256 GENETICS AND EUGENICS 



more male offspring occur. It is possible that the chemical 

 composition of the egg changes when fertilization is delayed, 

 the total energy content decreasing and so diminishing the 

 probability that the egg will develop into a female. Riddle's 

 work with pigeons suggests such an interpretation. 



Further frog's eggs may by various means be caused to 

 develop parthenogenetically. Loeb (1918) has raised twenty 

 leopard frogs from unfertilized eggs artificially stimulated 

 into development by the prick of a needle. Among these 

 frogs both sexes were represented and the chromosome num- 

 ber was found to be diploid. Accordingly sex differentiation 

 in this case would seem not to have depended upon chro- 

 mosome reduction of the ordinary sort. Again King (1912) 

 has shown that keeping toad's eggs out of water for several 

 hours after fertilization raises the percentage of female 

 young from fifty to over seventy. Hence it may be, as 

 Riddle thinks, that the natural sex tendencies of the gametes 

 may under certain conditions be overbalanced or counter- 

 acted by other agencies influencing metabolism, the eggs 

 perhaps developing parthenogenetically without passing 

 into the haploid chromosome state of ordinary gametes. 



What are we to understand by the expression, '* natural sex 

 tendencies of the gametes " ? Obviously what is meant is the 

 genetic constitution of the gametes, that is their content of 

 genes. But we have seen that strong reasons exist for believ- 

 ing that genes are found exclusively in the chromatin. If this 

 is so, "natural sex tendencies of the gametes," can mean only 

 composition of the gametes as regards chromatin. One of the 

 most important generalizations reached in recent years by 

 cytologists is this, that the chromatin composition of the 

 gamete does in reality determine its natural sex tendencies. 



In a great many animals, possibly in all, the chromosome 

 composition of the individual's cell-nuclei bears an interest- 

 ing relation to its sex. Thus in bees, ants, wasps, and related 

 insects, as well as in small Crustacea and rotifers, only females 

 develop from fertilized eggs, i. e., from zygotes, whereas males 

 develop from unfertilized eggs which have the nuclear con- 



