These observations on Ammocoetes lead most definitely to the conclusion that the starting-point of the whole cartilaginous skeleton of the vertebrate consisted of a series of transverse cartilaginous bars, for the purpose of supporting branchial segments; these were connected with two axial longitudinal cartilaginous rods, which at first contained cartilage only near the places of junction of the branchial {129}bars. This system may be called the mesosomatic skeleton, as it is entirely confined to the branchial or mesosomatic region. In addition to this primitive cartilaginous framework, which was formed for the support of the mesosomatic or respiratory segments, but at a slightly later period in the phylogenetic history, a separate cartilaginous system was formed for the support of the prosomatic segments, viz. the trabeculæ and parachordals with the auditory capsules: a system which was at first entirely separated from the mesosomatic, and, as we shall see, is more advanced in structure than the branchial system. Later still, the story is completed at the time of transformation to Petromyzon by the formation of the simple cartilaginous skull and the rudimentary vertebræ, the structure of which is also of a more advanced type. THE STRUCTURE OF THE SOFT BRANCHIAL CARTILAGE. Having considered the topographical position of the primitive branchial cartilaginous skeleton, we may now inquire, What was its structure and how was it formed? In the higher vertebrates various forms of cartilage are described, viz. hyaline, fibro-cartilage, elastic cartilage, and parenchymatous cartilage. Of these, the parenchymatous cartilage is looked upon as the most primitive form, because it preserves without modification the characters of embryonic cartilage. Embryology, then, would lead to the belief that the earliest form of cartilage in the vertebrate kingdom ought to be of this type, viz. large cells, each of which is enclosed in a simple capsule, so that the capsules of the cells form the whole of the matrix, and thus form a simple homogeneous honeycomb-structure, in the alveoli of which the cartilage-cells lie singly. If, then, the branchial cartilages of Ammocoetes are, as has just been argued, the representatives of the cartilaginous skeleton of the primitive vertebrate, it is reasonable to suppose that they should resemble in structure this embryonic cartilage. Such is undoubtedly the case: all observers who have described the branchial basket-work of Ammocoetes or Petromyzon have been struck with the extremely primitive character of the cartilage, and the last observer (Schaffer) describes it as composed of thin walls of homogeneous material, in which there are no lines of separation, which form a simple honeycomb-structure, in the alveoli {130}of which the separate cells lie singly. These branchial cartilages are each surrounded by a layer of perichondrium, and in Fig. 55, A, I give a picture of a section of a portion of one of the bars.