THE NUCLEUS 



69 



these statements is furnished by the trypanosomes parasitic in 

 vertebrate blood : on the one hand, these parasites often contain 

 in their cytoplasm so-called " chromatoid grains," probably of the 

 nature of volutin (Swellengrebel , 514), which stain in a similar 

 manner to the nucleus ; on the other hand, the nuclei of the 

 parasites react to stains in a manner very different from the 

 nuclei of the blood-cells amongst which they live. In short, it 

 is not possible to name any stain or class of stains which can be 

 relied upon either to combine \vith chromatin alone, or to stain 

 chromatin in the same manner and to the same degree, at all times 

 and in all cases* (compare. Fig. 33). When, 

 therefore, the adjectives " chromatinic " and 

 ' achromatinic ' are used in the course of 

 this work, it must be clearly understood 

 that these terms signify that the bodies or 

 substances to which they are applied con- 

 sist or do not consist, as the case may be, 

 of chromatin, and not that they stain or 

 do not stain with certain dyes. 



As regards the chemical nature of chro- 

 matin, it is characterized by containing 

 protein-substances more complex in com- FIG. 33. Diagram to repre- 



position than any other part of the cell ; it s f nt in a graphic manner 



... * /. .. i the action of colouring 



is not possible to say definitely, however, matters that stain chro- 



whether it is to be regarded as a single matin. The circle drawn 



i . , i , . ,. with an uninterrupted line 



chemical substance or as a combination or 

 mixture of several. Its most salient feature 

 is its variability ; judged by microchemical 

 tests, no two samples of chromatin can be 

 considered identical in composition, whether 

 from different cells or even from the same 

 cell at different times. Certain substances, 

 especially phosphorus-compounds, are espe- 

 cially characteristic of nucleo-proteins, but 

 it is not possible at the present time to 

 define or identify chromatin by its chemical 

 properties or composition. 



All experience at the present time tends to show that the final 



test for the identification of chromatin in the cell' is its relation to 



the vital activities and life-history of the organism. The term 



' chromatin ' is thus to be regarded as denoting a biological or 



physiological, but not a chemico-physical, unity. A given body 



* Methyl-green, acidulated with acetic arid, has sometimes been indicated as a 

 most distinctively nuclear stain ; but Hertwig (64) has shown that in the nuclei 

 of Actinosphcerium this stain colours the plastin-framework, and not the chro- 

 matin, and this author casts doubt on the alleged value of this stain as a reagent 

 for demonstrating chromatin in the nucleus. 



is supposed to represent 

 a theoretically perfect 

 chromatin - stain , which 

 would stain chromatin 

 always, and nothing else 

 but chromatin ; the circles 

 drawn with interrupted 

 lines represent the action 

 of chromatin stains actu- 

 ally ; they will stain chro- 

 matin as a general rule, 

 thojigh not invariably, but 

 they will also stain other 

 things which are not chro- 

 matin. 



