360 CHAPTER XXV11. 



much as two or three days, especially if the operation be con- 

 ducted on a slide and under a cover-glass, which is the safer 

 plan. 



It must be remembered that these operations must be per- 

 formed on frexli cells, for hardening agents bring about very 

 considerable modifications in the nature of chromatin, render- 

 ing it almost insoluble in ammonia, potash, or sodic phosphate, 

 etc. Hydrochloric acid, however, still swells and dissolves 

 it, though with difficulty. 



Partial digestion may render service in the study of the 

 chromatic elements of nuclei. Chromatin resists the action 

 of digestive fluids much longer than the albumins do ; so that 

 a moderate digestion serves to free the chromosomes from any 

 caryoplasmic granulations that may obscure them, whilst at 

 the same time it clears up the cytoplasm. 



Concerning the microchernistry of the cell see further last edition ; 

 also CAENOY & LEBHUN, La Cellule, xii, 2, 1897, p. 94 ; ZIMMEBMANN, 

 Die Morphologie u. Physiologic des Pflanzlichen Zellkernes, Jena, 189(> 

 (treats also of the animal cell) ; HAECKER, Praxis u. Theorie der Zellen- 

 und Befruchtungslehre, Jena, 1899. 



648. Cytological Fixing Agents. It does not follow that a 

 fixing agent that is good for one element of a cell is also good 

 for all others. That which is good for cytoplasm is not 

 necessarily good for the nucleus, and vice versa. 



As regard* the nucleus, it is a rule that admits of no 

 exception that all fixatives must be acid for if not they 

 will not satisfactorily preserve either chromatin or nucleoli. 



For instance, bichromate of potash, if not rendered acid, 

 should be banished from the study of nuclei, because it 

 causes chromatin and nucleoli to swell, so that clear images of 

 them are not obtained. (I do not myself think that, as 

 regards chromosomes at all events, the images given by bi- 

 chromate are so unnatural as they are held to be by Flemmmg 

 and the majority of authors. Chromosomes during life are 

 always in a state that may be fittingly described as xirollen 

 by comparison with their state after fixation by acids. 

 During life, in the equatorial and polar stages of division, 

 they are mostly compacted into plates or pectiniform figures 

 in which the separate elements are not clearly discernible, 

 and which are more like the images given by fixation with 



