qa The effect of the Parasites on their hosts. 



Now in the face of this contrae! ictory evidence let us examine the case of Inachus 

 scorpio and Sacculina. We see here that the infected males may develope the perfect secon- 

 dary sexual characters of the female without the presence of an ovary' at ali. The full deve- 

 lopment of the female secondary sexual characters is therefore not dependent on the presence 

 of a difFerentiated ovary. But we know frorn the subsequent history of these males that they 

 possess the potentiality of forming an ovary, because they are capable of regenerating an 

 hermaphrodite gonad with mature ova. Now we also know that infected females, which have 

 the ovaries degenerate, do not possess fully formed female secondary sexual characters. It is 

 clear therefore that what causes the development of the female secondary sexual characters 

 is the potentiality to forni a differentiated ovary and vice versa in the male. Now we 

 do not know in what this potentiality lies, but if we frame some material hypothesis to account 

 for the faets, we may safely suppose that this potentiality lies in the presence of some for- 

 mative substance in the body which causes the differentiation of the primary and of the 

 secondary sexual characters. In other words, the differential development of the secon- 

 dary sexual characters is not due to the presence of a corresponding differen- 

 tiated gonad, but the differential development of both primary and secondary 

 sexual characters is due to a third common factor, which may be assumed to be the 

 presence in the body of a sexual formative substance. This is the only theory 

 which can account for the assumption in infected males of I. scorpio of the secondary sexual 

 characters proper to the female when they do not possess an ovary, and for their subsequent 

 development of an ovary. 



In the light of this theory also ali the apparently contradictory evidences from ordinary 

 castration etc. are brought into harmony. For we suppose that the differentiation of the gonad 

 consists in its taking up from the body the sexual formative substance; the removal of the 

 gonad therefore after it has begun to be differentiated means the removal of a large quantity 

 of the formative substance, and this will react on the development of the secondary sexual 

 characters, according to the degree in which the formative substance was centred in the gonad; 

 but the removal of the gland at a very early stage before differentiation has set in, as in 

 Kellogg's experiment, need not influence the development of the secondary sexual characters, 

 because but little of the formative substance will have been removed. In any case by referring 

 the undoubted correlation that exists between the primary and secondary sexual characters to a 

 third factor, we explain to a large extent why this correlation appears so imperfect and erratic. 



We have arrived then at a conception of sexual differentiation which points to a for- 

 mative substance at work in the body causing the visible differentiation of the germinai cells 

 into male and female, and also calling forth the development of the appropriate secondary 

 sexual characters. 



The question, therefore, that was first put as to how the parasite brings about the 

 parasitic castration, comes to be, how does the parasite affect the sexual formative substance? 

 We imagine this substance to be a product of the general metabolism of the body which 



