CYTOLOGICAL METHODS. 359 



HENKING'S mixture, which has been given above ( 625), 

 may also be found useful. 



Other fixing agents, such as picric acid or weak sublimate 

 solution, may of course be used, and in some cases doubtless 

 should be preferred. Other stains, too, such as Bismarck 

 brown, may be used as occasion dictates ; and of course other 

 examination media than solution of Ripart may be employed. 

 But, for general purposes, the methyl-green-osmium-and- 

 Ripart'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. 



647. Some Microchemical Eeactions. Methyl green is a test 

 for chromatin, in so far as it colours nothing but the chro- 

 matin in the nucleus. It is, however, not a perfect test, for 

 the intensity of the coloration it produces varies 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. The following sugges- 

 tions are taken from CARNOY, who is, I believe, the only 

 writer on the zoological side, at all events who has insisted 

 on the necessity of applying microchemical methods in a 

 systematic manner to the study of cells. 



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 (Bio I. Cell. f 

 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 prolonged, sometimes to as 



