24 FIXING AND HARDENING. 



fixatives causing the maximum disturbance and destruction in the 

 individual cell, those in (c) the least. 



A great deal, however, depends on the accessibility of the cells to 

 the fixative, and as to whether vertebrate or invertebrate material 

 is being used. 



(a) Carnoy, Petrunkewitsch, alcohol, Gilson, picro-nitric, etc. 



Fat, mitochondria, Golgi apparatus, and often delicate yolk discs 

 do not show after these. (Using alcohols and xylol subsequently.) 



(6) Bouin, Zenker, corrosive acetic, Flemming-with-acetic acid, etc. 



Mitochondria and Golgi apparatus rarely show after these, except 

 possibly in mammals, where these cell inclusions are more resistant 

 than in invertebrata. Fats show with the last-mentioned fixative. 



(c) Osmic acid, Flemming- without-acetic, Champy, Altmann,' 

 formalin, Mann's mercury-osmic liquid, SjovalFs method, etc. 

 Preserve all formed granules (except glycogen). (Using fluids 

 subsequently as above.) 



In section (c) the formol alone will not preserve fat ; but see 

 Sjovall's method ( 696). 



The fixatives have not been classed according to how they them- 

 selves alone affect the contents of the cell, but according to how they 

 preserve the cell preparatory to its treatment in the liquids necessary 

 for embedding and sectioning. 



Injurious liquids which should never be used in cytological fixation 

 (3, vide supra) are acetic acid, chloroform and alcohol. Acetic acid is 

 nearly the most destructive to delicate lipins, and its use, except 

 where chromosomes are being studied, is rarely indicated ; any 

 worker who uses acetic acid in his fixing mixtures cannot hope to get 

 a correct picture of any part of his cell, possibly excepting the chromo- 

 somes (not the resting nucleus). The most valuable fixatives are 

 osmium - tetroxide, bichromate of potassium, chromium - trioxide, 

 and formaldehyde, possibly in the order named ; the most valuable 

 mixtures are Muller-formol (or Helly), Flemming- without-acetic, 

 Altmann, and Champy ; the three latter approach as near perfection 

 as present-day technique allows. Altmann's fluid (K^Cr^O; + 

 Os0 4 ) I find to be a splendid mixture. In no case, except in small 

 invertebrates, do these fixatives (excluding formol) give a true 

 fixation of cell aggregates ; this is due to their inferior penetrating 

 powers, and to an unevenness of penetration. Small invertebrates, 

 both marine and fresh- water, and small pieces of tissue, are usually 

 exquisitely preserved in chrome- osmium mixtures, but are not then 

 generally suitable for staining and mounting whole, especially for 

 staining in carmine mixtures. 



