CHANGES IN THE OVARY 131 



tains only half the original number of chromosomes. This 

 number varies in the different species, but is constant in each. 1 

 In Man it is twenty-four, so that in the mature human ovum 

 there are only twelve chromosomes. 2 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. It has been sup- 

 posed, therefore, that the reduction in the number of chromo- 

 somes 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. 3 



It is commonly believed that the chromatin material is 

 the substance which has the potentialities of development, and 

 which plays the principal part in perpetuating the hereditary 

 structure and qualities of the particular animal or plant, but 

 there is no real proof that this is the case (see p. 199 below). 



The maturation phenomena may take place within the ovary 

 prior to the discharge of the egg, or they may be postponed 

 until after ovulation has occurred. In the rabbit, as has been 



1 Van 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 chromosomes chez l'Homme," Anat. Anz., 

 vol. xxviii., 1900. 



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

 Cell, 2nd Edition, New York, 1900. See also Doncaster, "On the Matura- 

 tion of the Unfertilised Egg, &c., in the Tenthredinidas," Quar. Jour. Micr. 

 Science, vol. xlix., 1906 ; " Gametogenesis, &c.," Quar. Jour. Micr. Science, 

 vol. li., 1907. 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 Partheno- 

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

 Parthenogenesis," Science Progress, vol. iii. (July) 1908. These papers contain 

 further references. 



