PRRSTDENTTATi ADDRKSS. 443 



different stains before a combinationi 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 

 cytoplasDi of many Protists, ai-e identified by the fact tiiat thoy have a. 

 stroniger affinity for ' chromatiii-stains ' than chromatin itself. 



When, moreover, chromatin is compared with regard to its staining-reactions, 

 both in different organisms, and in the same organism at different times, it 

 is found to react very differently to one and the same stain. A striking 

 example of this capriciousness is seen when a preserved film is made of the 

 blood of some vertebrate which has nucleated blood-corpuscles, such as a bird 

 or fish, and which contains also parasitic trypanosomes. It is easy to stain 

 the nuclei of the blood-corpuscles with various stains, as for example carmine- 

 stains such as picro-carmine or alum-carmine, which will not colour the nuclei 

 of the trypanosomes in the slightest. Moreover, every cytologist knows that 

 the ' chromaticity ' of the chromatin varies enormously in different phases of 

 the nuclear cycle of generation ; it is often difficult to stain the chromatin 

 in the ' resting ' nucleus, but the first sign of impending nuclear division is a 

 marked inci-ease in the staining powers of the chromatin. There is no dye 

 known which can be relied upon to stain chromatin always, or wherever it 

 occurs. Methyl-green has been claimed to be the most reliable and certain 

 of nuclear stains, but R. Hertwig, in his classical researches upon Actino- 

 sphccrium, showed that it sometimes fails to stain chromatin. It is perfectly 

 conceivable that there might be varieties of cliromatin which could not be 

 stained by any dye whatsoever. 



I have felt bound to insist strongly upon the inadequacy of staining-methods 

 for the detection and identification of chromatin, well known though these 

 facts are to every cytologist, because here also I note a tendency amongst 

 biological chemists to regard staining-properties as the sole criterion of 

 chromatin. In reality such properties are of entirely secondary importance. 

 To use the terminology of formal logic, staining-properties are an ' accident,' 

 though it may be an 'inseparable accident,' of chromatin, not a 'difference' 

 which can be used to frame a logical definition, fer genus et difftrentias, of 

 this substance. If chromatin were nothing more than ' stainable substance,' 

 as Professor Armstrong terms it,* some of the most important results of 

 cytological investig;ation would be deprived of all real significance and reduced 

 to the merest futilities. 



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 complexity 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 reactions, for example, resistance to peptic digestion. E. B. Wilson, 

 in his well-known treatise, has emphasised the 'cardinal fact . . . that there 

 is a definite and constant contrast between nucleus and cytoplasm.' The 

 outstanding feature of the nucleus is the constant presence in abundance of 

 nuclein and nucleonroteins. Nuclein, which is probably identical with chro- 

 matin, is a complex albuminoid substance rich in phosphorus. It is the 

 phosphorus-content of chromatin that is its most characteristic chemical 

 peculiarity as contrasted with the cytoplasm. 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 cytologist 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 objective examples. If I make a preparation of Arcella vulgaris by suitable 

 methods, I see the two conspicuous nuclei and also a ring of granules lying 



• Science Progress, vol. vii. p. 327. 



