86 THE DETERMINATION OF SEX 



important generalizations. How great the vaaiations are is 

 shown by the following table: 



PER CENT. OF MALES. 



Loligo 16.6 Man 106.9(105.3?) 



Octopus 33.3 Domestic dog 138 . o 



Horse 98.3 Cottus 188.0 



Songbirds 100. o Lophius 385.0 



Herring loi.o Latrodectus 819.0 



Cat 105.0 



There are two series of cases known in which the sex 

 is determined in advance. The first series comprises several 

 species of animals of various classes which produce two sorts 

 of eggs, differing in size. Such eggs occur for example in 

 the worm Dinophilus, in many rotifers, as, for instance, 

 Hydatina, in daphnids, in Phylloxera, and other forms. The 

 large eggs produce only females, the smaller only males.^" 

 Oskar Schultze was induced by these facts to maintain that 

 sex is determined in the ovum. More recent discoveries have 

 rendered Schultze's theory superfluous. 



The second series of cases is afforded by the eggs especially 

 of various insects which may be developed parthogeneticaUy, 

 as occurs, for example, in Phylloxera. The fertilized ova 

 produce females only, the unfertilized on the contrary, 

 according to conditions, either males or females. For a long 

 time it was hoped, though in vain, to secure the explanation of 

 the determination of sex by the exact study of such ova. 



Naturahsts have long directed their efforts toward dis- 

 covering external conditions, the action of which determines 

 sex. It appears now to be established that imder certain 

 conditions the proportions of the sexes may be altered by ex- 

 ternal conditions. The experiments of Richard Hertwig, 

 which he published in 1907, excited great interest. They have 

 been extended by his pupil, Kuschkakewitz.*^ Hertwig 



