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 



Fig. 45. — Diagram to illustrate the two reduction divisions of sporm- 

 atogonial cells, a, first spermatocyte with two tetrads; b and c, division 

 of last; d and /, division of two cells of c; e and g, completion of second 

 division. 



first maturation spindle, eacli lialf of the d()u])le 

 chromosome splits lengtliwise so that four j")arallel 

 strands are present (Figs. 45, 46). One of the two 

 longitudinal si)lits in the tetrad is generally thought 



