68 A. RICHARDS. 



with a few exceptions have not proven suitable for a study of 

 cytological details. 



With the idea of finding more exactly what occurs to individual 

 cells when exposed to X-rays, the writer studied the eggs of the 

 freshwater snail, Planorbis lentus, in relation to this problem. 

 By choosing such a form, several advantages are gained. The 

 eggs divide in a very definite manner and the normal course of 

 their development has been carefully observed, as has that of 

 many related gasteropods. It is, therefore, a comparatively 

 simple matter to study at least the more gross effects of the 

 exposures, and to compare with experiments of most varied 

 character upon forms not dissimilar in the details of their develop- 

 ment to the one here employed. 



Furthermore, there is hope that the use of radioactivity in 

 experiments on eggs of well-known type may lead to further 

 knowledge of the principles of egg structure and organization. 

 The reactions which the eggs give to exposure to X-rays must be, 

 if constant, the expression of some quite definite mechanism within 

 the egg to which the X-rays act as a stimulus. A comparison of 

 these reactions with the responses of this mechanism to other 

 stimuli of different nature very possibly may lead to interesting 

 conclusions as to the nature of the mechanism itself. This, of 

 course, is the much broader biological problem. 



In most of the work which has been done recently from the 

 standpoint of pure biology radium has been the agent used for 

 experimentation. In general one would expect that the results 

 obtained from radium rays would be similar to those from X-rays; 

 but it is not possible to predict that such is the case and the 

 results with radium have been comparatively meager. Radium 

 rays are of three kinds, a, /3, and 7; of these the 7 rays are the 

 more penetrating and to them are probably due most of the 

 effects on living forms. From comparative studies made by 

 physicists it is well known that the 7 rays of radium are quite 

 similar in many particulars to the X-rays, and it is stated by 

 Rutherford that they are in fact the more penetrating X-rays. 

 In view of the facts, therefore, that it is perhaps easier to un- 

 derstand something of the nature of the disturbances caused by 

 the X-rays, and that this form of radio-activity is more easily 



