1 6 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



The way in which the number of chromosomes is reduced may be seen in the 

 spermatogenesis of Ascaris (Fig. 12). Four chromosomes are typical for Ascaris 

 megalocephala bivalens and each spermatogone contains this number. In the 

 early prophase of the primary spermatocyte there appears a spireme thread con- 





Fig. 12. — Reduction of chromosomes in the spermatogenesis of Ascaris megalocephala bi%'alens 

 (Brauer, Wilson). X about 1100. A-G, successive stages in the division of the primary spermatocyte. 

 The original reticulum undergoes a very early division of the chromatin granules which then form a 

 doubly split spireme {B, in profile). This becomes shorter (C, in profile) and then breaks in two to 

 form two tetrads (D, in profile), {E, in end). F, G, H, first division to form two secondary spermato- 

 cytes, each receiving two dyads. /, secondary spermatocyte. J, K, the same dividing. L, two 

 resulting spermatids, each containing two single chromosomes. 



sisting of four parallel rows of granules (B). This thread breaks in two and 

 forms two quadruple structures known as tetrads (D-F) ; each is equivalent to two 

 original chromosomes spht lengthwise to make a bundle of four. At the meta- 

 phase (G) the two tetrads spUt each into two chromosomes which already show 

 evidence of longitudinal fission and are termed dyads. One pair of dyads goes to 



