MATURATION 



17 



each of the daughter cells, or secondary spermatocytes (G-I). Without the 

 formation of a nuclear membrane, the second maturation smndle appears at once, 

 the two dyads split into four monads, and each daughter spermatid receives two 

 single chromosomes, or one-half the number characteristic for the species. The 

 tetrad, therefore, represents a precocious division of the chromosomes in prepara- 

 tion 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. 13. The first maturation division in 

 Ascaris is probably reductional,- each daughter nucleus receiving two complete 



Proliferation 



Spermatocyte i 

 Spermatocyte 2 



Spermatids 

 Spermatozoa 



Oocyte 2 (ovum 

 and polocyte i) 



Omim and 

 three polo- 

 cylcs 



12 3 4 12 3 4 



Fig. 13. — Diagrams of maturation, spermatogenesis and oogenesis (Boveri). 



Growth 

 period 



Maturation 

 period 



Transforma- 

 tion period of 

 spermatozoa 



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 chromo- 

 somes, these being split lengthwise. In the latter case the division is equational, 

 each daughter nucleus reciving 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 vertebrates it has 

 been shown that the number of chromosomes in the oogonia is even, the number 

 in the spermatogonia odd, and that all the mature ova and half the spermatids 

 contain an extra or accessory chromosome (see p. 32). 



During oogenesis, the ova undergo a similar process of maturation. Two 



