346 THE PRESIDENTS ADDRESS. 



case some writers use the word "protoplasm" as synonymous with 

 cytoplasm that is to say, to denote the ground-substance of the 

 living matter in contradistinction to the nucleus or chromatin. 

 I think it is best to use the term "protoplasm" for the living 

 substance as a whole, regarding it as composed of two principal 

 parts namely, the cytoplasmic matrix and the chromatin-grains 

 or nucleus, which, as universal and permanent constituents, are 

 to be distinguished from temporary products of vital activity 

 such as the metaplastic grains. 



I must now enter into some further details with regard to the 

 chromatin-grains. The usual test for chromatin is its staining 

 properties, but this is, in reality, a very inadequate method of 

 identification, for two reasons. In the first place, there may be 

 present in the protoplasm formed masses of substances of various 

 kinds which are not chromatin, but which may stain more 

 intensely than the true chromatin, even with the so-called nuclear 

 stains. In the second place, true chromatin often reacts very 

 differently towards the same stain in different cases. A particular 

 method of staining will colour the chromatin of one organism 

 very deeply, that of another not at all. There is no dye known 

 which can be relied upon to stain chromatin always, or which 

 will stain nothing but chromatin in the protoplasm. 



From the chemical point of view, all that can be said of 

 chromatin is that it appears to consist of, or to contain proteins 

 more complex than any part of the living substance ; the so-called 

 nucleo-proteins, characterised chiefly by being rich in phosphorus- 

 compounds. The infinite variability of the proteins, already 

 mentioned, is seen especially in the case of chromatin. It is 

 highly probable that no two samples of chromatin are ever 

 perfectly similar a statement which applies not only to organ- 

 isms of different species, but even to different individuals of the 

 same species. 



Chromatin, therefore, is not to be i*egarded as a substance 

 which can be characterised or defined by chemico-physical reactions 

 or properties. It would perhaps be better to speak of chromatins, 

 or of the chromatin-class of substances, than to use the word 

 "chromatin "in a manner that might convey the idea that a definite 

 chemical compound was meant by it. The conception of chromatin 

 is founded essentially upon its biological properties ; it must be 

 recognised and identified by its relation to the vital activities and 



