y<: The effect of the Parasites on their hosts. 



becoming ova under the influence of the sexual formative substance, while others are capable 

 of becoming male cells. 



In this manner I would account for the peculiar lateral or transverse correlation in 

 gynandromorphous animals ; in them also it is, I think, possible to hold that a sexual forma- 

 tive substance is present circulating in the whole body, which is the condition of the herma- 

 phroditic development, but this substance affects different parts of the body in a different 

 manner owing to a determining differentiation already present in the cells of these different 

 parts. In the special case of gynandromorphous insects, theories have been suggested by both 

 Boveri and Morgan (see 22) to account for such a cellular differentiation; thus Boveri 

 suggests that in a gynandromorphous insect the spermatozoon, bearing the determination of 

 one sex, may have united, not with the egg nucleus, but with one of the products of its first 

 division: then supposing that the right and left sides of the body correspond to the first two 

 blastomeres it might result that the two halves of the body would be sexually different. 

 Morgan has suggested a slightly different but analogous interpretation. 



If we admit therefore that sexual differentiation as a whole is due to two interacting 

 factors, namely the factor of a generally distributed formative substance, and the factor of a 

 local and specific differentiation of particular cells, we obtain what is probably an approxi- 

 matelv true hvpothesis, and moreover an hypothesis which is in excellent accord with the 

 whole tendency of modern experimental embryology. Thus E. B. AVilson 20 , in whose 

 recent papers may be found not only the most judicious experimentation but the most critical 

 analysis of contemporary results, has adopted a view of development which in the combina- 

 tion of the idea of formative stuffs and of localized germinal areas, is fundamentally similar 

 and was indeed suggestive of our view of sexual differentiation : while the analogous position 

 of this author in relation to the particular problem of sex as being perhaps determined pri- 

 marily by the presence or absence in the germ cells of particular structural elements but 

 being also partly a question of metabolism, exhibits the same sympathetic grasp of the just 

 claims of both ejiigenetic and evolutionary ideas in embryological theory. 



Returning to our special subject of parasitic castration, we must next fully discuss the 

 remarkable and, as far as I know, hitherto unsuspected difference between the male and 

 female sex in their reaction to their parasites. This difference, which is fully sustained by 

 all my observations, consists in the fact that, whereas the male sex or at any rate a great 

 number of the male sex in infected crabs can under proper conditions assume the full female 

 characters, the female sex on the other hand cannot assume any distinctively male character 

 under similar conditions. In this opinion I again run slightly counter to the views expressed 

 by Giard, but I hope that in the light of the evidence produced he may modify his con- 

 clusions. It is of course undeniable that the secondary sexual characters of infected females 

 may be in a degenerate condition, and that this degenerate condition may be an apparent 

 approach to the male condition, but there is no secure evidence, as far as I can see, that the 

 females under the influence of parasitic castration can assume characters distinctively and 



