326 OYTOLOGICAL METHODS. 



these, and such energetic hardening agents as Flemming's 

 mixture, come such light fixing agents as picric acid or very 

 dilute acetic acid. These it is whose employment is indicated 

 for the study of fresh isolated cells. 



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

 out a piece of living tissue in a drop of acidulated solution of 

 methyl green (0*75 per cent, of acetic acid). This is a deli- 

 cate fixing agent, killing cells instantly without change of 

 form. Complete the fixation by exposing the preparation for 

 a quarter of an hour to vapour of osmium, and add a drop of 

 solution of Ripart and Petit and a cover. 



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 osmium 

 if you like), and afterwards stain with methyl green. 



HENKING'S mixture, which has been given above ( 617), 

 may also be found useful. 



FLEMMING (Arch. f. mik. Anat. } xx, 1881, p. 3) found the 

 following method useful for the study of the division of the 

 ova of Echinodermata : The ova are stained on the slide by 

 adding the stain at the edge of the cover. Safranin or other 

 nitro- or anilin colours may be used. As soon as the entire 

 ovum is of a dark colour the stain is drawn off with blotting- 

 paper, and acetic acid of 1 per cent, added. Schneider's 

 acetic carmine (Zool. Anzeig., 1880) (see 158) is very con- 

 venient, and gives good results. (For the details of the 

 manipulation by which these reagents are added and drawn 

 off on the slide it is well to consult the article quoted, p. 6.) 



Another good method is as follows : Segmenting ova are 

 treated with a mixture of 40 to 50 parts of concentrated 

 nitric acid with 60 to 50 parts water. Wash with water until 

 all the yellow stain of the nitric acid has disappeared ; stain 

 with Schneider's acetic carmine, and mount in glycerin. (The 

 preparations cannot be said to be permanent, as after a time 

 the stain darkens in such a way as to render the nuclear 

 figures unrecognisable.) 



Other fixing agents, such as picric acid or weak sublimate 



