VI 



CHROMATIN AND IDIOPLASM 



155 



though here a complete complement of hereditary substance is bound 

 to be present. This is probably a universal rule in oogenesis (cf. Fig. 22, 

 etc.). Thus Gardiner (1899) calculated that in Polychaems not more 

 than 5^0" part of the chromatin of the germinal vesicle at its most 

 chromatic stage is used up in the formation of the chromosomes of the 

 meiotic divisions. 



In the sea-urchin Strongylocentrotus, Erdmann (1909) finds that the 

 volume of the chromosomes in the pluteus is only ?V of their volume in 

 the two-cell stage. 



Such considerations have led to the hypothesis of two kinds of 

 chromatin idiochromatin, the essential hereditary substance, or idio- 

 plasma proper, and tropho chromatin (see later under chromidia). Prob- 



FIG. 71. 



Equatorial plates of spermatogonia of three Vertebrates. A, Lepidosiren ; B, salamander ; C, rabbit. 

 The three figures are drawn to the same scale to show the relative amounts of chromatin present in each. 



ably, however, the idioplasm cannot, strictly speaking, be identified with 

 any substance. The hereditary factors should probably be considered as 

 elementary organisms, consisting mainly indeed of chromatin but possessing 

 an organic structure on which their activities depend. 



The argument therefore from equality or otherwise of mass, though 

 weighty, is not conclusive in deciding the claims of the various constituents 

 of the gametes to be considered as the idioplasm. There is no doubt that 

 in animals, at any rate, a certain very small amount of cytoplasm is 

 introduced by the male gamete along with the nucleus (see also under 

 chondriosomes, below). This has, however, been denied in the case of 

 some of the higher plants where it has been held that no cytoplasm 

 enters the egg cell with the nucleus (Strasburger, 1908), but a negative is 

 proverbially hard to prove. 



