THE EVIDENCE OF THE SKELETON 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 trabeculee and parachordals with the auditory cap- 
sules: a system which was at first entirely separated from the mesoso- 
matic, 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 cartila- 
ginous skull and the rudimentary vertebre, the structure of which 
is also of a more advanced type. 
THE STRUCTURE OF THE Sorr 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 eartilage-cells lie singly. If, then, the branchial cartilages of 
Ammoccetes 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 car- 
tilage. Such is undoubtedly the case: all observers who have 
described the branchial basket-work of Ammoccetes or Petromyzon 
have been struck with the extremely primitive character of the car- 
tilage, 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 
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