The Cellular Basis 151 



cells. The growth -period of a spermatocyte lasts for a briefer 

 time than does that of an oocyte so that the former remains 

 relatively small (Fig. 41). 



Sy nap sis. All of the cell divisions which take place during 

 the division period are of the usual kind, in which every chromo- 

 some splits lengthwise into two and the two halves then separate 

 and move to opposite poles of the spindle where they swell up 

 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 spenmatocytes the chromosomes form a closely 

 wound coil of long chromatin threads (Fig. 50 A and B), and 

 when these threads uncoil later it is seen that the chromosomes 

 have united in pairs (Figs. 50 D and E, 50 a, 51 B, 52 B) ; this 

 process is known as synapsis, or the conjugation of the chromo- 

 somes, and there is evidence that one member 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. 

 50 a, 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- 



