THE DETERMINATION OF SEX 477 



But ordinary twins, which arise from two distinct ova developing 

 simultaneously, are often of different sexes. Why is there this 

 difference ? The same question arises when we contrast the 

 " poly-embryony " (i.e. numerous embryos from one ovum) 

 which occurs in some insects with the ordinary simultaneous 

 production of many offspring from as many ova. In poly- 

 embryony the offspring are all of the same sex ; in ordinary 

 multiparity both sexes occur in varying proportions. As we shall 

 see, this particular case of the general problem is very suggestive. 



In one household the family consists of boys and girls, in a 

 second of boys only, in a third of girls only — what determines 

 this ? A setting of hen's eggs gives rise to cocks and hens in 

 varying proportions — is the proportion practically modifiable ? 

 A guillemot usually lays a single egg in a season — what determines 

 the sex of the offspring ? It is well known that the unfertilised 

 eggs of a queen-bee develop into drones, while the unfertilised 

 eggs of aphides produced all through the summer months develop 

 into parthenogenetic females, until at the end of the season, in 

 autumn, males are produced. What does this mean ? 



A great step would be gained if we could narrow the issue in 

 various cases by answering the question, When is the sex of the 

 offspring finally determined ? How long may a germ-cell 

 remain with the potentiality of either sex ? Is there sex-deter- 

 mination before fertilisation or during fertilisation, or not until 

 after fertilisation ? Are there cases where we must admit that 

 the embryo has the potentiality of either sex ? Is the deter- 

 mination early in some types, such as Mammals, and later in 

 other types, such as Amphibians ? 



Prof. V. Haecker has proposed a useful terminology. Sex- 

 differentiation implies that one of the two sex-primordia in the 

 germ-cell is activated, while the other remains latent, (a) This 

 may occur before fertilisation — progamic sex-differentiation — as 

 in the large and small ova of Dinophilus, Rotifers, and Phylloxera. 

 (b) Or it may occur at the moment of fertilisation — syngamic 



