THE BIOLOGICAL BACKGROUND OF GENETICS 



205 



which results in a reduction of the number of chromosomes charac- 

 teristic of the zygote to one-half (Fig. 47). The other division simply 

 separates twin chromosomes as in ordinary mitosis. The result is 

 that each gamete has only one of each kind of chromosome and there- 

 fore only one-half the number possessed by the zygote and the pri- 

 mordial germ cells. It is customary to speak of the somatic or zygotic 



Fig. 47. — Diagram to illustrate spermatogenesis, a, showing the diploid num- 

 ber of chromosomes (six is arbitrarily chosen) as they occur in divisions of ordinal} 

 cells and spermatogonia; b, the pairing (synapsis) of corresponding mates in the 

 primary spermatocyte preparatory to reduction; c, each secondary spermatocyte 

 receives three, the haploid number of chromosomes; d, division of the secondary 

 spermatocytes to form e, spermatids, which transform into/,, spermatozoa. {From 

 Guyer.) 



number as diploid, or 211, and the gametic number as haploid, or n. 

 The final phase of spermatogenesis consists of an elaborate specializa- 

 tion of the male gamete, consisting of the development of locomotor 

 organs and adaptations for penetrating the egg. 



Oogenesis. — The period of multiplication is essentially the same as 

 in spermatogenesis except that fewer and larger cells are produced. 

 The period of growth is very marked, for it is during this period that 

 the egg accumulates the yolk, which is usually massed at the vegetal 

 pole. The nucleus and central body, confined to a small region of 



