356 THE GERM-PLaSM 



the male or female part of which may be external, and conse- 

 quently dominant. 



This is only a metaphor, and is not intended to represent the 

 actual occurrences. We are ignorant of the forces and sub- 

 stances which are here concerned, but we at any rate know that 

 the idioplasm of the primary germ-cells in the higher animals is 

 still capable, in by far the most cases, of giving rise to either 

 kind of orerm-cells, and that the decision as to whether the 

 germ-cells will develop into ova or spermatozoa occurs at some 

 early stage in embryogeny : in the eggs of the bee it takes place 

 at the beginning of embryonic development, long before the 

 first primary germ-cell is differentiated, while in other animals 

 it perhaps occurs at a later stage. The well-known researches 

 of Siebold and Leuckart, prove that at any rate in the case of 

 bees, this decision undoubtedly rests upon the occurrence or 

 omission of fertilisation, — it occurs, that is, at the time when the 

 germ-plasm which controls the new organism is constituted ; and 

 this fact seems to me to be of great importance. If fertilisation 

 takes place, the organism develops into a female, and if not, 

 a male will result. This at least proves that the decision jnay 

 take place at such an early stage, and I doubt whether it can in 

 any case occur later : in some animals, at any rate, it occurs 

 still earlier, during the period of maturation of the egg. The 

 Phylloxera lays large eggs which produce females, and small 

 ones from which males arise. Both become fertilised, so that 

 in this case fertilisation takes no part in the determination 

 of sex. 



These questions cannot be discussed more fully here. We 

 are only concerned in making it clear that sexual determinants 

 in the sense indicated must be assumed to exist, and that both 

 kinds are contained in the primary germ-cells. The following 

 considerations will render it apparent why we have assumed 

 these determinants to be double, — i.e.^ to consist of two groups 

 of biophors of a common origin lying close to one another. Even 

 apart from such low organisms as Volvox, it is well known that 

 the male and female individuals only differ from one another as 

 regards the kind of sexual cells they produce in many of the 

 lower Metazoa — viz., the sponges and Hydromedusae. In these 

 forms the sexual determinants alone are double. In most other 

 animals, however, the difference of sex is not confined to the germ- 

 cells, but affects the soma itself to a greater or less extent. Hence 



