The study of Ammocoetes gives yet another clue to the nature of the earliest skeleton, for these two marked groups of cartilage--the branchial and basi-cranial--are characterized by a difference in structure as well as a difference in topographical position. J. Müller was the first to point out that these two sets of cartilages differ in appearance and constitution, and he gave to them the name of yellow and grey cartilage. Parker has described them fully under the terms soft and hard cartilage, terms which Schaffer has also used, and I shall also make use of them here. The whole of the branchial cartilaginous skeleton is composed of soft cartilage, while the basi-cranial skeleton, consisting of trabeculæ, parachordals, and auditory capsule, is composed {127}of hard cartilage, the only soft cartilage in this region being that which forms the nasal capsule, not represented in Fig. 53, B. These two groups of cartilage arise independently, so that at first the basi-cranial system is quite separate from the branchial, and only late in the history of the animal is a junction effected between the branchial system and the trabeculæ and parachordals, an initial separation which is especially striking when we consider that in this animal all the cartilaginous structures of any one system are continuous: there is no sign of anything in the nature of joints. Of these two main groups, the branchial cartilages are formed first in the embryo, a fact which suggests that they are the most primitive of the vertebrate cartilages, and that, therefore, the first true formation of cartilage in the invertebrate ancestor may be looked for in the shape of bars supporting the branchial mechanism. The evidence of the origin of the cartilaginous structures in Ammocoetes is given by Shipley in the following words:-- "The branchial bases are the first part of the skeleton to appear. They arise about the 24th day as straight bars of cartilage, lying external and slightly posterior to the branchial vessel. "The first traces of the basi-cranial skeleton appear on the 30th day as two rods of cartilage--the trabeculæ." Our attention must, in the first place, be directed to this branchial basket-work of Ammocoetes.