Irving Hardesty 251 



A comijarison of results obtained by the silver method with those 

 obtained by other methods is interesting. The silver method can be 

 considered little more than an aid in the interpretation of appearances 

 obtained by the more general stains, and should" always be used collater- 

 ally. Four of the figures given are combination drawings comparing 

 appearances given by the Benda method with silhouettes resulting from 

 the application of the silver method to spinal cords of the same respec- 

 tive stages. It is well known that the silver method often differentiates 

 certain structures very clearly, but from time to time varies greatly in 

 its selectiveness for reasons not well understood. Furthermore, beinsr 

 a precipitation method, the structures it does show are coated with the 

 reduced salt and consequently are of unnatural size and coarseness. The 

 amount of detail depends wholly upon the extent to which the salt is 

 allowed to precipitate. Certain detail as to the external form may be 

 obtained, or practically none at all, for the specimen may be so clogged 

 as to appear as a black, indefinite mass. These facts have been especially 

 impressed in my experiments with the method upon the spinal cords of 

 embryo pigs. The external limiting membrane and, indeed, the whole 

 mantle layer is often clogged beyond recognition of detail or outline, 

 when the other layers are practically unaffected. Looking over the 

 illustrations of others, one is convinced they experienced the same diffi- 

 culties and the fact is further impressed of the folly of accepting, un- 

 controlled, the results of the silver method as giving either the whole 

 or even the true story. The preparations from which the accompanying 

 drawings were made are the result of considerable experimentation, and 

 I think they show about all the method is capable of showing. 



One seeming peculiarity of the silver method shown in the drawings 

 is that in pigs up to 70 millimeters, the reduced salt shows a marked 

 preference for only those nuclei situated in the ependymal layer. With 

 the exception of an occasional nerve-cell when present, all other nuclei 

 are unlocated. Fortunately only a small percentage of the ependymal 

 nuclei are selected. In these young stages the necessarily short segment 

 of the very tender spinal cord is usually left intact in the vertebral canal 

 and the ependymal nuclei, being nearer the solution in the ventricle and 

 connected with the internal limiting membrane by the heavier inner ends 

 of the axial filaments, are perhaps the first to be reached by the silver 

 solution. The potassium bichronuite being already in the specimen, 

 the surfaces of the nuclei may act something like nodal points or centers 

 of crystallization of the resulting compound. The deposit once started 

 upon a surface, it continues from that surface along the lines of least 

 resistance, which here seem to be the radial axes of the syncytium or the 



