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 (Ammoceetes), in which vertebre are not yet formed, but the 
cranial and branchial skeleton is well marked. 
The embryological 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 Ammocoetes 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 trabecule 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 trabecule 
and parachordals, from which the whole complicated basi-cranial skeleton of the 
higher vertebrates arose. 
The mesosomatic skeleton, which is formed before the prosomatic, 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 branchie are 
situated. 
The cartilages which compose these two skeletons respectively are markedly 
different in chemical constitution, in that the first (hard cartilage) 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 Ammocetes 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, 
which 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 Ammocecetes 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. 
