CHAPTER XXVI. 307 



Glacial acetic acid lOc.c. 



Urea crystals J grm. 



Use for twenty-four hours. Stain in iron haematoxylin or Flemming's 

 tricolour mixture (Jour. Morph., xxviii., 1916 17). 



661. Precautions in Dehydrating and Clearing. It is well known 

 that too rapid dehydration or clearing will cause shrinkage and 

 distortion. Material to be used for chromosome study should be 

 dehydrated gradually, either by a syphon or some drop method 

 ( 3). For clearing xylol is not good, as it causes much shrinkage. 

 The least shrinkage occurs with some vegetable oil, like bergamot, 

 origanum, cedar wood, or cassia (cinnamic aldehyde), which, if it 

 will not mix with paraffin, should be washed out subsequently in 

 some paraffin solvent, like benzole or xylol. EZRA ALLEN (Anat. 

 Record, x, 1915 16), following Suchannek ( 134), uses distilled 

 aniline oil as a substitute for the higher alcohols. One gradually 

 brings brain and genital or such tissue to 75 per cent, alcohol, and 

 then adds distilled anilin oil by the drop method, shaking frequently, 

 or using some system for agitating the fluids, or by some diffusion 

 apparatus ( 3). When nearly pure anilin oil has replaced the 

 alcohol, one transfers to pure oil until the tissue is cleared. From 

 anilin oil one embeds as follows : warm the oil and tissue slightly, 

 adding every ten minutes a few drops of melted paraffin, mixing 

 thoroughly with a pipette ; continue till the mixture has 85 to 90 

 per cent, paraffin. Transfer to melted paraffin. If bergamot oil 

 has been used for largish objects, at least four changes of pure 

 paraffin must be made, one half-hour to each, and the fifth bath 

 for at least one hour. In most cases gradual clearing in cedar-wood 

 oil will give satisfactory results. See also 5. 



662. Mounting Sections between Coverslips. AGAR (Quart. Jour. 

 Micr. Sci., 1911) has devised a method for mounting prepara- 

 tions between coverslips in order that they can be observed on both 

 sides. Use one larger coverslip as if it were a slide. Carriers may be 

 made by stamping out a square in stiff cardboard, or thin metal. 

 See also, C. CEPEDE, C. R. Soc. BioL, civ, 1913. 



663. Smear Preparations of Gonads. In some cases smear pre- 

 parations of testes especially may provide useful evidence in a 

 research on chromosomes. One may be fortunate enough to find 

 nuclei at the prophase or metaphase of mitosis, with all the chromo- 

 somes spread out so as to be counted with ease. In many cases, 

 to study early stages in synapsis for which very rapid penetration 

 is essential, smear preparations are a sine qua non. 



202 



