Internal 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 reexamina- 

 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 to a male. The female produces eggs 

 again of two kinds, male and female eggs ; the male two kinds of 



