208 CHARLES PACKARD 



stop the radiations than protoplasm. This is probably true, 

 since the former substance contains more phosphorus and iron 

 than the latter. But the difference in composition is not so 

 great as to account for the striking differences in response. 



Furthermore, the changes in the apparent susceptibility of 

 chromatin during the different phases of mitosis cannot be ex- 

 plained by the statement that its chemical constitution has al- 

 tered so as to render it more or less absorptive. The time elaps- 

 ing between the prophase and the metaphase is so brief that we 

 cannot imagine any profound chemical change, involving a 

 dropping out of elements, to occur. It is obvious therefore 

 that to account for the observed phenomena we cannot assume 

 that the chromatin is more able to stop the rays, or that it varies 

 in its ability to stop them, at different periods of its activity. 

 It is necessary to take into account the physiological processes 

 occurring during mitosis. 



During the periods in which the radium was appHed to the 

 eggs two phenomena which are of interest in this connection 

 occur. In the first place the chromatin, which, during the rest- 

 ing stage, or the stage previous to the fusion of the germ nuclei, 

 is diffuse and semi liquid, becomes condensed and gelatinous in 

 the metaphase. After this time it gradually becomes more 

 and more diffuse up to the period of the resting stage. On ac- 

 count of this increase in its density we might suppose that it 

 becomes more absorptive to the rays. But a consideration of 

 other experiments shows that this cannot be true. The sperm, 

 in which chromatin is condensed to the greatest degree, is far 

 more resistant to radiations than is the egg in which the chroma- 

 tin is diffuse. I have exposed Nereis sperm for five hours yet 

 at the end of that time they were not only actively motile but 

 were able to bring about normal fertilization of the fresh egg. 

 The fact that in the egg and the sperm there is an equal amount 

 of chromatin, yet the egg is much more easily affected than the 

 sperm, argues that something beside chromatin is involved. 



The second phenomenon which occurs during this period is 

 the breaking down of the nuclear wall with the liberation into 

 the egg protoplasm of the nuclear sap. This is followed by a 



