THE EVIDENCE OF THE SKELETON 1 29 



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 trabecular 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 vertebrae, 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 w r hich 

 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 car- 

 tilage. 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 car- 

 tilage, and the last observer (Schafi'er) 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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