At present I am inclined to think that both soft and hard cartilage originate in a very similar manner, viz. by the formation of capsular {136}material around the invading chondroblasts, and that the difference in the resulting cartilage is mainly due to the difference in chemical composition of the matrix of the connective tissue which is invaded. Thus the difference may be formulated as follows:-- The hard cartilage is formed by the invasion of chondroblasts into a fibrous tissue, which contains a gelatinous rather than a mucoid substratum, in contradistinction to the soft cartilage which is formed, probably also by the invasion of chondroblasts, in a tissue--the muco-cartilage--which contains a specially mucoid substratum. Such, then, is the very clearly defined starting-point of the vertebrate skeleton--two distinct formations of different histological and chemical structure,--the one forming a segmented branchial skeleton, the other a non-segmented basi-cranial skeleton. THE CARTILAGINOUS SKELETON OF LIMULUS. Among the whole of the invertebrates at present living on the earth, is there any sign of an internal cartilaginous skeleton that will give a direct clue to the origin of the primitive vertebrate skeleton? The answer to this question is most significant: only one animal among all those at present known possesses a cartilaginous skeleton, which is directly comparable with that of Ammocoetes, and here the comparison is very close--only one animal among the thousands of living invertebrate forms, and that animal is the only representative still surviving of the palæostracan group, which was the dominant race when the vertebrate first made its appearance. The Limulus, or king-crab, possesses a segmented branchial internal cartilaginous skeleton (Fig. 53, A), made up of the same kind of cartilage as the branchial skeleton of Ammocoetes, confined to the mesosomatic or branchial region, just as in Ammocoetes, forming, as in Ammocoetes, cartilaginous bars supporting the branchiæ, and these bars are situated externally to the branchiæ, as in Ammocoetes. In addition this animal possesses a basi-cranial internal semi-cartilaginous unsegmented plate known as the entosternite or plastron situated, with respect to the oesophagus, similarly to the position of the trabeculæ with respect to the infundibulum in Ammocoetes. Moreover, the cartilaginous cells in this tissue differ from those in the branchial region, in precisely the same manner as the hard cartilage differs from the soft in Ammocoetes.