Luternal Factors of Sex Determination 413 
finds, moreover, that “identical” twins may be of opposite sexes. 
He believes, therefore, that the closer resemblance sometimes 
observed in children born at the same time is due to the similar 
conditions of the germ-cells of the parent at the time of concep- 
tion, or to the more nearly similar environment under which the 
twins are reared. These two conditions he includes in what 
he calls the “heredity” of the offspring. In general, the term 
“heredity”’ has a different meaning to biologists, viz. to express 
the idea of the inheritance derived from the germ-cells apart 
from the special environment to which they or the offspring 
that they produce have been subjected. Disregarding, however, 
this difference of definition, the facts recorded by Thorndike 
cast serious doubts on the assumption that there is a sharp dis- 
tinction between the two kinds of twins. If the conclusion is 
substantiated, we have little evidence left on which to base the 
assumption that identical twins owe their resemblance and their 
sex to a common origin. If then, as seems probable, identical 
twins, double monsters, and the like are more often of the same 
sex, it would appear that special external conditions existing at 
the time in one or in both parents determine the sex of the 
embryo. Improbable as this may seem, a careful reéxamina- 
tion of the evidence should be made. 
Two Recent Theories of Sex Determination based on the 
Assumption of Male and Female Eggs 
Beard has proposed an hypothesis of sex determination that 
rests on the assumption of male and female eggs. He suggests 
that the sex of the individual is determined by the egg alone. 
According to his view, the egg and its two polar bodies are of the 
same sex, male or female. Just as there are two kinds of eggs, 
there are assumed to be also two kinds of spermatozoa; but these 
have no function in respect to sex determination, and Beard as- 
sumes, in fact, that one kind has even lost its power to fertilize 
the eggs. When the female egg is fertilized it gives rise to a 
female, and the male egg toa male. The female produces eggs 
again of two kinds, male and female eggs; the male two kinds of 
