FIXING AND HARDENING 19 



penetration to ensure that small pieces of tissue be equally fixed 

 by it throughout. No single substance or chemical compound 

 fulfils all that is required of a good fixing agent ; hence it is that 

 all the best fixing agents are mixtures. Osmic acid, for instance, 

 fulfils some of these conditions, but not all of them. It kills 

 rapidly and preserves admirably the elements of cytoplasm, but 

 nuclei not so well. But the optical differentiation that it gives, 

 though sometimes good, is often very inferior. For osmic acid, 

 by coagulating in nearly equal degrees alike the spongioplasm 

 (the plastin reticulum) and the hyaloplasm (the enchylema) of 

 the cell-body, and the chromatin of nuclei, raises alike the 

 refractive indices of all of them ; so that if the fixing action have 

 been in the least degree overdone, the cells acquire a homogeneous 

 aspect in which the finer details are obscured by the general 

 refractivity of the whole. If now, instead of using it pure, it be 

 used in combination with chromic acid, a better differentiation is 

 obtained ; for chromic acid, whilst enhancing, and at all events 

 not interfering with the fixation of chromatin, serves to facilitate 

 penetration and to counteract the excessive action of the osmic 

 acid on the protoplasm, so that the cells come out less homogeneous 

 and with more detail observable in them. 



Descriptive embryologists often use strange illogical mixtures 

 containing both reducible substances and violent reducers, both 

 fat-solvents and fat-preservers, mixed together without regard 

 for the chemistry of fixation. It is only the logically planned 

 fixative that is found generally useful, and which stands the 

 test of time. Fixation falls under three broad headings : — 



1. Micro-anatomical, in which correct preservation of cell 

 aggregates, without shrinkage or expanding, is the desideratum. 

 Such is the aim of most descriptive embryologists. 



2. Cytological from the point of view of the chromosome or 

 nucleus. 



3. Cytological from the point of view of fixing the cell in a 

 state which most resembles its condition ivhen alive ; also so as to 

 identify the cell elements, especially in the cytoplasm. 



In most cases the results attained by workers belonging to 

 sections 1 and 2 can truly be said to give a caricature of the cell 

 intra vitam. We give below a general classification of fixatives, 

 those in (a) being 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-nitrie, etc. 



Fat, mitochondria, Golgi apparatus, and often delicate yolk 

 discs do not show after these. (Using alcohols and xylol sub- 

 sequently.) 



