CHEMICAL RELATIONS OF NUCLEUS AND CYTOPLASM 331 



characterized by the presence of nuclein (chromatin) which has been 

 proved by chemical analysis to differ widely from the cytoplasmic 

 substances, 1 while the various forms of plastids are centres for the 

 formation of chlorophyll, starch, or pigment. These facts give ground 

 for the conclusion that the morphological differentiation of cell-organs 

 is in general accompanied by underlying chemical specializations 

 which are themselves the expression of differences of metabolic ac- 

 tivity ; and these relations, imperfectly comprehended as they are, are 

 of fundamental importance to the student of development. 



I. The Proteids and their Allies 



The most important chemical compounds found in the cell are the 

 group (A protein substances, and there is every reason to believe that 

 these form the principal basis of living protoplasm in all of its forms. 

 These substances are complex compounds of carbon, hydrogen, nitro- 

 gen, and oxygen, often containing a small percentage of sulphur, and 

 in some cases also phosphorus and iron. They form a very extensive 

 group of which the different members differ considerably in physical 

 and chemical properties, though all have certain common traits and 

 are closely related. They are variously classified even by the latest 

 writers. By many authors (for example Halliburton, '93) the word 

 " proteids " is used in a broad sense as synonymous with albuminous 

 substances, including under them the various forms of albumin (egg- 

 albumin, cell-albumin, muscle-albumin, vegetable-albumins), globulin 

 (fibrinogin vitellin, etc.), and \hz peptones ( diffusible hydrated proteids). 

 Another series of nearly related substances are the albuminoids 

 (reckoned by some chemists among the " proteids "), examples of 

 which are gelatin, mucin, and, according to some authors also, 

 nuclein, and the nucleo-albumins. Some of the best authorities how- 

 ever, among them Kossel and Hammarsten, follow the usage of 

 Hoppe-Seyler in restricting the word proteid to substances of greater 

 complexity than the albumins and globulins. Examples of these 

 are the nucleins and nucleo-proteids, which are compounds of nu- 

 cleinic acid with albumin, histon, or protamin. The nucleo-proteids, 

 found only in the nucleus, are not to be confounded with the nucleo- 



1 It has long been known that a form of " nuclein " may also be obtained from the nucleo- 

 albumins of the cytoplasm, e.g. from the yolk of hens' eggs (vitellin). Such nucleins differ, 

 however, from those of nuclear origin in not yielding as cleavage-products the nuclein bases 

 (adenin, xanthin, etc.). The term " paranuclein " (Kossel) or " pseudo-nuclein " (Ham- 

 marsten) has therefore been suggested for this substance. True nucleins containing a large 

 percentage of albumin are distinguished as nucleo-proteids. They may be split into albumin 

 (or albumin radicals) and nucleinic acid, the latter yielding as cleavage-products the nuclein 

 bases. Pseudo-nucleins containing a large percentage of albumin are designated as nucleo- 

 albumins, which in like manner split into albumin and paranucleinic or pseudo-nucleinic acid, 

 which yields no nuclein bases. (See Hammarsten, '94.) 



