THE CHROMOSOMES 161 
polar spindle, which develops in the egg as the 
nuclear membrane breaks down. 
At or before the time when the double chromo- 
somes of the sperm and of the egg pass onto the 
Fic. 45.—Diagram to illustrate the two reduction divisions of sperm- 
atogonial cells. a, first spermatocyte with two tetrads; b and c, division 
of last; d and f, division of two cells of c; e and g, completion of second - 
division. 
first maturation spindle, each half of the double 
chromosome splits lengthwise so that four parallel 
strands are present (Figs. 45, 46). One of the two 
longitudinal splits in the tetrad is generally thought 
