THE DETERMINATION OF SEX 477 



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 



