THE MORPHOLOGY OF FERTILIZATION 45 



divisions of the spermatocytes by which the four 

 functional spermatids, or immature spermatozoa, are 

 formed. These divisions in the history of both spermat- 

 ozoon and ovum, known as the maturation divisions, 

 are concerned in reduction of the chromosomes to one- 

 half that characteristic of the species (haploid number), 

 an event that always precedes fertilization; so that 

 the union of the two germ cells hi fertilization restores 

 the species or diploid number. 



The reduction divisions are preceded in both sper- 

 matogenesis and ovogenesis by a pairing of chromo- 

 somes in the so-called growth period to form bivalent 

 chromosomes (tetrads); it is believed that the two 

 members of a pair are always maternal and paternal, 

 respectively, in their origin. In one of the reduction 

 divisions the members of each pair separate again and 

 pass into different daughter-cells, while in the other 

 division each chromosome divides in the usual fashion. 

 The germ cells are thus prepared, not only by re- 

 duction in number of the chromosomes, but by differ- 

 ential distribution of the latter for their subsequent 

 union. 



The maturation divisions of the sperm cells always 

 occur prior to the special differentiation of this cell as 

 a locomotor cell, and thus long before fertilization; 

 but those of the ovum, which requires no subsequent 

 differentiation to function as a fully mature gamete, 

 do not occur until the time of fertilization or immedi- 

 ately preceding it. Both kinds of cells lose their capa- 

 city for division after maturation unless they unite in 

 fertilization; but many ova lose this capacity prior to 

 maturation, or during the course of the maturation 



