THE CELLULAR BASIS 155 



they have entered the oviducts, the proportion 

 of males to females is enormously increased. 

 A wholly similar result has been observed by 

 Pearl and Parshley in the case of cattle, where 

 delayed fertilization of the egg leads to a great 

 preponderance of males. Hertwig attempts 

 to explain his extremely interesting and im- 

 portant observations as due to the relative size 

 of nucleus and cytoplasm of the egg; but in 

 general this nucleus-plasma ratio may vary 

 greatly irrespective of sex and there is no 

 clear evidence that it is a cause of sex 

 determination. 



Miss King also, working on toad's eggs, has 

 increased the proportion of females by slightly 

 drying the eggs or by withdrawing water from 

 them by placing them in solutions of salts, 

 acids, sugar, etc., but the manner in which 

 drying increases the proportion of females is 

 wholly unknown. Quite recently Whitney has 

 shown that in certain species of rotifers a 

 scanty diet produces only female offspring 

 while a copious diet produces as high as 95 

 per cent of male offspring, thus confirming the 

 conclusions of many earlier investigators that 



