 

 serves to generate in the chromatin a poison which brings about 

 abnormalities in growth. 



I have been unable to find any evidence of parthenogenetic 

 development either in Nereis or in Arbacia. When the eggs 

 are radiated development is either diploid or does not occur at 

 all. The same is true if the sperm is radiated. If the eggs 

 or sperm are not greatly injured development is diploid but 

 abnormal. 



It is evident that no generalization on the effect of radiations 

 can be based on the behavior of a single form, for it has been 

 shown that there are several types of response among the cells 

 already studied. Nor can we assume that the effect is directly 

 on the nucleus or on the protoplasm. If it were on the former 

 we should expect that exposure of the cells during the resting 

 stage of the nucleus would be followed by greater abnormalities 

 than would obtain when the radiation is made during mitosis, 

 since in the former period the chromatin is more finely divided 

 and presents a larger surface to the rays. But the reverse is the 

 case. Mottram ('13) has shown that Ascaris eggs are eight 

 times more susceptible during division than during the resting 

 stage; that is, there are eight times as many deaths following 

 an exposure made during mitosis than during the latter period. 

 It has also been observed that cancer tissue is much more suscepti- 

 ble to the rays when it is growing rapidly than when it is nearly 

 stationary. This indicates that an explanation for these phenom- 

 ena must take into account the differences between the physio- 

 logical state of the cell constituents during these periods. 



During the resting period the interchange of material between 

 nucleus and protoplasm is small compared with the amount 

 which takes place during cleavage. At the latter time the 



