FIXATION AND FIXATIVES 7 



It is vertebrate embryological material which suffers most from distortion, 



and the preservation of its external form is best accomplished by solutions 



containing formaldehyde, potassium dichromate, or potassium dichromate plus 



sodium sulfate. It must be made quite clear that "distortion" is used here to 



indicate a change which produces a definite change of "shape." Uniform 



"shrinking" and "swelling" without an accompanying change of "shape" are 



perfectly distinct processes; potassium dichromate (inter alia) produces the 



former and formaldehyde the latter, so that combinations of these two or of 



formaldehyde with Miiller's fixing fluid are indicated for the preservation of 



the external form of delicate mammalian tissues. 



Preservation of Nuclear Detail. Much which must be omitted here has 



been written upon the chemical aspect of this problem. From the practical 

 point of view, the question is bound up with that of penetration. The most 

 universally employed penetrating agent is acetic acid, the swelling action of 

 which is usually restrained by the addition of picric or chromic acid. It should 

 be pointed out that penetration and distortion are usually produced by the 

 same agents, and under this heading must be included many of the ether- 

 alcohol fixatives. The distorting action of these upon whole animals is to a 

 certain extent restrained by the addition of mercuric chloride (cf Carnoy and 

 Lebrun's fluid, p. 10). In general, however, it is impossible to obtain the finer 

 details of nuclear fixation in entire animals or organs, the shape of which it 

 is desired to preserve. 



Preservation of Cytoplasmic Detail. This is largely a question of the 

 chemical or physical coagulation of relatively large masses of protoplasm. This 

 coagulation is brought about by various reagents: e.g., alcohol; compounds of 

 chromium, copper, osmium, and mercury; picric acid; and formaldehyde. The 

 choice of the agent employed must be governed by the particular detail which 

 it is desired to preserve and by the effect which the chosen agent is likely to 

 exert upon the shape and size of the object. The question of transparency 

 should also be included here. Many objects may be preserved conveniently and 

 permanently in neutral formol, where they will retain much of their original 

 transparency; this transparency is usually destroyed by previous fixation in any 

 fluid other than weak osmic acid solutions. 



Dichromate Fixatives. Potassium dichromate was one of the earliest sub- 

 stances employed for fixing. It is, however, a very poor protein coagulant. In 

 its original use, in the solution of Miiller, it was used in combination with 

 sodium sulfate which is an excellent protein precipitant. Most of the modern 

 dichromate fixative solutions have been derived from Miiller, though in most 

 the addition of mercuric chloride has made the sodium sulfate unnecessary. 



