CHROMATIN, ANIMAL CHROMOSOMES, NUCLEOLI 259 



For staining such temporary mounts, Belling's iron-aceto- 

 carmine is highly recommended (see § 257), or 1 per cent, of methyl 

 green in a 1 per cent, solution of acetic acid may be used. 



A typical example of this kind of work is as follows : A bit of 

 living tissue is teased out in Ringer's solution (the cold-blooded 

 or warm-blooded formula, as the case may be), then placed on a 

 slide, a coverslip added, and examined. If a stain is needed, as 

 is usually the case, after placing the tissue on a slide the excess 

 of Ringer is drawn off and aceto-carmine is added in excess. 

 This fixative stain is allowed to act for two to ten minutes or more, 

 if need be, a coverslip is placed over the tissue, the excess stain is 

 withdrawn with filter paper and the cover-glass may be sealed 

 with vaseline or with melted paraffin, or in some other way. It 

 is often desirable to mash out the tissue after it has been stained. 

 This is done by pressing in the coverslip with a blunt needle or 

 with the tip of a finger. This should be done before the cover is 

 sealed. Smears may be made of testicular and similar tissues 

 and stained in this way, but in general crushed preparations are 

 much better. 



Or you may fix the preparation, after teasing, with vapour of osmium 

 for half a minute to two minutes, then add a drop of methyl green, 

 and after five minutes wash out with 1 per cent, acetic acid, and add 

 solution of Ripart and Petit and cover. 



Or you may kill and fix the cells by teasing in solution of Ripart 

 and Petit (to which you may add a trace of osmic acid if you like), 

 and afterwards stain with methyl green. 



We have found Pictet's chloride of manganese (§ 442) useful as an 

 examination medium. A little solution of dahlia may be added to it. 



Henking (Zeit. iviss. Mik., viii, 1891, p. 156) recommends a liquid 

 composed of — 



Water ....... 80 c.c. 



Glycerin . . . . . . 16 ,, 



Formic acid . . . . . . 3 ,, 



Osmic acid of 1 per cent. . . . 1 ,, 



Dahlia ....... 004 grm. 



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 either the aceto-carmine, or the 

 methyl green osmium and Ripart's medium methods give such 

 good results and are so easy and convenient to use that they 

 should always be tried first in studies of this sort. 



623. The " Nuclealfarbung " Method (Feulgen's Reaction). Of 

 the various methods for the demonstration of cliromatin, that of 

 Feulgen seems to be the most precise and the most satisfactory, 

 for general cytological ])urposcs. It depends upon the j^rcsence, 

 among the products resulting from a mild hydrolysis of nucleo- 

 proteids, of an aldehyde grouping and the brilliant violet colora- 



9—2 



