474 



THE REPRODUCTIVE SYSTEM 



eration. Its 'elements' are the same, except in so far as the complement 

 is halved during maturation and restored at fertilization by addition of an 

 equal quota from another line of descent. This halving and doubling is 

 the physical mechanism which underlies the phenomenon of variation in- 

 volved in heredity. Offspring thus resemble parents because both have 

 had origin largely from the same germ-plasm; lack of resemblance results 

 from the fact mainly that the germ-plasm of parents and offspring of neces- 

 sity differ in certain elements since different additions are made at fertili- 

 zation in the case of successive generations. 



We may now return to the metaphase (Fig. 

 16) and follow the further course of the chromo- 

 somes. The chromosomes are in typical tetrad 

 condition. The first division most probably sep- 

 arates the chromosomes that paired at synapsis. 

 The unpaired, odd, or accessory chromosome (x) 

 passes undivided and in advance of the other 

 chromosomes to one pole of the spindle. Thus 

 one of the resulting daughter-cells, prespermatid 

 or secondary spermalocyte, receives 11 chromo- 

 somes, the other, 11 plus the accessory, or 12 

 chromosomes. Fig. 17 is a side view and Fig. 18 

 a polar view (metaphase plate), of the metaphase 

 group, showing all 12 chromosomes. The acces- 

 sory is still recognizable at telophase, Fig. 19. 

 The chromosomes of the resulting secondary sper- 

 matocytes do not pass into a diffuse stage but re- 

 main more or less compact and chromatic a con- 

 dition characteristic of the later segmented spi- 

 reme stage of mitosis. In half the cells the acces- 

 sory appears in its typical shape, staining reac- 

 tion, and position, as shown in Fig. 20. This 

 shows also a remnant of the original spindle. 

 Attention should here be called to the fact that in 

 many forms chromatic particles have been de- 

 scribed passing from the nucleus into the cyto- 



FIG. 424. DIAGRAMS 

 ILLUSTRATING THE 

 BEHAVIOR OF THE 

 CHROMOSOMES DUR- 

 ING THE FIRST (A) 

 AND SECOND (B) 

 MATURATION Drvi- 



The bivalent chro- 

 mosome a + b enters 

 the first maturation 

 spindle and suffers a 

 reductional division, 

 chromosome a being 

 separated from chro- 

 mosome b. In the sec- 

 ond maturation divi- 

 sion, each chromosome 

 undergoes an ordinary 

 longitudinal division. 

 This division is already 

 foreshadowed during 

 the first or heterotypic 



division, producing the , 

 -. tetrad condition of the P lasm ' during the growth stages of the primary 

 bivalent chromosome. spermatocyte, particularly at the synapsis phase. 

 Similar conditions have occasionally been de- 

 scribed also for the spermatogonia, and in several instances even for the 

 secondary spermatocytes. These extrusion products are called chromidia, 



