CHAPTER XXIX. 361 



of mordanting with bichromate. The different lipoids, when pure, 

 differ greatly in the ease with which the stainable chromium com- 

 pounds is formed ; thus the cerebrosides and protagon stain after 

 a short mordanting ; the unsaturated true fats require a slightly 

 longer time, while lecithin, and especially cholesterin, are very 

 resistant and require prolonged mordanting. But mixtures of 

 cholesterin and the various fatty substances show quite a different 

 behaviour and reach the stainable stage very rapidly. This appears 

 to be due to the fact that such mixtures form colloidal solutions in 

 a peculiar physical condition (fluid crystals). It will be clear that 

 a histochemical identification of the various lipoids by means of 

 this method is not possible. Its value lies in the possibility of 

 demonstrating, first, the presence of fatty substances by a method 

 which gives good histological details, and secondly, by comparison 

 with normal tissues, the occurrence of chemical changes in these 

 substances under pathological conditions. 



As introduced by Weigert, the method stains the lipoids of the 

 normal myelin sheath. By prolonging the bichromating the 

 degenerating nerve fibres may be made to stain in the early stage of 

 degeneration, or the fat droplets in fatty organs may be stained. 

 The same principle underlies Altmann's method for the staining 

 of mitochondria, which are by some believed to consist of a central 

 core of protein covered by an envelope of fatty material. The 

 original method consists in mordanting with a bichromate solution 

 and staining with acid fuchsin, just as Weigert originally used acid 

 fuchsin for the staining of the myelin sheath. In the staining of 

 mitochondria, the acid fuchsin can again be replaced by hsematoxylin 

 (Heidenhain's hsematoxylin). 



(4) BEHAVIOUR IN POLARISED LIGHT. The true fats and the 

 fatty acids are isotropic, i.e., show no double refraction in polarised 

 light, so that with crossed nicols the field appears dark. The lipoids 

 are anisotropic. In fresh teased preparations they can be seen with 

 crossed nicols as luminous droplets with a varying degree of brilliancy. 

 The double refraction disappears on gentle heating to about 60 

 and reappears on cooling. In formol fixed frozen sections the aniso- 

 tropic lipoids appear chiefly in the form of needles and as droplets. 

 Heating and cooling produces the effect mentioned above. 



The behaviour in polarised light is, therefore, an easy and im- 

 portant means of differentiating the isotropic true fats from lipoids. 

 (For a detailed description of the technique of the polarisation 

 microscope, see AMBRONN, Anleitung zur Benutzung des Polarisa- 

 tions Mikroskopes bei Histologischen Untersuchungen ; ADAMI, 



