130 OUTLINES OF EVOLUTIONARY BIOLOGY 



differentiated from the germ cells. The result of this is that the 

 germ cells alone retain the full complement of chromatin 

 derived from the parents, and their nuclei are accordingly 

 actually much larger than those of the somatic cells. 



The differentiation into somatic cells and germ cells cannot 

 usually he traced so far back in the development of the individual 

 as in Ascaris, but in a great many animals the distinction can 

 be recognized at a very early stage. In certain insects, for 

 example, the primordial germ cells can be traced back to a large 

 " pole-cell " which lies at one end of the segmenting ovum, and 



in the arrow-worm, Sagitta, 

 they can be identified at the 

 gastrula stage (Fig. 63, p.//.r.). 

 As we shall see later on, this 

 early segregation of the germ 

 cells is of very great interest 

 from the point of view of the 

 theory of heredity. It can 

 hardly be said, however, at 

 any rate in the present state 

 of our knowledge, to be a 

 phenomenon of universal or 

 even general occurrence, and 

 in the majority of creloinate 

 animals the germ cells are first 

 recognizable in the ccelomic 

 epithelium at a compara- 

 tively late stage of develop- 

 ment (Fig. 62, B). In plants also, in cases where there is 

 a well developed gametophyte this appears to attain its full 

 development before the germ cells are recognizable, and the 

 entire life of the sporophyte is passed without any distinction 

 between somatic and germ cells manifesting itself. Moreover, 

 the fact that the ordinary cells of a fern prothallus can, on 

 occasion, act as germ cells, 1 prevents us from admitting any 

 absolute distinction between the two categories. 



The primordial germ cells may undergo extensive multiplication 

 by ordinary mitotie division before giving rise to the actual 

 gametes. In niulticellular animals the process of gametogenesis 

 (formation of gametes), which is either oogenesis or sperrnato- 



1 Vide, p. 105. 



FIG. 63. Section of the Gastrula of an 

 Arrow Worm (Sagitta) showing the 

 primordial Germ Cells. (After O. 

 Hertwig.) 



bp., blastopore ; ent., enteron ; ep., epiblast ; 

 hyp., hypoblast; p.g.c., primordial germ 

 cells. 



