476 - Heredity and Evolution 



CONTINUITY OF THE GERM CELLS 



In multicellular species, some but not all 

 of the cells retain the potentiality of trans- 

 mitting their chromosomes to the cells of the 

 next generation. These germ cells stand in 

 contrast to the somatic cells, which cannot 

 perpetuate their chromosomes beyond the 

 lifetime of the individual organism. The 

 somatic cells of an organism are destined to 

 die with the individual, but the germ cells 

 are potentially immortal (Fig. 26-1). Only 

 the germ cells establish continuity from in- 

 dividual to individual in each successive 

 generation, and only changes in the germ 

 cells are effective in changing the hereditary 

 destiny of the species. Accordingly, it is neces- 

 sary to follow the germ cells and to deter- 

 mine how these cells transmit their chromo- 

 somes to the gametes and to the offspring. 



Among sexual organisms, any cell that 



stands in line of descent of the eggs or sperm 

 is a germ cell. The germ cells include not 

 only the sperm and eggs but also the zygote 

 and some of the cells of the developing em- 

 bryo, even before the gonads become differ- 

 entiated (Fig. 26-2). These early primordial 

 germ cells eventually come to lie in the 

 gonads — but now they are called by a differ- 

 ent name. In the gonads, while they are mul- 

 tiplying by repealed mitotic divisions, the 

 germ cells of an animal are called gonia; 

 more specifically, in the testis of the male 

 animal the germ cells are called sperma- 

 togonia, in contrast to the oogonia, which lie 

 in the ovaries of the female. 



Gametogenesis: Maturation of the Gam- 

 etes. In the gonads of the adult, the eggs and 

 sperm begin to undergo maturation; and 

 during maturation the chromosomes of the 

 ripening eggs and sperm go through similar 

 stages of development. However, there are 



SUCCESSIVE 

 GENERATIONS 



GERM PLASM 

 (PROTOPLASM OF 

 THE GERM CELLS) 



SOMA 



(CELLS OF THE 



BODY) 



L 3 



RELATION BETWEEN GERM PLASM 



AND SOMA IN ASEXUAL 



REPRODUCTION 



RELATION BETWEEN GERM PLASM 



AND SOMA IN BIPARENTAL 



SEXUAL REPRODUCTION 



Fig. 26-1. The germ cells maintain continuity from generation to generation, whether the 

 reproduction is sexual or asexual. 



