THE ANIMAL CELL. 305 



a prominent position. This substance, while not definitely known, 

 is usually classed as a nuclein, but differs from the more common 

 forms in being soluble with difficulty. Especially abundant also is 

 a nucleo-albumin, which Kossel and Lilienfeld first obtained from 

 the thymus gland of the calf. This they termed nucleohiston, from 

 the fact that on treatment with hydrochloric acid it is decomposed 

 into a nuclein leukonuclein, and a special alburnose-like substance 

 histon, which differs from other albumins in being insoluble in am- 

 monium hydrate (see page 322). This substance is probably iden- 

 tical with the so-called tissue fibrinogen and cellular fibrinogen of 

 other observers, and, no doubt, is closely related . to the cy toglobin 

 and prseglobulin of Alexander Schmidt. 



In addition to these substances, we further meet with nucleinic 

 acids in the free state, and in some cells also with the basic radicles 

 of the nucleinic acids i. e., the xanthin bases, as such. 



Whether lecithins, protagon, and glycogen, which are constantly 

 found in the cellular protoplasm, likewise occur in the nucleus, is 

 not known. 



Of mineral constituents, iron is constantly present, and appar- 

 ently occurs in combination with the nucleins in organic form. 



