The Cellular Basis I g i 



into chromosomal vesicles and form the daughter nuclei, as is 

 shown in Figs. 7, and 43. But during the growth period of the 

 oocytes and spermatocytes the chromosomes form a closel) 

 wound coil of long chromatin threads (Fig. 50), and when these 

 threads uncoil later it is seen that the chromosomes have united 

 in pairs (Figs. 50 D and E, 51 B, 52 B, 58) ; this process is known 

 as synapsis, or the conjugation of the chromosomes, and there 

 is evidence that one memher of each synaptic pair is derived 

 from the father, and the other from the mother. The union of 

 these chromosomes is a temporary one and is not so close that 

 they lose their identity. By this union of the chromosomes into 

 pairs the number of separate chromosomes is reduced to half the 

 normal number; if there are usually 4 chromosomes, as in As- 

 caris, they are reduced to 2 pairs ; if 48 chromosomes, as in man, 

 there are 24 of these pairs.* 



Conjugation of Homologous Chromosomes. — In the conjuga- 

 tion of the chromosomes it is plain that, generally speaking, 

 those chromosomes unite which are similar in shape and size; 

 big chromosomes unite with big ones, little ones with little - ones, 

 and those of peculiar shape with others of similar shape (Figs. 

 51 B, 52 B, 54, 58). It is probable that the two members of a pair 

 of conjugating chromosomes are homologous not merely in shape 

 and size but also in function, though this homology does not 

 amount to identity. These homologous chromosomes may be com- 

 pared to the fingers of the two hands ; each digit differs from every 

 other one but the thumb, index finger and other fingers of the 

 right hand are homologous but not identical with the correspond- 

 ing digits of the left hand, and the conjugation of homologous 

 chromosomes may be compared to the placing together of the 

 two hands so that homologous digits come together. 



In some instances it can be proved that one member of each 

 conjugating pair of chromosomes comes from one parent and the 

 other from the other parent, and it is probable that this is always 



* See pp. 161-163. 



