50 GENETICS 



(figure 1 8, D). These are known as "groups of four," or 

 "tetrads." 



Since each pair of chromosomes becomes one tetrad, 

 there are n tetrads. In each tetrad, two lobes represent a 

 maternal chromosome, while two represent a paternal 

 chromosome. 



Some males have one chromosome (X) without a mate; 

 this does not conjugate, but otherwise goes through the 

 same process as the others, forming a two-lobed, instead 

 of a four-lobed body. 



17. Now the large cell containing the n tetrads divides 

 quickly, twice in succession ("maturation divisions"), thus 

 producing four final germ cells (sperms or ova) (figure 18, 

 EtoJ). 



18. In these two maturation divisions, each tetrad di- 

 vides twice, in such a way that the four lobes separate into 

 the four different germ cells formed. 



19. Thus two of the four germ cells receive from each 

 tetrad a maternal lobe only, while two receive a paternal 

 lobe only. 



20. These lobes transform each into a chromosome like 

 that from which it was derived. 



21. Thus each final germ receives either a maternal or 

 a paternal chromosome from each of the n pairs that were 

 present in the immature germ cells. Each therefore con- 

 tains but n single chromosomes, in place of the n pairs (or 

 2« single chromosomes) that were present in the parental 

 cells. This is the process of reduction of the number of 

 chromosomes, from 2w to n. 



As set forth in paragraph 13 above, the net result is the 

 same as would be produced by the division of each imma- 

 ture germ cell into two instead of four, one chromosome 

 from each pair passing into each of the two germ cells 

 produced. For many practical purposes, one can think of 

 germ-cell formation as occurring In this way (figure 4). 



