ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 507 



box. Whether plaster of Paris or metal cases are used, the gelatin 

 must not come in contact with the sides of the dishes. The tools neces- 

 sary to make these boxes have proved fairly expensive, but the boxes 

 themselves can be obtained at a comparatively small cost. 



Preparation and Mounting of Microscopic ObjectSs* — H. C. 

 Whitfield says that in cutting the tissues for fixing purposes, small 

 pieces are essential, in order that the tissues may be quickly per- 

 meated with the fixing fluid. The chief fixing agents are osmic acid, 

 chromo-acetic acid, picro-acetic acid, formalin, and alcohol. The first- 

 named is most efficient for small subjects, but is barred in many cases 

 owing to expense. The cheapest of the three is chromo-acetic acid, 

 which may be washed out in water afterwards, thereby preventing 

 shrinkage. The drawback to picro-acetic acid is that it necessitates 

 the use of alcohol for removing the fixing fluid from the specimen, 

 consequently the risk of shrinkage of the cells is much greater in this 

 method of treatment. The use of osmic acid is often followed by 

 blackening of the cell-contents. 



Chromo-acetic formula : — Chr. acid, 1 grain ; glac. acetic, 2 • 25 c.cm. ; 

 water, 3<»0 c.cm. 



There are many cases where fixing is not required, but in all 

 instances the material needs treatment, so that it may be preserved in 

 such a state that the cells retain their normal shape and do not 

 undergo any degeneration. This may be accomplished by treatment 

 with 50 p.c. alcohol, followed successively by 75 p.c, 90 p.c, and 

 absolute, at periods of six or seven hours. By this means the material 

 becomes impregnated with the spirit without causing shrinkage. They 

 may then be kept in spirit indefinitely, and used as required. 



Section Gutting. — This may be performed in various ways : by hand 

 with or without a microtome, or by a mechanically controlled section 

 cutter. The free-hand cutting is quite satisfactory in many cases for 

 sections for cursory examination, but if a really good specimen for per- 

 manent use is required the use of a microtome is advised. In the author's 

 own work the two which he made use of were Cole's hand section cutter, 

 and the Cambridge Rocking Microtome. Both of these necessitate the 

 use of embedding material. In the former, fresh carrot is the most suit- 

 able, while the latter requires a much longer preparation, as the subject 

 must be trftated for paraffin infiltration. This will be described later in 

 Ribbon Section Cutting and Staining. In cutting the sections a well- 

 sharpened razor must be used. The author usually got the cutler to 

 grind the razor, a stiff hollow-ground pattern, and then set it himself 

 on a hone. This is done by a slicing action, rubbing the edge of the 

 razor against the hone, as it were taking a slice from the stone. The 

 operation is easily completed in ten minutes or so. The razor must 

 then be well stropped in such a manner that the back of the razor never 

 leaves the strop. On placing the object in the carrot, a slicing action 

 is preferable to a planing action, as there is a tendency to tear the tissues 

 with the latter method. 



* Proc. Photomicrographic Soc, v. (1916) pp. 43-52 (3 figs.). 



