126 THE PHYSIOLOGY OF REPRODUCTION 



and extruded. The polar bodies undergo degeneration. Meanwhile 

 the nucleus of the ovum once more becomes surrounded by a 

 membrane and enters upon a resting stage. 



The process of formation of the first polar body differs from that 

 of the second in that the chromosomes do not undergo splitting. 

 Consequently the nucleus of the mature ovum contains only half 

 the original number of chromosomes. This number varies in the 

 different species, but is constant in each. 1 According to Duesberg 

 it is twenty-four in man, so that in the mature human ovum there 

 should be only twelve chromosomes. 2 



The evidence of other investigators is conflicting, von Winiwarter 3 

 stating that in the human female there are forty-eight chromosomes 

 and in the male .forty-seven. According to Guyer 4 and Montgomery 5 

 there are in the negro probably only half the number of chromo- 

 somes that there are in the white race. 



As will be shown in the next chapter, the spermatozoa, or 

 male germ-cells, undergo a similar process of maturation, the 

 conjugating cells containing only half the number of chromosomes 

 characteristic of the species, just as in the case of the conjugating 

 ova. 6 It has been supposed, therefore, that the reduction in the 

 number of chromosomes is a preparation on the part of the germ- 

 cells for their subsequent union, and a means by which the number 

 of chromosomes is held constant in each species. 



The discovery that the nuclei of the conjugating cells contain 

 only half the number of chromosomes possessed by the soma or 

 body-cells was made originally by van Beneden. It has since been 

 extended to so many animals and plants that it may probably be 

 regarded as a general law of development. 7 . 



1 Von Winiwarter, however, states that in the rabbit the number varies 

 from thirty-six to eighty, but is generally about forty-two (Arch, de Biol., 

 vol. xvi., 1900). 



2 Duesberg, " Sur le Nombre de chromosomes chez 1'Homme," Anat. Anz., 

 vol. xxviii., 1906. 



3 Von Winiwarter, "Etude sur la Spermatogenese humaine," Arch, de Biol., 

 vol. xxvii., 1912. 



4 Guyer, " Accessory Chromosomes in Man)" Biol. Bull., vol. xix., 1910 ; 

 Science, vol. xxxix., 1914. 



6 Montgomery, "Human Spermatogenesis," Jour. Acad. Nat. Science, 

 Philadelphia, vol. xv., 1912. 



6 But see below, footnote, p. 166. 



7 For details of the process in various forms of life see Wilson, The Cell, 

 2nd Edition, New York, 1900. See also Doncaster, "On the Maturation of 

 the Unfertilised Egg, etc., in the Tenthredinidse," Quar. Jour. Micr. Science, 

 vol. xlix., 1906; " Gametogenesis, etc.," Prof. Roy. Soc., B., vol. Ixxxii., 1910, 

 and vol. Ixxxix., 1916. Doncaster shows that in the sawflies there are two 

 types of maturation process, in one of which there is no reduction. It is 

 probable that only the reduced eggs are capable of fertilisation. In other 

 cases, however, the ova are able to undergo parthenogenetic reproduction 

 without forming polar bodies. See Hewitt, " The Cytological Aspect of 

 Parthenogenesis in Insects," Manchester Memoirs, vol. lx., 1906 ; Doncaster, 



