548 THE POPULAR SCIENCE MONTHLY 



action of two factors, both unknown quantities. By the union of an 

 egg and a sperm an organism is produced which has a certain sex. If 

 we let X represent the unknown sex tendency of the ova, and Y the 

 unknown sex tendency of the spermatozoon, then we may state the 

 process of sex production thus : X -j- Y = sex of offspring. If we 

 could discover the value of X or Y we could solve the equation and 

 discover the secret of sex. Correns, as the result of experimentation 

 with certain flowering plants since 1900, has found the value of X in 

 these particular forms and has thus contributed invaluable facts for 

 the reinterpretation of former observations and for the formulation of 

 a wider generalization regarding sex phenomena. 



Many facts are known which leave little doubt that in most animals 

 sex is absolutely fixed in the fertilized ovum. A real exception is the 

 case of the frog, where the embryo or even the larval tadpole is her- 

 maphroditic, i. e., it contains both ovaries and testes. In a later stage 

 of development one or the other of these pairs of organs degenerates 

 when the frog becomes a definitive male or a definitive female. 



In the human species twins are frequently of the same sex, either 

 male or female. Such twins are known as " identical twins " when 

 enveloped in a common chorion or foetal membrane. They are the re- 

 sult of an independent development of accidentally separated cells at 

 the two-cell stage of development. Double monsters likewise are 

 always of the same sex. " Ordinary twins " are as frequently of 

 opposite sex as of the same sex. In this case each foetus is envel- 

 oped in its own chorionic membrane. Such twins are the result of 

 the synchronous development of two ova simultaneously successfully 

 fertilized. These facts show that sex is already determined in the 

 fertilized egg and before the first segmentation. Similar evidence is 

 contributed by Jehring, who studied poly-embryony in a species of 

 armadillo (Tatusa hyhrida) found in Paraguay. Here as many as 

 eight offspring appear at a single birth. Jehring reports that all eight 

 fetuses are enclosed in a common fetal envelope. Hence the eight 

 offspring must result from the development of the products of division 

 as the eight-cell stage of segmentation. Since the offspring are all of 

 the same sex, sex must have been already determined in the fertilized 

 egg prior to the first cleavage. Again, Professor Silvestri, of Naples, 

 has quite recently contributed new facts which lead to the same posi- 

 tive conclusion. He has discovered that Litomastix, a kind of bee 

 (Chalcidse), lays its eggs in the egg of a moth, Plusia. As the latter 

 develops into a larval caterpillar, the egg of Litomastix segments into 

 a chain of many eggs, each of which gives rise to an embryo bee. The 

 caterpillar may contain a hundred such embryos and they are all of 

 the same sex — female if the egg was fertilized; male if unfertilized. 

 Sex must have been already fixed in the fertilized egg. But this is an 



