624 THE PHYSIOLOGY OF REPRODUCTION 



respective functions are to produce spermatozoa and ova. This 

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

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

 are three lands 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 in- 

 dividuals without conjugating with spermatozoa. This method 

 of reproduction is described as parthenogenetic (see p. 216). 



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 rela- 

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

 at the time of metamorphosis. Born, 3 and subsequently 



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



* Punnett, "On the Proportion of the Sexes among the Todas," Proc. 

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



3 Born, " Experimentelle Untersuchungen ueber die Entstehung der 

 Geschlechtsunterschiede," Breslauer tirztiiche Zeit., 1881. 



