The Cellular Basis 153 



true. In every cell of every individual which has developed 

 from a fertilized egg there are two full sets of chromosomes, one 

 of which came from the sperm and the other from the egg; but 

 when this individual in its turn produces germ cells homologous 

 chromosomes of each set unite in pairs, side by side, during the 

 growth period. This again may be compared to the union of 

 the two hands, the right digits, for example, representing the 

 paternal and the left the maternal chromosomes which come to- 

 gether in homologous pairs, corresponding joints lying opposite 

 each other, as corresponding genes in the chromosomes lie oppo- 

 site each other. 



These synaptic pairs are the bivalent chromosomes, and in ad- 

 dition to showing the line of junction by which they are united 

 they frequently show a longitudinal split through the middle 

 of each chromosome and at right angles to the line of junction. 

 It thus happens that these bivalent chromosomes are frequently 

 four-parted and such four-parted chromosomes are known as 

 tetrads (Figs. 51 B, 52 B, C). 



(c) The Maturation Period. Finally at the close of the 

 growth period both oocyte and spermatocyte undergo two peculiar 

 divisions, one following immediately after the other, which are 

 unlike any other cell divisions. These are known as the first and 

 second maturation divisions and they are the last divisions which 

 take place in the formation of the egg and sperm. 



Reduction Division. In one or the other of these two matura- 

 tion divisions the pairs of chromosomes separate along the line 

 of junction, one member of each pair going to one pole of the 

 spindle and the other to the other pole, so that in each of the 

 daughter cells thus formed only a single set of chromosomes is 

 present (Figs. 51 C, D, 54) ; but since the position of the pairs 

 of chromosomes in the spindle is a matter of chance it rarely 

 happens that all the paternal chromosomes go to one pole and all 

 the maternal ones to the other; thus each of the sex cells comes 

 to contain a complete set of chromosomes, though particular indi- 



