304 CYTOLOGICAL METHODS. 



it) ; differentiate in the iron alum solution for at least two minutes 

 after the chromosomes, examined in water, appear to have been 

 sufficiently extracted, for chromosomes always appear paler in 

 water than- after they have been got into balsam." Personally 

 I have never found iron hsematoxylin to clog chromosomes, and can 

 only assume that something was wrong with the fixation, or a bad 

 specimen of stain was used. 



Hot or Cold Fixation ? Some workers advocate the use of hot 

 fixatives, others believe that the best results are obtained by keeping 

 the capsule or vial of fixative on ice while the material is being fixed. 

 Possibly cold, not freezing, fixatives are indicated for invertebrates 

 and cold-blooded animals, and fixatives at body heat for warm- 

 blooded. This, however, must be left to the discretion of the 

 worker, who may find that either very cold or hot fixatives may improve 

 his preparations in an unexpected manner. EZKA ALLEN recommends 

 using his modified chromic Bouin and urea at 38 C., while he 

 believes that Flemming should be used in an ice-box (Anat. Record 

 x, 1915 16). COWDRY (Contrib. Carnegie Inst. Wash., viii, 1918, 

 recommends the use of Regaud's formol-bichromate on ice. See 

 also 31. 



653. Dissection of Animals for Chromosome Work. Some ob- 

 servers have claimed that dissecting out gonads of invertebrates, in 

 a dish of " tap- water," gives clearer chromosome figures than when 

 one uses Ringer or such salt solutions. If possible, avoid dissecting 

 out in any fluid. Fleas and lice, and such small insects, are to be 

 treated as follows : cut off the end of the abdomen, hold the insect 

 down on a glass slide by its head with the aid of a mounted needle, 

 and with another needle press the viscera out with a stripping 

 motion from the head backwards. Immediately transfer the viscera 

 to a fixative (DONCASTER, Quart. Jour. Micr. Sti., 1920). For 

 bigger insects one may qpen the body cavity after having cut off 

 their heads, and pipette fixative over the viscera before separating 

 away the gonads. Then transfer to a capsule of the fixative. Read 

 also directions in 12 and 676. 



654. Fusion of Chromosomes caused by Fixation. It should be noted 

 that unsuitable or inferior fixation may cause such artifacts as fusion of 

 chromosomes which, intra vitam, may have merely been closely paired. 

 With Diptera it has been found that bodies of mosquitoes, etc., should not 

 be thrown whole into a fixer, but either finely teased or the gonads 

 should be carefully dissected out. This obtains a more rapid and there- 

 fore a more efficient fixation (HANCE, Jour. Morph., 1917 ; METZ, 

 Jour. Exp. ZooL, 1916). 



