The structure of this hard cartilage of the trabeculæ and auditory capsules resembles that of the soft, in so far that it consists of large cells with a comparatively small amount of intercellular substance. Schaffer, who has described it lately, considers that it is a nearer approach to hyaline cartilage than the soft, but yet cannot be called hyaline cartilage in the usual sense of the term. Its peculiarities and its differences from the soft are especially well seen by its staining reactions. I have myself been particularly struck with the effect of picrocarmine or combined hæmatoxylin and picric acid {134}staining (Fig. 57). In the case of the soft cartilage the capsular substance stains respectively a brilliant red or blue, while that of the hard cartilage is coloured a deep yellow, so that the junction between the parachordals and the branchial cartilages is beautifully marked out. Then, again, with thionin, which gives so marked a reaction in the case of the soft cartilage, the hard cartilage of the auditory capsule is not stained at all, and in the trabeculæ the deep purple colour is confined to the mucoid cement-substance between the capsules, just as Schaffer has stated. The same kinds of reactions have been described by Schaffer: thus by double staining with hæmalum-eosin the hard cartilage stains red, the soft blue; and he points out that even with over-staining by hæmalum the auditory capsule remains colourless, just as I have noticed with thionin. He infers, precisely as I have done from the thionin reaction, that chondro-mucoid, which is so marked a constituent of the soft cartilage and of the muco-cartilage, is absent or present in but slight quantities in the hard cartilage. Similarly, he points out that double staining with tropoeolin-methyl-violet stains the hard cartilage a bright orange colour, and the soft cartilage a violet. The evidence, then, shows clearly that a marked chemical difference exists between these two cartilages, which may be expressed by saying that the one contains very largely a basophil substance, which we may speak of as belonging to the class of chondro-mucoid substances, while the other contains mainly an oxyphil substance, probably a chondro-gelatine substance. We may perhaps go further and attribute this difference of composition to a difference of origin; for whereas the soft cartilage is invariably formed in a special tissue, the muco-cartilage, which shows by its reaction how largely it is composed of a mucoid substance, the hard cartilage is certainly, in the case of the cartilage of the cranium where its origin has been clearly made out, formed in the membranous tissue of the cranium of Ammocoetes--_i.e._ in a tissue which stains light blue with thionin, and contains a gelatinous rather than a mucoid substratum.