70 The Preparation of Microscope Slides 



fixative. In the majority of cases, both cytological and histological detail 

 is ruined by its use, yet it remains of great value for many of the marine 

 Hydrozoa which are subsequently intended to serve as wholemounts or 

 museum preparations. Osmic acid is unquestionably the most useful im- 

 mobilizing agent that has yet been discovered, in that cytological, but 

 not always nuclear, detail is well preserved by its use. Many of the lower 

 invertebrates retain far more of their original transparency with this than 

 with any other fixative. However, it is both expensive to buy and dan- 

 gerous to use, so that it cannot be recommended to an elementary class. 



It is vertebrate embryological material that 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 that 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 Midler's fixing fluid are indicated 

 for the preservation of the external form of delicate mammalian tissues . 



Preservation of Nuclear Detail. Much that 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. T he 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 pro- 

 duced 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. In general, however, it is impossible to obtain the finer details of 

 nuclear fixation in entire animals or organs, the shapes 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; com- 

 pounds of chromium, copper, osmium, and mercury; picric acid; and 

 formaldehyde. The choice of the agent employed must be governed by 

 the particular detail that 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. 



