CYTOLOGICAL METHODS. 319 



Other fixing- agents, such as picric acid or weak sublimate 

 solution^ may of course be used. Other stains, too, such as 

 Bismarck brown, and of course other examination media 

 than solution of Ripart may be employed. But, for general 

 purposes, the methyl - green - osmium - and - Kipart's - medium 

 method gives such good results, and is so very convenient, 

 that it may be called a classical method for the study of fresh 

 cells. 



646. Some Microchemical Reactions. — Methyl green is a test 

 for chromatin, in so far as (with fixsh cells) it colours nothing 

 but the chromatin in the nucleus, see § 276. It is, however, 

 not a perfect test, for the intensity of the coloration it pro- 

 duces vai-ies greatly in different nuclei, and may in certain 

 nuclei be extremely weak, or (apparently) even altogether 

 wanting. In these cases other tests must be applied in order 

 to establish with certainty the presence or absence of that 

 element. 



Chromatin is distinguished from albuminoids by not being 

 soluble, as these are, in water and in weak mineral acids, 

 such as O'l per cent, 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 

 {Carboy, Biol. Cell., pp. 208 — 9). It is only partially digestible 

 (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 potassium, 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 pi-olonged, 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 



