REACTIONS OF FIXATIVES WITH TISSUES AND CELLS 73 



This method of embedding was chosen partly because it is so 

 much used, partly because it has a strong tendency to distort and 

 thus provides a stringent test. Sections are cut at 8/x and dyed v^ith 

 Hansen's haematein (so-called Trioxyhdmatein).^^^ When a piece 

 of tissue has been dehydrated, passed through toluene or other 

 antemedium, and embedded in paraffin, it has probably undergone 

 all the distortion that the processes of microtechnique can w^reak 

 upon it, and the choice of mounting medium is therefore of little 

 significance for the purpose of the test. Canada balsam was chosen, 

 mainly because it seems to remain the most popular mountant. 



It is to be wished that we had some means of estimating the 

 quality of fixation objectively. The degree of shrinkage or swelling 

 can be measured (p. 75), but we have no other numerical data 

 and for the present it is necessary to rely on subjective impressions. 

 Anyone who proposes to judge fixatives should equip himself 

 for the work by prolonged experience in the study of living 

 cells. 



People are often prejudiced in their beliefs about the value of 

 different fixatives. To prevent this from influencing the results of 

 tests, it is essential that the judge should never know what fixative 

 he has been judging until after he has given his opinion. He 

 should examine many preparations fixed in different ways and 

 report fully on each before being told which is which. Preparations 

 fixed in fluids that contain osmium tetroxide are usually darkened, 

 and they should therefore be bleached before dyeing, so as not to 

 be recognizable. 



Written records should be made under a standardized set of 

 headings before a definite opinion of the value of a fixative is 

 formed. The following will serve as examples of headings in tests 

 carried out with the testis of the mouse: — 



outlines of seminiferous tubules (whether smooth as in life, or 

 wrinkled by shrinkage) ; 



spaces betw^een tubules (whether artificially enlarged or dis- 

 torted) ; 



cohesion of spermatogenetic cells (whether maintained or lost) ; 



cytoplasm of spermatogenetic cells (whether homogeneously 

 fixed, coarsely coagulated, or disintegrated) ; 



chromosomes (whether fixed in life-like form in the meiotic 

 phases) ; 



dyeing (whether intense or weak, differential or diffuse). 



