OBSERVATIONS ON STRUCTURE OF CELLS AND NUCLEI. 173 



tides — nucleoli. Different reagents demonstrate in the 

 nucleus a network and nucleoli with varying distinctness, 

 and it is a question whether what these reagents reveal are 

 partly or wholly artificial products. Flemming suspects the 

 chromates especially as liable to produce artificial products ; 

 dilute chromic acid and picric acid, on the other hand, he 

 has great faith in, although reading his observations with 

 bichromate of potash on fresh blood-corpuscles of Salamandra 

 (p. SS6), the reader, I think, will come to a conclusion, viz. 

 that bichromate of potash is a very good reagent to demon- 

 strate the intranuclear network, which conclusion is opposite 

 to that arrived at by Flemming himself. 



That many things, although not visible in the perfectly 

 fresh state, but only after treatment with reagents, are 

 nevertheless preformed structures, is a proposition which it 

 is unnecessary to prove in the present state of histological 

 science ; the whole progress of modern histology contains 

 that proof. 



A thing is not visible at all, or only indistinctly so, in the 

 living and fresh state if its refractive index does not differ 

 from the parts surrounding it ; after adding hardening re- 

 agents many structures are brought out with greater or 

 less distinctness. But there are certain minute structures 

 which have been and can be demonstrated only with certain 

 particular reagents. I remind the reader of the results ob- 

 tained by Max Schultze with perosmic acid, by v. Keckling- 

 hausen with nitrate of silver, by Cohnheim with chloride of 

 gold, &c., then of the different dyes which are used in his- 

 tology on account of their selective power. 



The cornea, carefully cut out and quickly placed under 

 the microscope, shows absolutely no structure ; of the different 

 layers of epithelium on the anterior surface, of their nuclei, of 

 the corneal corpuscles and the lacunse in which they are 

 placed, of the innumerable fine nerves underneath the epi- 

 thelium, we see nothing, or next to nothing. After the 

 lapse of some time the cornea shows the outlines of some of 

 the epithelial cells, and in some of these we perceive faintly 

 the outlines of the nuclei. If we add a hardening reagent, 

 such as a chromate, chromic acid, or any other acid, alcohol, 

 &c., we at once see all the epithelial cells and their nuclei 

 very well defined ; the lacunse in which the corneal cor- 

 puscles lie, and these latter, as well as the fine nerves, are 

 seen very imperfectly and only in a few places. But when 

 treating the cornea, after Recklinghausen, with nitrate of 

 silver, we demonstrate the lacunse and their canals, and 

 when using Cohnheim's chloride of gold we trace with ease 



VOL. XIX. NEW SER. M 



