i6 



THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



by side and split lengthwise to make a bundle of four. At the metaphase 

 (G) each tetrad divides into its two original chromosomes which already 

 show evidence of longitudinal fission and are termed dyads. One pair of 

 dyads goes to each of the daughter cells, or secondary spermatocytes (G-I). 

 Without the formation of a nuclear membrane, the second maturation 

 spindle appears at once, the two dyads split into four monads, and each 

 daughter spermatid receives two single chromosomes (monads) , or one-half 

 the number characteristic for the species. The tetrad, therefore, repre- 



E F 



FIG. 10. Diagrams of the development of spermatozoa (after Meves in Lewis and Stohr). 

 a.c., Anterior centrosome; a./., axial filament; c.p., connecting piece; ch.p., chief piece; g.c., cap; 

 n., nucleus; nk., neck; p., protoplasm; p.c., posterior centrosome. 



sents a precocious division of the chromosomes in preparation for two 

 rapidly succeeding cell divisions which occur without the intervention of 

 the customary resting periods. The easily understood tetrads are not 

 formed in most animals, although the outcome of maturation is identical in 

 either case. A diagram of maturation is shown in Fig. 12. The first 

 maturation division in Ascaris is probably reductional, each daughter 

 nucleus receiving two complete chromosomes of the original four, whereas 

 in the second maturation division, as in ordinary mitosis, each daughter 

 nucleus receives a half of each of the two chromosomes, these being split 

 lengthwise. In the latter case the division is equational, each daughter 

 nucleus receiving chromosomes bearing similar hereditary qualities. 



In some animals the sequence of events is reversed, reduction occurring 

 at the second maturation division. In many insects and some vetebrates 



