MODIFICATION OF THE GERM-CELLS IN MAMMALS 197 



ject to fertilization by either the X or Y type of spermatozoa. 

 Even though the egg might be practically equally accessible to 

 both types under normal conditions, a peculiarly affected egg 

 might become much more readily fertilized by one class of 

 sperm than the other, and almost all male offspring might re- 

 sult in one case and females in the second. One might feel 

 that these are large suppositions on the basis of the minute 

 differences between the two groups of sperm. But it may be 

 replied that the differences are only minute from the standpoint 

 of the minuteness of the structure considered. Corresponding 

 differences between great things would necessarily* seem much 

 more important, but with present powers of observation only 

 very great differences between cellular structures are visible 

 at all. 



There is evidence from a study of the control of sex-ratios 

 in normal guinea-pigs to indicate that certain females have a 

 very strong tendency to produce male offspring regardless of the 

 male with which she is paired (Papanicolaou, '15). Other fe- 

 males have as decidedly marked tendency to produce female 

 offspring. Such females may be said to have either a male or 

 female tendency, while other females are in this regard indif- 

 ferent, producing as many offspring of one sex as of the other. 

 These tendencies may be explained in accord with the above dis- 

 cussion as due to a high affinity for one type of sperm on the 

 part of the ova of one female, while the ova of another female are 

 particularly susceptible to fertilization by the other class of 

 sperm. The indifferent females are those with ova which are 

 fertilized equally as well by one type of spermatozoa as the 

 other. There are striking cases among the ascidians and other 

 forms illustrating selective fertilization, and the above suggestions 

 are by no means without foundation. 



Certain male guinea-pigs are also known to have a strong 

 tendency to beget female offspring regardless of the females with 

 which they are paired. Other males have a high male-producing 

 tendency and still others are more or less indifferent in their 

 sex-determining quality. This may be readily imagined to re- 

 sult from a difference in the activity or fertilizing powers of the 



