4 SEX-DETERMINATION 



contrasted characters (details of structure and of behaviour) 

 and usually to such as are derived from two sexuallv con- 

 trasted individuals. 



The division of a cell into two by simple fission is no mere 

 casual cleavage; it is a process of precision, karyokinesis or 

 mitosis, the essential feature of which is the exact halving 

 of the two chief constituents of the cell, the nucleus and the 

 cytoplasm, so that the two daughter cells that result are 

 save in respect of initial size, exact copies of the cell that 

 produced them. 



The most striking features of mitosis are the exact and 

 precise division of the chromosomes and the precise distri- 

 bution of the daughter chromosomes so formed. In respect 

 of their chromosome constitution the daughter cells are 

 exact copies of the mother cell that produced them. 



In each and every species there is a characteristic number 

 ot chromosomes within the nuclei of its component cells 

 I;^or example, man has forty-eight (Evans and Swezy, 1020). 

 These exist in the form of pairs, the members of any given 

 pair (with one exception later to be considered) being alike 

 m size, shape and behaviour during cell-division. 



This constancy of the chromosome number could not 

 exist if at fertilization both the egg and the sperm brought 

 into the new zygote that number of chromosomes which is 

 characteristic of the species. Offspring have the same num- 

 ber as the parents (polymitotic forms and polyploidy being 

 disregarded). Constancy is maintained by a reduction of the 

 chromosome number to a half during gametogenesis. The 

 existence of this process was postulated by Weismann 

 u ''Vxr, ^^ hypothesis has been verified universally since 

 then. Wherever there is fertilization there is also reduction 

 which in essence consists of two divisions of the nucleus of 

 the cell associated with one division of its chromosomes 

 with the result that four daughter nuclei are produced, each 

 ot these having half the number (the haploid number) of 

 chromosomes characteristic of the species. Meiosis, this 

 process of reduction, is a modification of mitosis. 



Fertilization consists essentially in the bringing together 

 ot two half-sets of chromosomes and the consequent re- 



