The consideration of the differences in form, number, and histological structure, which the parts of the skeleton present during the various stages of development does not fall within the scope of this book: we have here but to deal with the adult frog.

The skeleton is made up of histologically different materials; these are:—(1) bone, (2) hyaline cartilage, and (3) so-called calcified cartilage. Concerning the last it is necessary to make some observations. I have chosen for it the name calcified cartilage in place of the more usual names ‘cartilaginous bone’ or ‘primordial ossification,’ as by this term its nature appears to be expressed without any ambiguity‍25: it is hyaline cartilage in which calcareous particles have been deposited to a greater or less extent: in the fresh state it has the appearance of moderately firm cartilage; when dry it becomes opaque and white, like the calcareous crusts on the cartilages of the Plagiostomata. The calcareous material is deposited in the cartilage in finer or coarser granules; after removal of the lime by means of acids, the cartilaginous structure becomes apparent although not so perfectly as in unchanged cartilage.

This calcified cartilage is widely distributed in the frog’s skeleton: very many parts, which in higher animals consist only temporarily of this substance during the transition from cartilage to bone, are in the frog formed of it throughout life. It is especially well-marked in the epiphyses of the long bones in the hand and foot, in the bones of the shoulder-girdle, etc. To avoid repetition later on I will briefly describe it as found in the first-mentioned situation. Dugès‍26 has described its external appearance, while Bruch‍27 has made us acquainted with its histological peculiarities. If a long bone of the frog be dried, the femur for example, the middle part is found to differ considerably from the epiphyses in colour and in other particulars. The shaft alone has the appearance of bone, the epiphysis consisting of a white, opaque, firm substance, resembling plaster of Paris or lime, but which in the fresh moist state is exactly like cartilage. The epiphyses, which are fitted to the ends of the diaphysis like the cap of a stick-handle, have sharply defined margins (Fig. 3), as is well seen in Figs. 36, 39, 45, and 46. If a section be made through the epiphysis and part of the diaphysis, the long tube of true bone is seen to cease abruptly above o, Fig. 3, and over the end of it the epiphysis E is fixed. This epiphysis consists almost entirely of calcified cartilage c′, and has merely a superficial layer of hyaline cartilage A. The bony cylinder of the diaphysis o, which contains the marrow M in its interior, is shut off from the epiphysis by hyaline cartilage, the cells of which are arranged in transverse layers, o.