444 XXXII. CELL-DIVISION AND NUCLEAR DIVISION. 



purpose is the Horse Chestnut, which otters the advantage that its 

 flower-buds rnay be obtained in quantity at any required develop- 

 mental stage. The fixed flower heads of many Compositae, such a.-, 

 Tanacetum vulgar e, the Tansy, taken in longitudinal median sect Ion, 

 also serve admirably ; and, owing to the centripetal opening of the 

 florets, a long sequence of stages in nuclear division, as also in 

 pollen formation and ovular development, can be found in the 

 same section. This material fixes well. 



Fixing and Embedding, and Section- Cutting by Microtome. Fc 

 a fuller insight into nuclear and cell-division preparations fixed 

 and stained as above do not suffice ; it is necessary to use the 

 most approved means of fixing, embed the object 'when fixed, 

 that it can be sectionised in the microtome, and then stain 

 perfectly as possible. 



The best fixing fluid for the contents of vegetable cells thi 

 far known is Flemminy's solution." This is used in very 

 degrees of concentration, according to the object to be fixed. The 

 weakest mixture l contains 0*25 per cent, chromic acid, 0*1 per 

 cent, osmic acid, and O'l per cent, glacial acetic acid ; the 

 strongest mixture consists of 15 parts by volume of 1 per cent, 

 chromic acid, 4 parts of 2 per cent, osmic acid, and 1 part, or less, 

 glacial acetic acid. The weaker fluid is made use of specially for 

 the smallest objects, and stronger in proportion to their size. It 

 is desirable that, wherever practicable, large objects should be cut 

 up into the smallest possible pieces before laying into the fixing 

 fluid ; and if cutting up is practicable, it is best to use the weakest 

 fluid as above. A large quantity of the fixing fluid should always 

 be used, at least a hundredfold the bulk of the objects to be fixed. 

 With small objects fixing is complete even after a few hours, with 

 somewhat larger ones about a day. After complete fixing the 



1 Assuming that the stock solutions of these acids are 1 per cent, chromic, 

 2 per cent, acetic, and 2 per cent, osmic, a mixture, e.g., of 20 c.c. of this 

 weakest solution would contain 5 c.c. of the 1 per cent, chromic, 1 c.c. of the 

 2 per cent, osmic, and 2 c.c. of the 2 per cent, acetic, to which 13 c.c. water 

 should be added. 



Osmic acid is an admirable fixing reagent, but has what is sometimes the 

 disadvantage of producing a black or brown precipitate with oils, tannic acid, 

 etc. In the present instance this is no disadvantage, but if it were, decolor- 

 ising of even microtome sections without injury can be effected by means of 

 a solution of 1 part peroxide of hydrogen in 10 to 25 parts of 70-80 per cent, 

 alcohol. Xylol, creosote, etc., if used in the mounting processes, may also 

 decolorise them. [ED.] 



