RELATION TO GENERAL BIOLOGICAL PROBLEMS 113 



in the body of a caterpillar, a single egg divides very 

 early into a very large number of separate embryonic 

 primordia, each of which produces an adult insect. 

 No matter how many individuals are derived from one 

 egg — the number may be even a thousand — they are of 

 the same sex. Some get more food than others, grow 

 faster, and become larger, but nothing environmental 

 affects the zygotically determined sex. The mode of zy- 

 gotic sex-determination is somewhat peculiar in that 

 eggs develop whether fertilized or not; if fertilized all 

 offspring from that egg are females; if not fertilized, 

 but developing parthenogenetically, all offspring from 

 a given egg are males. 



If then in two groups as far apart as mammals and 

 hymenoptera the fact of zygotic sex-determination is 

 proved by the phenomenon of polyembryony, it seems 

 probable that this is a very general principle; possibly 

 universal. 



SEX-RATIOS IN TWINS AND MULTIPLE BIRTHS 



Certain facts derived from a statistical study of the 

 sexes of twins of various species seem at first sight to be 

 opposed to the theory of rigid zygotic sex-determination 

 in mammals. It would appear that the proportion of 

 males to females in twins and multiple offspring, whether 

 monozygotic or dizygotic, should not differ from that 

 which holds for the species in general. In mammals 

 the ratio of males to females is in general quite close 

 to I : I, although a sHght preponderance of males is 

 usually present. 



In man, for example, it has been shown by several 

 writers that as the number of individuals to a birth 



