2 9 6 GENERAL BIOLOGY 



Chromosomes exhibit marked individuality of form, dif- 

 fering in different organisms in length, breadth, curvature, 

 etc., but in a given species, they are fairly constant in form. 

 In certain genera it is claimed that two species may be dis- 

 tinguished as well by the chromosomes of a single cell* as 

 by the external characters of the adult animal. 



The history of the germ cells. Since at the beginning of 

 embryonic life, the egg is already a new organism, charged 

 with the potentiality to develop all the characters of the 

 adult, we must seek the source of these characters farther 

 back. How does the egg come into being? It traces its 

 lineage from an antecedent egg, as we have already seen (fig. 

 174). That antecedent egg gives rise to both body-plasm 

 and germ-plasm, but the latter is very early set apart from, 

 although surrounded by the former; walls are built up 

 about the germ plasm (spermary or ovary walls) , by which 

 it is protected and through which it is nourished. Thus, the 

 germ plasm is removed from direct contact with environ- 

 ment, and also from direct relations with the functional cells 

 of the body, and in this isolation it develops. 



The primordial germ cells, thus segregated, pass through a 

 period of rapid divisions which succeed each other in quick 

 succession without much intervening growth, and the result 

 of which is great increase in numbers and great reduction in 

 size. The small germ cells thus produced are called sperma- 

 togones or oogones, according as they develop in spermary 

 or ovary. Then follows a growth period, without division, 

 in which the normal size is regained, and much new cyto- 

 plasm is formed. The differentiation of the cytoplasm into 

 the different materials that will subsequently be devoted to 

 the production of different parts of the embryo occurs dur- 



*They differ among themselves in size and form in the single 

 nucleus; wherefore, it is ordinarily the chromosome complex that 

 offers recognition characters, rather than single chromosomes. 



