424 COLLOIDS IN BIOLOGY AND MEDICINE 



without any thought whatsoever of the chemistry involved. For a 

 truly scientific " theory of fixation " it would be necessary first to con- 

 sider the condition of the solutions involved, so that even the funda- 

 mental facts would have to be experimentally determined, and then 

 only would we be in a position to determine their action on colloids. 

 At present we lack almost the very essentials for the development 

 of a rational method from this wilderness of directions. 



Hardening. 



Hardening follows fixation. For this purpose alcohol is almost ex- 

 clusively used; it is gradually concentrated, beginning with 50 per 

 cent alcohol and then increasing the strength 10 per cent until 96 

 per cent alcohol is reached. A preliminary washing out of the 

 fixative is required, only if it forms precipitates with alcohol. 



In order to make thin sections with the microtome or razor it is 

 usually necessary to embed the preparation; in structures contain- 

 ing lime, siliceous or chitinous deposits, these must be removed first 

 by employing suitable acids. Finally, the sections must be mounted. 

 We shall not discuss these manipulations at greater length, since 

 they are purely technical. 



From our viewpoint, however, a very important procedure is 



Staining. 



Unstained specimens are usually so uniformly transparent that it 

 is difficult or almost impossible to distinguihs their intimate struc- 

 ture. In order easily to recognize the individual structures, histol- 

 ogists make use of stains. As has been said, they are chiefly 

 concerned with a morphological classification; to see cells, it usually 

 suffices to stain the nuclei; cell division, spermatogenesis and secre- 

 tion require specific stains. It is remarkable that but few, especially 

 P. EHRLICH, P. G. UNNA among others, have considered what con- 

 clusions concerning the chemical nature of the stained substance may 

 be derived from staining. We are unacquainted with any conclusions 

 concerning the physical nature (density) of tissues. The elaboration 

 of these investigations would be of great importance, since staining 

 pictures for us the action of drugs, toxins and disinfectants. 



The Theory of Staining. 



Though supporters of the chemical and of the physical theories of 

 dyeing were until recently actively disputing, there are mutual con- 

 cessions at present. _We have recognized that dyeing does not occur 



