662 THE PHYSIOLOGY OF REPRODUCTION 



respective functions are to produce spermatozoa and ova. This 

 condition is described as dioscious. Again, in a relatively small 

 number of animals, of which the bee is a familiar example, there 

 are three kinds of individuals perfect females, imperfect females, 

 and males. In a few insects there are even more than three kinds. 

 Lastly, in certain of the lower animals the females can reproduce 

 ova which are capable of developing into adult individuals without 

 conjugating with spermatozoa. This method of reproduction is 

 described as parthenogenetic (see p. 230). 



Among dioecious animals the two sexual individuals are generally 

 produced in approximately equal numbers. Thus, in man the 

 number of male births is only slightly in excess of the number of 

 female births, the proportion varying very slightly in different 

 countries, 1 while in those races in which the numerical proportion 

 of the two sexes among the adult population is very unequal, 

 inequality is due to a higher death-rate of children belonging to one 

 sex. Thus among the Todas the pronounced preponderance of males 

 over females results from the practice of female infanticide. 2 



Theories of sex determination may be conveniently arranged 

 under three headings : (1) Those in which it is assumed that the 

 sexual condition of the individual is determined subsequently to 

 fertilisation and during embryonic or larval life ; (2) those which 

 assume that sex is established either at the moment of fertilisation 

 or prior to fertilisation ; and (3) those which limit sex-determination 

 to no particular period, or which definitely assert that sex may be 

 established at different periods of development in different animals. 



(1) THEORIES WHICH ASSUME THAT SEX-DETERMINATION TAKES 

 PLACE SUBSEQUENTLY TO FERTILISATION 



In tadpoles sex is apparently indeterminate until a relatively 

 late stage of development, but it is said to be established at the 

 time of metamorphosis. Born, 3 and subsequently Yung 4 and certain 

 other investigators, have adduced evidence in support of the view 

 that the sex is determined by the quantity and quality, of the food 

 supply. Thus they claimed that they could produce an excess of 

 females by feeding the tadpoles upon a meat or fish diet. The 

 conclusions of these authors, however, are hardly borne out by 



1 Bodio, "Movimento della Populazione," Gonfronti Internazionali, 1895. 



2 Punnett, "On the Proportion of the Sexes among the Todas," Proc. C<<il>. 

 Phil. Soc., vol. xii., 1904. 



3 Born, " Experimentelle Untersuchungen ueber'die Entstehung der Gesch- 

 lechtsunterschiede," Brest auer drztliche Zeit., 1881. 



4 Yung, " De 1'Influence de la Nature des Aliments sur la Sexualite," C. R. 

 de I'Acad. des Sciences, vol. xciii., 1881. 



