112 Transactions of the Society. 



the latter and the former by observing the shape and general 

 appearance. As we have seen, the black coloration of the fat and 

 the yolk is traceable to the reduction of the osmium tetroxide by 

 means of one of the oleic fat series ; but it is doubtful whether 

 the colour of the Golgi apparatus is due to the same reaction. 

 Ordinary fat blackens in a few hours ; the Golgi elements take 

 days to undergo this process. Moreover, Flemming without acetic 

 acid (i.e. chromium trioxide and osmic acid) will turn fai black, 

 while the presence of the chrome salt prevents the Golgi apparatus 

 from going black in the time it would have taken had the chromic 

 acid not been present. For these reasons I am led to consider 

 that the blackening of the Golgi apparatus is at present difficult 

 to explain by the assumption that the elements contain some 

 unsaturated fat. These remarks apply especially to Mollusca. 

 Another fact which must be noticed is that tlie presence in a 

 tis'sue of such a reducing substance as formalin may cause the 

 black effect to appear in almost all parts of the cell when osmic 

 acid is subsequently used (Sjovall's method). This fact must l)e 

 Ijorne in mind by those who would consider that osmic acid is in 

 any way specific for fat. 



The silver-nitrate method also presents much the same problem : 

 tissues are fixed in formalin and treated in silver nitrate ; some 

 additional reducer is then used, and sections are prepared and 

 toned. The black appearance in the majority of sections is strictly 

 confined to the Golgi apparatus, but in other parts of the section it 

 may have spread to the mitochondria or to the whole cell. This is 

 somewhat like the osmic acid reaction, because by Sjovall's method 

 the tissues are first fixed in formalin and then treated in osmic 

 acid. The black colour can be progressive as with silver nitrate ; it 

 may be strictly confined to the Golgi apparatus, or it may have 

 spread to mitochondria and gradually all over the cell. I do not 

 feel able to explain these curious facts, which are so important. 

 It may be pointed out that while Kopsch stains fat black, formol- 

 silver nitrate, while causing Golgi apparatus to become black, does 

 not stain yolk or fat black in successful preparations. 



Cytological Methods of the Futuke. 



It is not easy to forecast the future of a branch of Zoology such' 

 as Cytology, but one cannot read the papers by Cajal's school, 

 published in the "Trabajos del Laboratorio de XJniversidad de 

 Madrid," without feeling that the formol metallic methods (gold 

 and silver) have a great future before them. The chrome-osmium 

 tetroxide fixatives are at present giving the most fruitful results, 

 but neither metallic impregnations nor chrome-osmium prepara- 

 tions will take cytologists much farther without great improve- 

 ments in the manufacture of microscope lenses. 



