544 THE POPULAR SCIENCE MONTHLY 



metabolic fluctuations, now in favor of maleness (katabolic), now in 

 favor of femaleness (anabolic), the net result being an approximately 

 equal proportion of males to females. In other words, there must 

 be selective fertilization, that is, a female ovum must be fertilized by 

 a male sperm, and vice versa. The various assumptions made are not 

 pure speculations, but rest upon many facts. Some animals do possess 

 two kinds of eggs, of which one (the larger) develops into females and 

 the other kind (the smaller) into males; there are many instances of 

 selective fertilization; and many animals do produce two kinds of 

 spermatozoa. 



If there is only one kind of egg, as Correns suggests — as may well 

 be the case, since of the four potential ova, three (the polar bodies) 

 degenerate during maturation and only one becomes capable of fertiliza- 

 tion — and two kinds of spermatozoa, the explanation of sex becomes 

 very much simpler. One sex (female) must then be pure in respect to 

 sex (a homozygote) and the other must be a sex-hybrid (heterozygote). 

 If the egg is female in tendency, in order that there should appear 50 

 per cent, males, maleness must dominate over femaleness. In any case, 

 an interpretation of the facts involves the application of some phase of 

 Mendelism. As will appear farther on, the case of the honey-bee, ants 

 and plant-lice offer serious obstacles to the universal application of 

 Correns's interpretation, and even of the whole Mendelian scheme. 



Within the last decade three main positions have been advocated 

 by various investigators in regard to the cause of sex. The posi- 

 tion held by Beard and his school is to the effect that there are two 

 kinds of eggs, male and female, and one kind of spermatozoa ; and that 

 sex is determined exclusively by the egg, the spermatozoon having no 

 role in sex production. This position can be supported by various 

 facts. A certain worm (Dinophilus apatris) carefully studied by Kor- 

 schelt, is known to produce two kinds of eggs, large and small, the 

 former developing into females, the latter into males. Of course it 

 might be urged that in consequence of selective fertilization only the 

 small eggs are impregnated by a male-producing spermatozoan and the 

 large eggs by a female-producing, and an entirely different interpreta- 

 tion of the facts would be necessitated. But in the case of Hydatina 

 senta, a rotifer or " wheel animalcule," where large and small eggs are 

 also laid, the former without fertilization develop into females and 

 the latter into males. However, both kinds of females, the large-egg- 

 laying kind and the small-egg-laying kind, came originally from fer- 

 tilized eggs, and it may be that a difference in metabolic activity of the 

 several females kept the eggs of the one small and allowed those of 

 another to grow large and so gave the victory to the female determinant 

 in the well-nourished egg. This assumption is supported by the fact 

 that the amount of nourishment taken by the young female between 



