Heredity - 477 



Fig. 26-2. Continuity of the germ cells in the life cycle of the frog. 1, the 

 fertile egg— the shaded area becomes cytoplasm of the germ cell; 2, 8-celled 

 stage; 3, 32-celled stage; 4, blastula showing 4 germ cells; 5-7, successive stages 

 in the development of the gastrula, showing germ cells; 8-10, metamorphosing 

 frogs; 11, mature frog with eggs in ovary ready to be discharged. (After 

 Conklin. From Guyer, Animal Biology. Permission of Harper and Row.) 



certain differences, and this makes it neces- 

 sary to give a separate description of the 

 maturation of the two kinds of gametes. 



A very brief description of the maturation 

 of the sperm was given previously (Chap. 3), 

 but now it is necessary to follow the process 

 more closely. In the testes, when a spermato- 

 gonium stops multiplying by mitosis and be- 

 gins to grow, it is a sign that the meiotic 

 divisions are about to occur (Fig. 26-3). Now 

 each germ cell is called a primary spermato- 

 cyte and each primary spermatocyte is des- 

 tined to form four sperm (Fig. 26-3). In fact, 

 the growth of the primary spermatocyte pro- 

 ceeds simultaneously with the long prophase 

 of the first meiotic division; and during this 

 growth stage the pairs of chromosomes en- 

 gage themselves in synapsis (Fig. 26-4). After 

 growth and synapsis, the first and second 

 meiotic divisions usually proceed quite rap- 

 idly. First the two secondary spermatocytes 



are formed, and then each secondary sperma- 

 tocyte divides. This second of the meiotic 

 divisions forms the four spermatids, which 

 soon grow tails and become actively motile 

 sperm (Fig. 26-3). 



The diploid nature of the germ cells prior 

 to the meiotic divisions follows from the fact 

 that they are derived entirely by mitosis 

 from the original zygote that produced all 

 the cells of the individual. But when the 

 meiotic divisions have been completed all the 

 sperm are haploid (Fig. 26-4). The single 

 haploid set of chromosomes that is carried 

 by the sperm represents the essential contri- 

 bution of the male parent toward the heredi- 

 tary constitution of the offspring. 



Oogenesis differs from spermatogenesis 

 mainly as to the behavior of the cytoplasm 

 during the meiotic divisions. In the case of 

 the ripening egg (Fig. 26-3), the growth 

 period, during which synapsis occurs, is more 



