262 CHROMATIN, ANIMAL CHROMOSOMES, NUCLEOLI 



hydrochloric acid. It is easily soluble in concentrated mineral acids, 

 in alkalies, even when very dilute, and in some alkaline salts, such as 

 carbonate of potash and biphosphate of soda. In the presence of 10 per 

 cent, solution of sodium chloride it swells up into a gelatinous mass, or 

 even, as frequently happens, dissolves entirely (Carnoy, Biol. Cell. 

 pp. 208 — 9). It is only partially diaestible (when in situ in the nucleus) 

 in the usual laboratory digestion fluids. 



The solvents of chromatin that are the most useful in practice are 1 

 per cent, caustic potash, fuming hydrochloric acid, or cyanide of potas- 

 sium, or carbonate of potash. These last generally give better results 

 than dilute alkalies. They may be employed in solutions of 40 to 50 

 per cent, strength. If it be desired to remove all the chromatin from a 

 nucleus the reaction must be prolonged, sometimes to as much as two 

 or three days, especially if the operation be conducted on a slide and 

 under a cover-glass, which is the safer plan. 



These operations must be performed on fresh cells, for hardening 

 agents render chromatin almost insoluble in ammonia, potash, or sodic 

 phosphate, etc. Hydrochloric acid, however, still swells and dissolves 

 it, though with difficulty. 



Chromatin resists the action of digestive fluids much longer than the 

 albumens do ; so that a moderate digestion serves to free the chromo- 

 somes from any karyoplasmic granulations that may obscure them 

 whilst at the same time it clears up the cytoplasm. Unna (Monatschr. 

 prakt. Derm., xxxiii, 1901, p. 342) digests tissues in solutions of sodium 

 chloride, to remove the " granoplasm." 



ANIMAL CHROMOSOMES 



624. General Remarks. There is no fixative known, for either 

 animal or plant tissues, which will fix all parts of a cell equally 

 well, and for detailed cytological studies it is desirable, at the outset, 

 to decide whether nuclear or cytoplasmic structures are of paramount 

 importajice. The reason for this seems to be, in part, that nuclear 

 fixatives must be strongly acid in order to preserve the chromatin 

 well and give clean cut fixation images after staining. On the 

 other hand, the cytoplasm and its inclusions seem to require, in 

 general, fixatives which are nearer neutrality, though let it be 

 noted that all fixatives are on the acid side of ^H 7-0 (exceptions 

 to the general rule are Benda's fixative (pH 1-2) and Champy's 

 fluid (j9H 1-4). To illustrate the effects of hydrogen ion concen- 

 tration, ZiRKLE {Protoplasma, iv, 1928, p. 201) has shown that 

 when bichromate fixatives are used which are on the acid side of 

 the critical range, pH 4-2-J9H 5-2, the nucleolus, chromatin, 

 spindle fibres and spongioplasm are fixed, while nuclear lymph, 

 mitochondria and hyaloplasm are dissolved. But on the basic 

 side of the critical range the cliromatin and spindle fibres are 

 destroyed while the other elements persist. 



YoMAHA {Bot. Mag. Tokyo, xxxix, 1925, p. 167) has measured 

 the hydrogen ion concentration of most of the common fixatives 

 and finds this to range from ^H 1-1 to pH 3-0. (For a discussion 

 of the j:;H see Atkins in the last edition.) 



