There are two constituents of the cell which have, during the last few years, attracted a great deal of attention, namely, the nucleins and paranucleins.

The nucleins are essential constituents of the cell nucleus, appearing there in different forms of combination. The true nucleins possess at least three characters in common.

In the first place, they have a high percentage of phosphorus. Secondly, they offer great resistance to the action of peptic digestion.

Thirdly, they furnish nuclein bases on being subjected to the action of weak acids at moderate temperatures (about 100° C). They may be regarded as representing combinations between a rich phosphorus-holding acid—nucleic acid—and albumin. These combinations may either be of a firm or loose character, that is to say, in the case of the former it is difficult to split off the nucleic acid from the nuclein by the action of hot saturated baryta solutions, while in the latter the decomposition takes place easily.

It seems probable that there are four or more different nucleic acids, according as they give on decomposition certain definite nuclein bases. In other words, each nuclein base may have its corresponding nucleic acid from which it is derived. From this it follows that there may be also different nucleins corresponding to each nucleic acid. As examples, one may take thymus nuclein, which gives in decomposition with weak acids adenin and guanin, while, from the nuclein of the pancreas, only guanin has been obtained. This acid constituent of the nuclein possesses the property of precipitating proteids out of their solutions.