260 CHROMATIN, ANIMAL CHROMOSOMES, NUCLEOLI 



tion this gives with fuchsin-sulphurous acid. Needless to say, 

 the vahdity of this test depends upon the absence of any aldehyde 

 in the tissue, either produced by the cells themselves, or as the 

 indirect effect of the fixative, and on the prevention of an oxida- 

 tion of the fuchsin-sulphurous acid, which might cause the latter 

 to act as a stain. Feulgen {Zeit.f. physiol. Chemie, cxxxv, 1924) 

 advises against the use of any fixatives which contain aldehydes 

 or oxidising agents and recommends alcohol, acetic-alcohol and 

 sublimate acetic. This last, which has been widely used, consists 

 of a 6 per cent, corrosive sublimate solution to every 100 c.c. of 

 which 2 c.c, of glacial acetic acid is added. But the fixatives 

 recommended by Feulgen, unfortunately, are not very satisfactory 

 cytological reagents for chromosomes. Bauer {Zeit. f. Zellforsch. 

 u. mikr. Atiat., xv, 1932, p. 225) has shown, however, that, with 

 the exception of picro-formol, Bouin's and Hermann's fluids, this 

 test may be satisfactorily applied to tissues preserved with many 

 of the common nuclear fixatives provided : (1) that the tissue is 

 thoroughly washed after fixation and (2) that sections are subjected 

 to hydrolysis for the proper length of time, Bauer has shown 

 that there is a relatively short period during hydrolysis of animal 

 tissues when the maximum staining reaction is obtained. Thus, 

 following corrosive-acetic fixation, the stain is most intense when 

 hydrolysis goes on for from four to eight minutes with the optimum 

 at five minutes. After eight minutes the chromatin stains less 

 deeply, and after twenty-nine minutes of hydrolysis it will not 

 stain at all. The optimum time of hydrolysis for the commoner 

 fixtives, as determined by Bauer, is indicated below : 



Hydrolysis. The hydrolysing fluid is prepared by adding 

 82-5 c.c. of HCl (S.G. 1-19) to 1 litre of distilled water. The acid 

 is placed in a staining jar or other suitable container, and brought 



