18 DR. G. THIN. 



found free in potash and seen in sections the following facts 

 are to be borne in mind : — In silver preparations of the 

 surface of articular cartilage a layer of cells can be demon- 

 strated in which the intermediate dark substance is so scant 

 as not only to suggest but to justify their classification as 

 epithelial. In potash preparations layers of cells are found 

 free which, making due allowance for the difference in the 

 mode of preparation, can be identified Avith a probability 

 which amounts almost to certainty as being similar cells to 

 those which are seen in the silver preparations. The dificr- 

 ence between these cells, when demonstrated by silver, and 

 ordinary epithelium similarly treated, the author attributes, 

 not to a difference in the cells themselves, but to their rela- 

 tion to the cartilage-substance, on the surface of which they 

 may be considered to be firmly imbedded. Set free by 

 potash, no difference can be observed between them and 

 ordinary epithelium. 



In regard to the epithelial layers seen by silver in sections 

 it is to be considered that in potash preparations layers are 

 seen presenting two kinds of appearance. They are either 

 free in perfectly flat sheets, or they are seen applied to the 

 convex prominences and concave depressions of fragments 

 of the cartilage-substance. It is impossible to be certain 

 that the free sheets in the one case are not the same cells 

 seen still undetached in the other. The analogy of the 

 cornea w^ould seem to suggest that they are not the same. 



In the cornea the layers of polygonal flat cells are of two 

 kinds. One kind is most easily demonstrated by silver, and 

 consists of flat layers composed of large cells ; another kind 

 is most easily seen by gold, and, although in fortunate cir- 

 cumstances demonstrable as continuous broad layers, is most 

 often seen partially as forming a broad network of small poly- 

 gonal cells following the channels formed by the separation 

 of the lamellae in the position of the stellate cells. The 

 former suggests the idea of a thin fascia investing the larger 

 portions of the cornea-substance, the latter of layers of cells 

 investing its larger bundles. Similarly it is suggested that 

 the layers of flat cartilage-cells found free in the potash solu- 

 tion, and those Avhose outlines and nuclei are represented in 

 Fig. 6, as they were seen in a silver preparation, belong to a 

 thin layer of substance of the nature of a fascia, and that the 

 cells seen adhering to the cartilage-substance in potash pre- 

 parations invest more closely the cartilage-substance proper. 

 The explanation is hypothetical, but it is a hypothesis sug- 

 gested by facts. 



In regard to the elongated narrow cells nothing can be 



