PRIMARY fixatives: NON-COAGULANTS 125 



preceding one, except formaldehyde and ethanol. The reaction 

 with formaldehyde is slow: no darkening occurred within 24 hours 

 at 20° C in the circumstances of Bahr's experiments.^^ Mercuric 

 chloride, chromium trioxide, and potassium dichromate prevent 

 the reduction of osmium tetroxide by daylight. 



Unclassified remarks. It w^as first pointed out by Altmann * in 

 1889 that lipid globules are sometimes only blackened by osmium 

 tetroxide on the outside, so that they appear as rings in optical 

 section. He called these Ringkorner. He found that they were not 

 seen initially, but only when tissues fixed in osmium tetroxide had 

 been brought into ethanol. He attributed their formation to partial 

 solution of the lipid droplet by ethanol.^ 



Mmm^..,-----I(apuze (rettkornrest) 

 4^^g. Lucke ■ ■ 'Wmm- Proioplasmo. 



FIG. 16. Three Ringkorner and a cap or hood (Kapiize) formed by partial 

 solution of lipid globules: osmium preparations. 



(From Starke.-'^') 



This subject was carefully investigated by Starke. ^^^ He found 

 that when lipid droplets that had been treated with osmium tetrox- 

 ide were set free in ethanol, they never became Ringkorner, but 

 shrank into irregular shapes, blackened all through. When similar 

 droplets were treated with osmium tetroxide while still contained 

 in the tissues, the result was different; for when the tissues were 

 subsequently placed in ethanol, Ringkorner were formed. Starke 

 concluded that lipid droplets consisted of a part that was rendered 

 insoluble in ethanol by osmium tetroxide, and a part that was not. 

 When the latter was dissolved out by ethanol, the droplet shrank if 

 it could ; but if it were surrounded by fixed cytoplasm it could not 

 shrink, and a spherical hole was left, to the walls of which the 

 fixed and blackened lipid material attached itself (fig. 16). 



The black rings and crescents commonly seen in osmium pre- 

 parations are in many cases to be attributed to the cause sug- 

 gested by Starke. His paper, published more than 60 years ago, has 

 unfortunately been overlooked by many authors. 



Pieces of tissue that have already been fixed in another fixative 

 (generally a mixture containing osmium tetroxide) may be soaked 

 for several days in a simple aqueous solution of osmium tetroxide. 



