EFFECT OF RADIUM ON CELL DIVISION 207 



is also evident that in these experiments no profound change 

 took place which involved a dropping out of any of the elements 

 present in the protoplasm at the beginning of the experiment, 

 since the eggs developed perfectly normally after the treatment. 

 This fact is important in the explanation of the results here 

 presented, for at first sight they seem to indicate that the re- 

 verse is true, namely, that at certain periods of their activity 

 the eggs are more absorptive to the rays than at others. In- 

 asmuch as this cannot be true we must look to some physiologi- 

 cal changes in the egg which render it more susceptible at one 

 time than at another. 



When protoplasm is intensely radiated enough energy is 

 absorbed to bring about a marked physical and chemical 

 change. This may consist in partial ionisation of the mole- 

 cules, or in the breaking down of compounds present. Schwarz 

 has shown that when an egg, rich in yolk, is intensely radiated, 

 the lecithin is decomposed into cholin which acts as a poison. 

 It is doubtful whether this explanation can suffice to account 

 for some of the effects reported, since not all of the cells which 

 are injured contain lecithin in large amounts. This is particularly 

 true in the case of tumor cells which contain, according to recent 

 analyses, no more lecithin than the surrounding normal cells 

 which are uninjured by the radiation, although the tumor cells 

 are obviously affected. 



Other effects of severe radiation, such as cytolysis, have al- 

 ready been mentioned. When the unfertihzed Nereis egg is 

 radiated the cell wall is greatly weakened and the vitelline mem- 

 brane altered to such an extent that it allows many sperms to 

 enter. Prolonged exposures of the sperm cause in the chroma- 

 tin a change which shows itself in the fragmentation of the 

 sperm nucleus after it has entered the egg (Hertwig '12). 



Gunther Hertwig has suggested that radium acts directly 

 on the chromatin. That chromatin is more affected by equal 

 amounts of radiation is undoubtedly true, but it is also true that 

 protoplasm is actually affected. I have elsewhere (Packard 

 '14) pointed out some objections to this theory. In terms of 

 absorptive powers it assumes that chromatin is better able to 



