1206 NUCLEIX. 



admit of more than a somewhat synoptic statement of the views 

 at present held. 



The nucleins are all obtained as an undissolved residue, left 

 when the cells, tissue, or other parent-substance is subjected to 

 artificial digestion with pepsin and hydrochloric acid. This 

 residue is then purified by solution in very dilute alkali, pre- 

 cipitation with hydrochloric acid and washing with water, 

 alcohol and ether. As thus prepared the nucleins are white 

 amorphous powders, characteristically rich in phosphorus, whicli 

 may be split off as phosphoric acid by boiling with acids or 

 alkalis. During this decomposition some proteid is simultane- 

 ously obtained so that the nucleins are regarded as compounds 

 of proteid with a substance to which the name of nucleic acid 

 has been given. In accordance with these facts the nucleins 

 exhibit distinctly acid properties, give the reactions character- 

 istic of proteids and have a marked affinity for basic staining 

 reagents. 



Further study of the products obtained when nucleins ai 

 boiled with mineral acids has led to their being divided into 

 two classes; of these the true nucleins, or as it may best be 

 called simply 'nuclein,' yields not merely proteid and phos- 

 phoric acid, but characteristically members of the xanthine 

 group (see p. 1263), the so-called xanthine bases or nuclein 

 bases. These nucleins occur preponderatingly in nuclei, are 

 often ferruginous and some of them yield a reducing substance 

 when decomposed by boiling acids, while others do not. The 

 other form of nuclein has received the name of pseudo- or para- 

 nuclein ; it gives the same general reactions as does true nuclein, 

 but is sharply characterized by not yielding any of the xanthine 

 bases when decomposed by boiling with dilute mineral acids. 

 This form of nuclein is known chiefly as forming the undissolved 

 residue left during a peptic digestion of casein or other typical 

 nucleo-albumin. 



As already stated the true nucleins are regarded as com- 

 pounds of a proteid with nucleic acid. This acid is the source 

 of the xanthine bases obtained by the cleavage of nuclein, and 

 since the nucleins from various sources yield preponderatingly 

 and hence characteristically some one of the xanthine bases, it 

 appears probable that there are several forms of nucleic acid. 

 The nuclein of spermatozoa is regarded by many observers as 

 being a native nucleic acid, since it yields only xanthine bases 

 and phosphoric acid but no proteid when decomposed by acids. 



Nucleo-albumins . 



While nuclein is now regarded as a compound of nucleic acid 

 with a proteid, it is found that the nucleins may themselves 

 unite with a further and often large amount of proteid, to form 



