298 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



is accomplished by a modification in the mitotic division in which 

 instead of each chromosome spHtting lengthwise, as in ordinary mito- 

 sis, the chromosomes unite in pairs (Fig. 47, b), a process known tech- 

 nically as synapsis, and then apparently one member of each pair passes 

 entire into one new daughter cell, the other member going to the other 

 daughter cell (Fig. 47, c). In the pairing preliminary to this reduction 

 division, leaving out of account certain special cases to be considered 



Fig. 47. — Diagram to illustrate spermatogenesis, a, showing the diploid 

 number of chromosomes (six is arbitrarily chosen) as they occur in divisions of 

 ordinary cells and spermatogonia; h, the pairing (synapsis) of corresponding 

 mates in the primary spermatocyte preparatory to reduction; c, each secondary 

 spermatocj'te receives three, the haploid number of chromosomes; d, division of 

 the secondary spermatocytes to form e, spermatids, which transform into /, sper- 

 matozoa. {Frovi Gayer.) 



later, according to the best evidence at our command the union always 

 takes place between two chromosomes which match each other in size 

 and appearance. Since one of these is believed to be of maternal and 

 the other of paternal origin, the ensuing division separates correspond- 

 ing mates and insures that each gamete gets one of each kind of chro- 

 mosome although it appears to be a matter of mere chance whether or 

 not a given cell gets the paternal or the maternal representative of 

 that kind. 



