No. 589] 



THE EVOLUTION OF Till'. CELL 



17 



the cell has been stained with one of the dyes ordinarily 

 in use for coloring the chromatin, there are often seen in 

 the cytoplasm grains that are colored in exactly the same 

 manner as the chromatin-grains lodged in the nucleus. 

 Is an extranuclear grain which stains like chromatin to 

 be identified, ipso facto, as chromatin? By no means; it 

 may or it may not be chromatin. Simple inspection of a 

 stained preparation is altogether inadequate to deter- 

 mine whether such a body is or is not chromatin. Any 

 so-called chromatin-stain colors many bodies which may 

 occur in a cell besides the chromatin, and it may be nec- 

 essary to try a great many different stains before a com- 

 bination is found which will differentiate a given cyto- 

 plasmic enclosure from a true chromatin-grain by its 

 color-reactions. The so-called volutin-grains, for ex- 

 ample, 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. 



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 stri- 

 king example of this capriciousness is seen when a pre- 

 served 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-car- 

 mine or alum-carmine, which will not color the nuclei of 

 the trypanosomes in the slightest. Moreover, every cy- 

 tologist 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 increase in the staining 

 powers of the chromatin. There is no dye known which 

 can be relied upon to stain chromatin always, or wherever 



