282 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



ture of the chromosomes remains visible. Eachpaired 

 chromosome now divides not only by splitting lon- 

 gitudinally, but also by a median transverse division, 

 thus producing four fragments, each group of four 

 fragments being known as a tetrad. The number of 

 tetrads are thus equal to one-half the original num- 

 ber of chromosomes. The tetrad groups now ar- 

 range themselves in the equatorial plane of the 



Four chromosomes. f6/\V 



Primary spermalocytes. 



Two tetrads 



----- Somatic mitosis. 



Secondary spermatocytes. 

 Reduction mitosis. 



Two diad groups in , 



each cell. 



f I T I Young spermatozoa. 



Fig. 2ioa. Diagram of spermatogenesis. 



spindle, and then two fragments or chromosomes 

 from each tetrad pass to opposite poles of the 

 spindle to form two daughter cells or the secondary 

 spermatocytes. The chromosomes of each daughter 

 cell, therefore, form in groups of twos instead of 

 fours, and each group is now called a diad. The 

 number of diads is equal to the number of tetrads, 



