146 THE ORIGIN OF VERTEBRATES 



Summary. 



The skeleton considered in this chapter is not the notochord, but that 

 composed of cartilage. The tracing- downwards of the vertebrate bony and 

 cartilaginous skeleton to its earliest beginnings leads straight to the skeleton of 

 the larval lamprey (Amnioccetes), in which vertebrae are not yet formed, but the 

 cranial and branchial skeleton is well marked. 



The embryologies! and phylogenetic histories are in complete unison to show 

 that the cranial skeleton is older than the spinal, and this primitive branchial 

 skeleton is also in harmony with the laws of evolution, in that its structure, even 

 in the adult lamprey (Petromyzon). never gets beyond the stage characteristic 

 of embryonic cartilage in the higher vertebrates. 



The simplest and most primitive skeleton is that found in Animoccetes and 

 consists of two parts : (1) a prosomatic, (2) a mesosomatic skeleton. 



The prosomatic skeleton forms a non-segmented basi-cranial skeleton of the 

 simplest kind — the trabecular and the parachordals with their attached auditory 

 capsules, just as the embryology of the higher vertebrates teaches us must be 

 the case. There in the free-living, still-existent Ammoccetes we find the manifest 

 natural outcome of the embryological history in the shape of simple trabecular 

 and parachordals, from which the whole complicated basi-cranial skeleton of the 

 higher vertebrates arose. 



The mesosomatic skeleton, which is formed before the prosoniatic, consisted, 

 in the first instance, of simple branchial bars segmentally arranged, which were 

 connected together by a longitudinal subchordal bar. situated laterally on each 

 side of the notochord. These simple branchial bars later on form the branchial 

 basket-work, which forms an open-work cage within which the branchiae are 

 situated. 



The cartilages which compose these two skeletons respectively are markedly 

 different in chemical constitution, in that the first (hard cartilag'e) is mainly 

 composed of chondro-gelatin, the second (soft cartilage) of chondro-mucoid 

 material. 



The same kind of difference is seen in the two kinds of connective tissue 

 which are the forerunners of these two kinds of cartilage. Thus, the cranial 

 walls in Ammoccetes are formed of white fibrous tissue, an essentially gelatin- 

 containing tissue ; at transformation these are invaded by chondro-blasts and 

 the cartilaginous cranium, formed of hard cartilage, results. On the other hand, 

 the forerunner of the branchial soft cartilage is a very striking and peculiar 

 kind of connective tissue loaded with mucoid material, to which the name 

 muco-cartilage has been given. 



The enormous interest of this muco-cartilage consists in the fact that it 

 forms very well-defined plates of tissue, entirely confined to the head-region, 

 wliich are not found in any higher vertebrate, not even in the adult form 

 Petromyzon, for every scrap of the tissue as such disappears at transformation. 



It is this evidence of primitive non-vertebrate tissues, which occur in the 

 larval but not in the adult form, which makes Ammoccetes so valuable for the 

 investigation of the origin of vertebrates. 



The evidence, then, is extraordinarily clear as to the beginnings of the 

 vertebrate skeletal tissues. 



