436 CHARLES RUSSELL BARDEEN 



Fertilization 



There is no definite evidence that exposure to the X rays affects 

 the power of spermatozoa to fertiUze ova. The percentage of 

 ova fertiUzed by spermatozoa in different experiments varied 

 greatly; in some experiments nearly all, in others few or no ova 

 were fertilized. The percentage of the ova fertilized seems to 

 depend chiefly on the maturity of the sperm and the ova at the 

 time of making the experiment. If the ova are mature nearly 

 all are fertilized by spermatozoa obtained from males caught not 

 too long after emerging from hibernation. Males, however, 

 before being caught and brought to the laboratory may have dis- 

 charged so large a proportion of their mature spermatozoa that 

 an emulsion made from the testicles can fertilize comparatively 

 few eggs. If the ova are in the right condition those fertilized 

 seem to develop in essentially the same manner whatever be the 

 proportion of the fertilized to unfertilized eggs. Apparently only 

 those spermatozoa can fertilize which can fertilize normally. 

 Uterine ova early in the breeding season fertilized by ripe sperm 

 practically all develop normally. Late in the season the uterine 

 ova are frequently over-mature. The capacity of these over- 

 mature eggs for fertilization seems frequently to be reduced and 

 some of them, even when fertilized by normal sperm develop 

 abnormally. In studying the effects of the X rays or other agents 

 on spermatozoa and ova it is therefore necessary to take the phys- 

 iological conditions of the sex cells into consideration and to make 

 careful control experiments. When careful control experiments 

 are made it is usuall}^ found that the percentage of eggs fertilized 

 by the control sperm corresponds closely with the percentage of 

 those fertilized by the sperm exposed to the X rays. Thus for 

 instance, in one experiment where frog sperm was exposed for 

 twelve minutes to the X rays the percentage of eggs fertilized 

 by the X ray sperm was 25.3 and that of those fertilized by the 

 control sperm 26. In some of my first experiments a greater 

 proportion of the eggs were fertilized by the control than by the 

 exposed sperm, but all subsequent experiments have led me to 



