GENERAL OUTLINE 



l8l 



spcruiatogotiia from which the spermatozoa are ultimately formed 

 (Fig. 90). Like the oogonia, the spermatogonia continue for a time 

 to divide with the usual (somatic) number of chromosomes ; i.e. four 

 in Ascaris vicgaloccpliala bivalcns. Ceasing for a time to divide, they 



Fig. 91. — Diagrams showing the essential facts of reduction in the male. The somatic num- 

 ber ot chromosomes is suppos(;d to be four. 



A. B. Division of one of the spermatogonia, sliowing the fuH number (four) of chromosomes. 

 C. Primary spermatocyte prt- p.iiing for division ; tlie chromatin forms two tetrads. D, E. F. First 

 division to form two secondary spermatocytes each of which receives two dyads. G. H. Division 

 of the two secondary spermatocytes to form four spermatids. Each of the latter receives two 

 single chromosomes and a centrosome which persists in the middle-piece of the spermatozoon. 



now enlarge considerably to form s/^n-inaton'fc\s\ each of which is 

 morphologically equivalent to an unripe ovarian ovum, or oocyte. 

 Each spermatocyte finally divides twice in rapid succession, giving 

 rise first to two daughter-spermatocytes and then to four spermatids, 

 each of which is directly converted into a single spermatozoon. The 



