298 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
is accomplished by a modification in the mitotic division in which 
instead of each chromosome splitting lengthwise, as in ordinary mito- 
sis, the chromosomes unite in pairs (Fig. 47, 6), 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 
Fic. 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; b, the pairing (synapsis) of corresponding 
mates in the primary spermatocyte preparatory to reduction; c, each secondary 
spermatocyte receives three, the haploid number of chromosomes; d, division of 
the secondary spermatocytes to form e, spermatids, which transform into f, sper- 
matozoa. (From Guyer.) 
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. 
