34 JOURNAL OF THE [April, 



by development of contrast. Of this common examples are 

 found in the use of acetic or chromic acid in differentiation of 

 nuclear structure ; of picric acid in direct staining of tissues ; of 

 osmic acid in fixation of tissues, and in coloration of fat through 

 deoxidation. But there is every reason to believe that the j^roto- 

 plasmic elements, in the tissues or organisms so accentuated or 

 colored, no more remain in their original condition than, for 

 example, the threads of muscular fibre teased out with needles 

 by a laboratory student for the purpose of more easy distinction. 

 In this connection we may remark the significant change of 

 opinion ' of such authorities as Berthold, Schwartz, Kolliker, and 

 others, on the subject of the true structure of protoplasm, who 

 now look upon the reticulum and fibrils, recognized by From- 

 mann, Arnold, and others in sections of tissues so treated, as 

 being only artificial products. 



All these acids, then, together with their salts, such as ammo- 

 nium and potassium dichromates, and mixtures like Miiller's fluid^ 

 Erlicki's fluid, Lang's solution, etc., need to be rejected for our 

 present purpose. The only ones likely to be found useful, in 

 high dilution, are such organic acids as exist in living tissues, 

 possibly such as oxalic, malic, citric, etc., in plants, and sarcodic, 

 lactic, butyric, etc., in animals. As a rule, the mounting medium 

 for which we are now searching should probably be neutral in its 

 reaction, for most objects. 



This conclusion condemns at once and altogether, for preserva- 

 tion of protoplasmic forms, the use of the resinous media, so 

 excellent for general purposes of mounting — Canada balsam (so 

 often the refuge of the lazy microscopist, wishing to avoid the 

 construction of a cell), gum dammar, copaiba, copal, and styrax. 

 The objection to these is founded not only on their slight content 

 of organic acids, as well as of turpentine or similar solvent, but 

 also on the complicated series of processes required for the pre- 

 liminary dehydration and subsequent clearing. A glance at the 

 evidences of tedious torture of protoplasm in these long-drawn- 

 out processes ought to be sufficient to account easily for their 

 failure in the permanent preservation, without distortion, of the 

 natural forms of contractile matter within the interior of cells. 



We have also to guard against the shrinking of living proto- 



1 O. Blitschli, Sep. Abd. Verb. Deutsch. Zool. Gesell., 1891, 14-20. 



