658 THE PHYSIOLOGY OF REPRODUCTION 



When once we admit the existence of latent {i.e. recessive) 

 sexual characters in individuals in which the characters of one 

 sex are dominant, and that under certain circumstances those 

 of the latent sex can develop at the expense of the dominant ones, 

 in response to appropriate physiological stimuH, we are compelled 

 to acknowledge also that the sex of the future individual is 

 not always predetermined in the gametes or even in the fertihsed 

 ovum, but may be called into being at a later stage of life. 



Such an admission is of course opposed to some extent to 

 the modern tendency to believe that sex is fixed irrevocably in 

 the fertihsed ovum or in the gametes before fertihsation ; but 

 while there is evidence amounting to proof that this is the case 

 in some forms of life, it does not necessarily follow that it is true 

 of aU metazoon animals, or even that it is uniformly so of the 

 particular species which have been investigated. On the other 

 hand, many of the facts enumerated above point to the con- 

 clusion that the sex of the future organism is determined in 

 different cases by different factors and at different stages of 

 development — either in the unfertihsed gamete, or at the 

 moment of fertihsation, or in the early embryo, while the effects 

 of castration indicate that an alteration in the metabohsm, even 

 in comparatively late hfe, may initiate changes in the direction 

 of the opposite sex.^ 



APPENDIX 



Braem ' has described an experiment in which he divided in half a 

 mature female of the annelid, Uphryotrocha puerilis. The head portion 

 after some weeks regenerated and produced spermatozoa, but the ova 

 almost disappeared. There was no sign of hermaphroditism at the 

 outset, and Braem regards the case as one of change of sex resulting from 

 altered conditions. 



In a recent paper Potts' has adduced evidence that in certain 

 hermaphrodite Nematodes, in Rhabdocoel Turbellarians and in Rhim- 

 cephala the monoecious condition has arisen through the sperms 

 developing in the ovaries in gradually increasing numbers in successive 

 generations. 



Smith * states that in the male of Inachus affected by Sacculina the 

 assumption of adult female characters is due to the formation of a yolk 

 substance (or female generative substance) similar to that normally 

 elaborated in the ovaries. 



' Such changes are notoriously more difficult to effect after puberty than 

 before it. ' Braem, Anat. Anz., vol. xxxiii., 1908. 



' Potts, Quar. Jour. Micr. Scisnce, vol. Iv., 1910. 

 * Smith, Quar. Jour. Miar. Science, vol. Iv., 1910. 



