128 



THE CHROMOSOMES 



polar spindle, which develops in the egg as the 

 nuclear membrane breaks down. 



At the time when the double chromosomes of the 

 sperm and the egg are about to pass onto the first 



d 



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

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

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

 division. 



maturation spindle each half of the double chromo- 

 some splits lengthwise so that four parallel strands 

 are present (Figs. 45 and 46) ; such a group of strands is 

 known as a tetrad. It is usually held, although there 



