HEREDITY 187 



when the contents of the egg are cut in two during the early periods of 

 development, e.g. when the first two cleavage spheres become separated 

 from each other. In the case of mammalian or human twins, we possess 

 a criterion for the fact whether they come from one or two eggs in the 

 condition of the egg membranes. Twins coming from different eggs 

 have as a rule separate chorions. This follows from the development 

 of the chorion. In all cases where twins have a common chorion they 

 have also identical sex. This indicates that the sex of the embryos was 

 determined before the germ was split into two parts, and as this must 

 occur in the earliest stages of development, it follows that the sex of an 

 embryo is definitely determined very early; how early can only be 

 guessed at in mammalians, but in certain lower forms it can be shown 

 that the sex is already preestablished in the egg before the egg is even 

 fertilized. 



A striking example for this assertion was discovered by Korschelt 

 in Dinophilus apatris, a worm of the group of Turbellarians. As 

 Korschelt's paper is not accessible to me, I quote the observation after 

 Lenhossek.* Dinophilus lays two kinds of eggs, the one large and 

 opaque, the other small and transparent. The eggs are fertilized inside 

 the body of the female, and are afterward deposited in the sea water. 

 Korschelt separated the two types of eggs, and found that the large 

 opaque eggs give rise to females, the small transparent eggs to males. 

 If this observation is correct, there can be no doubt left that in this case 

 sex is already determined in the egg before the egg is fertilized. 



Facts of a somewhat analogous character seem to exist in a number 

 of forms such as plant lice, Cladocera and Rotifers. The Aphides are 

 viviparous as long as the temperature is not too low and the plant is 

 not drying out. Under such conditions they give rise to offspring of one 

 sex only, namely, females. These reproduce females parthenogeneti- 

 cally which possess no receptacle. When the plant dries out, or the 

 temperature becomes low, in addition to females, males are also produced. 

 The females which originate at this time possess a receptacle, and hence 

 can pair. After pairing they are not viviparous, but lay fertilized eggs, 

 the so-called winter eggs. From such eggs parthenogenetic females 

 invariably arise, and now the cycle may be repeated. It is obvious that 

 at least the sex of the winter egg is determined as soon as it is formed. 

 The same is probably true also for the sex of the embryo which pro- 

 ceeds from the summer eggs. In Cladocera conditions are not very 

 different. From the winter egg females invariably arise, and these 

 give rise parthenogenetically to females until, under conditions which 

 have not been sufficiently investigated, males and females are formed, 



* Lenhossek, Das Problem der geschlechtsbestimmcnden Ursachen, Jena, 1903. 



