THE MECHANISM OF REPRODUCTION 37 



made in the segregation of particular tissues. But when 

 the germ cells are formed, at spermatogenesis and oogen- 

 esis, the chromosomes unite in pairs, a process technically 

 known as synapsis, and at division one member of each 

 pair passes entire to one of the two new daughter cells, 

 thus reducing the number of the chromosomes in the 

 gamete to one-half of those possessed by the body cells. 

 Subsequently there is an equating or halving division 

 similar in appearance to the cell divisions in ordinary 

 growth. Four gametes are thus formed. Leaving out of 

 account the behavior of certain chromosomes believed to 

 control the distribution of sex, there is good evidence that 

 this union of chromosome pairs at synapsis always takes 

 place between two chromosomes, one of which had been 

 received from the father and one from the mother. In 

 other words, it seems clear that each gamete obtains one 

 of each kind of chromosome, although it is a matter of 

 chance whether the cell receives the maternal or paternal 

 representative of any type. Thus, if the chromosomes 

 of the body cells of a particular species are six in number, 

 and we represent them as ABC abc, regarding A, B, and 

 C as of maternal and a, b, and c as of paternal origin, at 

 synapsis A only pairs with a, B with b and C with c. 

 This procedure, however, will yield eight types of gam- 

 etes, ABC, ABc, AbC, aBC, Abe, aBc, abC, and abc, since it 

 is a mere matter of chance which daughter cell receives 

 either member of any pair. 



In spermatogenesis four sperms are formed from each 

 immature germ cell, but in oogenesis the maturation of 

 the egg only one functional gamete is produced, the 

 other three being aborted. Nevertheless, the two processes 



