532 



P. W. WHITING 



first spermatocytes, diagrams will be helpful. Text figure 2 

 represents a tetrad with its four elements, the chromatids. A 

 and B represent the chromatids from one homologue and C and 

 D the chromatids from the other. 



The formation and existence of tetrads mark the post-spireme 

 stage which ends, according to Wenrich ('16), "with the estab- 

 lishment of the tetrad-chromosomes upon the mitotic spindle of 



<^ 



Text Figures 2-7. Separation of the tetrad 



the first spermatocyte division." Since, however, in Culex 

 pipiens, each tetrad passes through late prophase stages, through 

 the metaphase, and up to the anaphase independently of the 

 others, the post-spireme stage must be considered separately for 

 each tetrad. 



In text figure 3 the tetrad has split at each end and the splits 

 are at right angles to each other. To the right the homologues 

 are separated by the primary split of Wenrich. To the left is 

 shown the secondary split, separating the fused daughters of 



