i86 



NATURE 



[October 14, 19 15 



plasm." Unless I do so, my position is certain to be 

 misunderstood, as, indeed, it has been already by 

 some of my critics. 



The term cell was applied originally to the proto- 

 plasmic corpuscles building up the bodies of the 

 Metazoa and Metaphyta, each such corpuscle consist- 

 ing of a minute individualised mass of the living sub- 

 stance and containing a nucleus. Hence a complete 

 cell is made up of two principal parts or regions, the 

 nucleus and the remainder of the protoplasmic body, 

 termed the cytoplasm. By some authors the term 

 protoplasm is restricted to the cytoplasmic portion of 

 the cell, and protoplasm is then contrasted with 

 nucleus ; but it is more convenient to consider the 

 whole cell as composed of protoplasm divided into 

 two regions, nucleus and cytoplasm. 



We come now to the consideration of the body 

 termed the nucleus, which undoubtedly possesses an 

 importance in the life and functions of the cell far 

 greater than would be inferred from the name given 

 to it. I have described in general terms the typical 

 nucleus of the text-books, as found commonly in the 

 cells that build up the bodies of ordinary animals 

 and plants. The minutiae of the details of structure 

 and arrangement of the constituent parts may vary 

 infinitely, but the type remains fairly constant. 

 When we come, however, to the nuclei of the Protista, 

 such pronounced modifications and variations of the 

 type are met with that a description in general terms 

 is no longer possible. I will direct attention now only 

 to one point. In the protist cell the chromatin is not 

 necessarily confined to the nucleus, but may occur 

 also as extranyclear grains and fragments, termed 

 chromidia, scattered through the protoplasmic body; 

 and the chromatin may be found only in the chro- 

 midial condition, a definite nucleus being temporarily 

 or permanently absent. 



The essential part of the nucleus is the chromatin, 

 and the other structural constituents of the nucleus, 

 namely, membrane, framework, and plastin or 

 nucleolar bodies, are to be regarded as accessory com. 

 ponents built up round, or added to, the primary 

 nuclear material, the chromatin. Even with regard 

 to the nuclei of Metazoa it is maintained by Vejdovsky 

 that at each cell-generation the entire nucleus of the 

 daughter-cell is produced from the chromosomes alone 

 of the mother-cell. The simplest body which can be 

 recognised as a nucleus, distinct from the chromidia 

 scattered without order or arrangement throughout 

 the protoplasmic body, is a mass of chromatin or a 

 clump of chromatin-grains supported on a framework 

 and lodged in a special vacuole in the cytoplasm. 



This brings me to a point which I wish to 

 emphasise most strongly, narnely, that the conception 

 of a true cell-nucleus is essentially a structural con- 

 ception. A nucleus is not merely an aggregation of 

 chromatin ; it is not simply a central core of some 

 chemical substance or material differiner in nature 

 from the remainder of the protoplasm. The concepts 

 "nucleus" and "chromatin" differ as do those of 

 "table" and "wood." Although chromatin is the 

 one universal and necessary constituent entering into 

 the composition of the cell-nucleus, a simple mass of 

 chromatin is not a nucleus. A true nucleus is a cell- 

 organ, of greater or less structural complexity, which 

 has been elaborated progressively in the course of the 

 evolution of the cell ; it is as much an organ of the 

 cell as the brain is an organ of the human body. As 

 a definite cell-organ, it performs in the life and 

 economy of the cell definite functions. As an organ 

 of the cell, however, it has no homologue or analogue 

 in the body of the multicellular animals or plants ; 

 there is no organ of the human body, taken as a 

 whole, similar or comparable to the nucleus of the 

 cell. 



The foregoing brief consideration of the nucleus 

 leads me now to discuss in more detail the nature and 

 properties of the essential nuclear substance, the so- 

 called chromatin. To define, or characterise ade- 

 quately, this substance is a difficult task. The name, 

 chromatin is derived from the fact that this substance 

 has a peculiar affinity for certain dyes or stains, so 

 that when a cell is treated with the appropriate colour- 

 ing reagents — with so-called nuclear stains — the 

 chromatin in the nucleus stands out sharply, by reason 

 of being coloured in a different manner from the rest 

 of the cell. In consequence, the statement is fre- 

 quently made, in a loose manner and without reflec- 

 tion, that chromatin is recognised by its staining 

 reactions, but in reality this is far from being true. 

 When a preparation of an ordinary cell is made by 

 the methods of technique commonly in use, the 

 chromatin is recognised and identified by its position 

 in a definite body with characteristic structure and 

 relations to the cell as a whole, namely, the nucleus, 

 and this is equally true whether the chromatin has 

 been stained or not. Any so-called chromatin-stain 

 colours many bodies which may occur in a cell besides 

 the chromatin, and it may be necessary to try a great 

 many different stains before a combination is found 

 which will differentiate a given cytoplasmic enclosure 

 from a true chromatin-grain by its colour-reactions. 

 The so-called volutin-grains, for example, which are 

 found commonly in the cytoplasm of many protists, 

 are identified by the fact that they have a stronger 

 affinity for " chromatin-stains " than chromatin itself. 



What, then, is the true criterion of the chromatin- 

 substance of living organisms? From the chemical 

 point of view the essential substance of the cell- 

 nucleus would appear to be characterised by a com- 

 plexity of molecular structure far exceeding that of 

 any other proteins, as well as by certain definite 

 peculiarities. Especially characteristic of chromatin 

 is its richness in phosphorus-compounds, and it stands 

 apart also from other cell-elements in its solvent re- 

 actions, for example, resistance to peptic digestion. 

 How far these features are common, however, to all 

 samples of chromatin in all types of living organisms 

 universally, cannot, I think, be stated definitely at 

 present ; at any rate, it is not feasible for a cytologlst 

 of these days to identify a granule in a living 

 organism or cell as chromatin solely by its chemical 

 reactions, although it is quite possible that at some 

 future time purely chemical tests will be decisive upon 

 this point — a consummation devoutly to be wished. 



The only criterion of chromatin that is convincing 

 to the present-day biologist is the test of its behaviour, 

 that is to say, its relations to the life, activity, and 

 development of the organism. I may best express my 

 meaning by an objective example. If I make a pre- 

 paration of Arcella vulgaris by suitable methods, I 

 see the two conspicuous nuclei and also a ring of 

 granules lying in the cytoplasm, stained in the same 

 manner as the chromatin of the nuclei. Are these 

 extranuclear granules to be regarded also as chro- 

 matin? Yes, most decidedly, because many laborious 

 and detailed investigations have shown that from this 

 ring of granules in Arcella nuclei can arise, usually 

 termed "secondary" nuclei for no other reason than 

 that they arise de novo from the extranuclear chro- 

 matin and quite independently of the "primary" 

 nuclei. The secondary nuclei are, however, true 

 nuclei in every respect, as shown by their structure, 

 behaviour, and relations to the life-history of the 

 organism ; they may fuse as nuclei of gametes (pro- 

 nuclei) in the sexual act, and they become, with or 

 without such fusion, the primary nuclei of future 

 generations of Arcella ; they then divide by karyo- 

 kinesis, when the organism reproduces itself in the 

 ordinary way by fission, and are replaced in their 



NO. 2398, VOL. 96] 



