22 



TEXT-BOOK OF EMBRYOLOGY. 



lumen or with their heads embedded in the supporting cells. The supporting 

 cells extend from the basement membrane to the lumen of the tubule where 

 they frequently spread out in a fan-like shape. 



The developmental cycle through which the spermatogenic cells pass, in- 

 cluding reduction of chromosomes, occurs in a definitely progressive manner 



I -r K L 



Fig. 14. — Reduction of chromosomes in spermatogenesis in Ascaris megalocephala (bivalens). 

 Brauer, Wilson. 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). This becomes shorter (C), and then breaks in two to form the 2 tetrads (D, in profile, 

 E, on end). F, G, H, First division to form 2 secondary spermatocytes, each receiving 2 dyads. /, 

 Secondary spermatocyte. /, K, The same dividing. L, Two resulting spermatids, each containing 

 2 single chromosomes. 



along length-segments of the seminiferous tubule. Thus a particular cross 

 section may contain only the earlier stages of spermatogenesis, the succeeding 

 serial sections containing the middle and later stages. After the sections con- 

 taining fully developed spermatozoa, the next succeeding sections contain the 

 early stages again. 



