GENERAL OUTLINE 



241 



2. Reduction in the Male. Spermatogenesis 



The researches of Platner ('89), Boveri, and especially of Oscar 

 Hertwig ('90, i ) have demonstrated that reduction takes place in the 

 male in a manner almost precisely parallel to that occurring in the 

 female. Platner first suggested ('89) that the formation of the polar 

 bodies is directly comparable to the last two divisions of the sperm 

 mother-cells (spermatocytes). In the following year Boveri reached 

 the same result in Ascaris, stating his conclusion that reduction in 

 the male must take place in the "grandmother-cell of the sperma- 

 tozoon, just as in the female it takes place in the grandmother-cell 

 of the tgg,'' and that the egg-formation and sperm-formation really 

 agree down to the smallest detail ('90, p. 64). Later in the same 

 year appeared Oscar Hertwig's splendid work on the spermato- 

 genesis of Ascaris, which established this conclusion in the most 

 striking manner. Like the ova, the spermatozoa are descended from 

 primordial germ-cells which by mitotic division give rise to the 

 spermatogonia from which the spermatozoa are ultimately formed 

 (Fig. 118). Like the oogonia, the spermatogonia continue for a time 

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

 in Ascaris megalocepJiala bivalens. Ceasing for a time to divide, they 

 now enlarge considerably to form spermatocytes, 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 

 history of the chromatin iji these two divisions is exactly parallel to 

 that in tJte formation of the polar bodies (Figs. 119, 120). From the 

 chromatin of the spermatocyte are formed a number of tetrads equal 

 to one-half the usual number of chromosomes. Each tetrad is halved 

 at the first division to form two dyads which pass into the respective 

 daughter-spermatocytes. At the ensuing division, which occurs with- 

 out the previous formation of a resting reticular nucleus, each dyad 

 is halved to form two single chromosomes which enter the respec- 

 tive spermatids (ultimately spermatozoa). From each spermatocyte, 

 therefore, arise four spermatozoa, and each sperm-nucleus receives 

 half the usual number of single chromosomes. The parallel with the 

 egg-reduction is complete. 



These facts leave no doubt that the spermatocyte is the morpho- 

 logical equivalent of the oocyte or immature ovarian Qgg, and that 

 the group of four spermatozoa to which it gives rise is equivalent 

 to the ripe ^gg plus the three polar bodies. Hertwig was thus led to 

 the following beautifully clear and simple conclusion : " The polar 

 bodies are abortive eggs which are formed by a final process of 



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