MATURATION 



2 7 



The way in which the number of chromosomes is reduced may be seen in the 

 spermatogenesis of Ascaris (Fig. 11). Four chromosomes are typical for Ascaris 

 megalocephala bivalens and each resting primary spermatocyte contains this 

 number. When the first maturation spindle appears only two chromosomes are 

 formed, but each of these is double, so four are really present. Each represents 

 the union of two chromosomes, shows a quadruple structure, and is termed a 

 tetrad (Fig. 11 E, F). At the metaphase (G) the two tetrads split each into two 

 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 sper- 



Spermatogonium 



Proliferation 

 'period. 



Spermatocyte 



Spermatids 

 Spermatozoa 



Growth 

 period 



Oocyte 2 (ovum 

 and polocyte 1) 



Maturation 

 period. 



Ovum and 

 three polo-' 

 cytes 



Fig. 12. — Diagrams of maturation, spermatogenesis and oogenesis (Boveri). 



matocytes (Fig. n G, I). Before 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, or one-half the 

 number characteristic for the species. A diagram of maturation in the male As- 

 caris is shown in Fig. 12 A. The first maturation division is 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 chro- 

 mosomes bearing similar hereditary qualities. In many insects and some ver- 



