SUSCEPTIBILIIY OF AMPHIBIAN OVA lO X-RAYS 433 



the ova to alterations in the fertihzing spermatozoa or to both 

 factors. Since not all of the molecules of a gas exposed to the 

 rays are ionized, we find in inorganic as well as in organic com- 

 pounds variations in susceptibility. 



To some extent, at least, idiosyncracy depends upon the general 

 health of the exposed organism. When fertilized by exposed 

 sperm ova which are overripe are much more prone to show 

 marked deformities early in development that normal ova do. 

 The susceptibility of cancer cells to the rays may be due in part 

 to a weakening of the cancer cells produced by the reaction of 

 the healthy tissues against the cancer tissue. In most instances, 

 however, it is at present impossible to determine just what internal 

 conditions make one organism at a given stage more susceptible 

 to the X rays than a sister organism of the same stage ip. 



In the following study of variations in susceptibility to irradia- 

 tion of amphibian sex cells and larvae at different stages of devel- 

 opment, individual as contrasted with specific sensitiveness has 

 to be taken into account. When a given lot of organisms has 

 been exposed some individuals will show the effects far more than 

 others. Those most affected will show the effects first, those' 

 least affected will show the effects late in developments or not 

 at all. In a given group the greater the percentage of organisms 

 severely affected the greater we may assume the susceptibility 

 of that group. Thus we may compare the susceptibility of organ- 

 isms at different stages of development by comparing the per- 

 centage of severely affected organisms in the groups exposed. 

 In the various experiments the developing eggs and larvae were 

 kept in large shallow glass dishes. The water was either fre- 

 quently changed or was kept pure by a constant small stream of 

 aeriated water. A small amount of various kinds of lake vege- 

 tation was kept in the dishes and the older larvae and tadpoles 

 were fed with various kinds of food. Control experiments were 

 carried on in all cases and every effort was made to keep the con- 

 trol and experiment specimens under equivalent conditions. By 

 the methods used it is easy to carry the developing organisms up 

 to the stage of well developed tadpoles, when there are not too 

 many organisms in the dish. Where not otherwise stated, 100 



