432 CHAKLES RUSSELL BARDEEN 



either within a few cell generations or not until a cytologically 

 remote period. The destruction of ova, spermatocytes of the 

 first order and spermatogonia is an illustration of the former, the 

 failure of a limb to develop after exposure of a spermatozoon is 

 an example of the latter. 



Apparently the most sensitive substances in the ovum are 

 those destined to determine the final stages in somatic morpho- 

 genesis. The more severe the exposure the earlier the cell gener- 

 ations in which the effects of exposure appear. 



The sensitiveness of tissues to the rays depends upon the amount 

 of differential morphogenesis which they are undergoing, or are 

 destined to undergo, upon the rapidity of the production of nu- 

 clear material and upon peculiarities either individual or specific. 

 The most susceptible tissues are those undergoing differential 

 morphogenesis accompanied by a rapid production of determin- 

 ative nuclear material (germinating seeds, ova during the earl} 

 cleavage stages, germinative epithelium of the testicles.) On the 

 other hand, the least susceptible of organisms seem to be some of 

 the unicellular plants and animals (bacteria, paramoecia.) In 

 these, in spite of rapid production of nuclear material, resistance 

 to the X rays is marked. This resistance may be attributed in 

 part to specific characteristics (unicellular organisms are known to 

 vary greatly in their sensitiveness to light) and in part it may, 

 as pointed out above, be due to the relative lack of disturbance 

 of organic stability in reproduction through simple cell division. 



In the higher organisms much idiosyncracy is shown in the 

 sensitiveness of tissues to the X rays. The human skin, for in- 

 stance, of some individuals is relatively resistant, in other indi- 

 viduals it is "burned" by relatively slight exposures, while in 

 rare cases the epithelium is altered so that it gives rise to epithel- 

 iomata. If frog eggs be fertilized by exposed sperm most of the 

 ova develop into abnormal larvae. A few may pass through appar- 

 ently normal tadpole stages and then show abnormalities during 

 metamorphosis (failure of one or more legs to develop) and some 

 may become apparently normal frogs. These differences must 

 depend either upon individual differences in susceptibility of 

 the exposed spermatozoa or upon differences in susceptibility of 



