280 CYTOLOGICAL METHODS. 



especially if the operation be conducted 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 fresh cells, for hardening agents bring about very 

 considerable modifications in the nature of nuclein, rendering 

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

 &c. 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. Nuclein resists the action of 

 digestive fluids much longer than the albumens 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. 



In the presence of iodine, of hot nitric acid, and of MILLON'S 

 test, nuclein gives the reactions of protein matters. 



(MILLON'S test consists of 1 part mercury, 1 part fuming 

 nitric acid, and 2 parts water.) 



Another statement of the micro-chemistry of the cell, by ZACHAEIAS, is to 

 be found in Zeit.f. wiss. Mik., iv, 3, 1887, p. 409, and Journ. Roy. Mic. 

 Soc., 1888, p. 505. 



603. Cytological Fixing Agents. The following is in great 

 part taken from the numerous papers of FLEMMING in the 

 Arch.f. mik.Anat., from the year 1879 onwards, and from his 

 Zellsiibstanz, Kern- und Zelltheilung. 



Osmic acid (^ to 2 per cent.) preserves the form of the 

 entire cell, but swells the nuclei and rounds off nucleoli. It 

 renders the nuclear "reticulum" undiscernible. Picric acid, 

 either concentrated or dilute, and chromic acid, 0-1 to 0*5 per 

 cent., are to be preferred to alcohol and other agents for the 

 study of the cells of Vertebrates. Shrinking and distortion of 

 the nuclear figures (and, with picric acid, swellings of them) 

 are to be expected, but other agents have the same defect to 

 a much greater degree ; alcohol especially causes entanglement 

 of the filaments. Acetic acid does the same, and causes 

 swelling besides. Stronger chromic acid solutions cause shrink- 

 ing. Neither of these reagents is harmless as regards the 

 nuclei of red blood-corpuscles. The salts of picric acid (potash-, 

 soda-, and baryta-salts) are most harmful. Weak (i. e. not 

 more than 1 per cent.) acetic, hydrochloric, or nitric acid 



