CHAPTER XV 



THE FACTORS WHICH DETERMINE SEX 

 " What was a question once is a question still." — Bacon. 



A WORK upon the Physiology of Eeproduction would be incom- 

 plete without some reference to the problem of sex-determina- 

 tion, and- some account of the more recent attempts which 

 have been made towards its solution. A rSsumi of some of the 

 more important papers and memoirs is given by Morgan in his 

 work on Experimental Zoology} and the reader is referred 

 to this volume for further references and fuller information in 

 regard to certain of the points discussed. It is hoped, however, 

 that the present summary may prove useful if only as a supple- 

 ment to Morgan's discussion, since certain important papers 

 deahng with sex-determination and containing an account of 

 experimental investigations have been published since the 

 appearance of Morgan's volume, and these papers I have en- 

 deavoured to summarise here. Moreover, some of the more 

 recent observations, and more particularly those relating to 

 " parasitic castration," have necessitated a further revision 

 of the conclusions previously arrived at. 



Reproduction in organisms may occur by simple fission or 

 budding, in which case it is said to be asexual, or it may involve 

 the union of two conjugating ceUs, which in Metazoa and Meta- 

 phyta are specially differentiated for the purpose, and are known 

 as ova and spermatozoa. In some animals these two types of 

 cell are produced by the same individual, which is then said to 

 be hermaphrodite or monoecious, but such a condition is rare 

 or absent altogether among the highest forms of life. In the 

 vast majority of animals there are two sexes — that is to say, 

 two kinds of sexual individuals, the male and the female, whose 



^ Morgan, Experimental Zoology, New York, 1907. See also Geddes 

 and Thomson, The Evolution of Sex, Revised Edition, Londoi-, 1904, 

 and Thomson, Heredity, London, 1908. 



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