CHAPTER III. 23 



Gilson-Petrunkewitsch and corrosive acetic, but these all destroy ] 

 much of the cell-contents, and give an incorrect picture of the cell, | 

 excepting of chromatinic structures, for which they are indicated. 



A good fixing agent should first of all preserve all the elements it 

 is desired to fix. But that is not enough ; it should also give good 

 optical differentiation, and should have sufficient power of penetra- 

 tion 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 differ- 

 entiation 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 homo- 

 geneous 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 witjiout 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 aggre- 

 gates, 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 when 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. 

 I give below a general classification of fixatives, those in (a) being 



