GENERAL METHODS, 2 7 



stained preparations with several different fluids is necessary 

 to obtain well-differentiated images. 



39. To wash microtome sections in running water the 

 apparatus described and figured in 35 may be .conven- 

 iently used by placing the slides in an oblique position in 

 the space h of the zinc vessel, so that the sides bearing the 

 sections are directed downward. To provide for the con- 

 stant escape of water from the bottom of this space, the 

 zinc strip i is perforated like a sieve near its lower edge, 

 while the somewhat lower strip, k, is unperforated, but 

 serves to prevent the too rapid fall of the fluid in the 

 space h. 



D. FIXING AND STAINING MICROSCOPICALLY SMALL 

 OBJECTS. 



40. The fixing and staining of microscopically small 

 objects offers certain technical difficulties, especially if one 

 has but a limited quantity of material. These are best 

 overcome by the methods devised by Overton (I, 13), which 

 can be variously modified to meet the peculiarities of the 

 objects concerned. Thus it seems to me more cenvenient 

 to carry on the manipulations to be described on the slide 

 rather than on the cover-glass, as Overton recommends, 

 unless for special reasons culture in the hanging drop is 

 necessary. 



In the first place, for fixing the objects in a drop of cul- 

 ture fluid an easily removable fixing medium should be 

 used. For this purpose the fumes of iodine are well 

 adapted. They are poured out upon the preparation from 

 a heated test-tube, and are easily driven off again by subse- 

 quent warming (2 to 5 minutes in the paraffine bath). For 

 the same purpose the fumes of osmic acid may be used. 

 They are applied by holding the slide, with the objects 

 downward, over the mouth of a bottle containing a dilute 

 solution of osmic acid.* 



* [Where this inversion of the slide cannot be safely risked, the addition to 

 the culture fluid of a drop of a \% solution of osmic acid may serve the same 

 purpose.] 



