132 THE ORIGIN OF VERTEBRATES 
its chemical composition, for unlike white fibrous tissue it contains a 
large amount of mucin, and this tissue is the forerunner of the earliest 
cartilaginous vertebrate skeleton, the branchial bars of Ammoccetes. 
The conclusions to which we are led by the study of the structure, 
position, and mode of origin of these primitive cartilages of 
Ammioccetes may be thus summed up :— 
1, The immediate ancestor of the vertebrate must have possessed 
a peculiar fibrous tissue—the ground-substance of which stained deep 
purple with thionin—in which cartilage arose. 
2. The cartilage so formed was not like hyaline cartilage, but 
resembled in a striking manner parenchymatous cartilage. 
3. This cartilage was situated partly in two axial longitudinal 
bands, partly as transverse bars, which supported the branchial 
apparatus. 
THE PROSOMATIC OR BASI-CRANIAL SKELETON OF AMMOCC:TES. 
Before searching for any evidence of a similar tissue in any 
invertebrate group, it is advisable to consider the other portion of 
the cartilaginous skeleton of Ammoccetes, which consists of the tra- 
beculee, parachordals and auditory capsules—the basi-cranial skeleton 
—and is composed of hard, not soft cartilage. 
This basi-cranial skeleton represented in Fig. 53, B, is confined to 
the region of the notochord, the cranial walls being composed entirely 
of a white fibrous membrane. It is separated at first entirely from 
the sub-chordal portion of the branchial basket-work, and is com- 
posed of a foremost part, the trabecule (77.), and of a hindermost 
part, the parachordals (Pr.ch.), which are characterized by the 
attachinent on each side of the large auditory capsule (4w.), In 
Ammoccetes the trabecular bars are continuous with the parachordals, 
the junction being marked by a small lateral projection on each side, 
which at transformation is seen to play an important part in the 
formation of the sub-ocular arch. The trabecular bar lies close 
against the notochord on each side up to its termination; it then 
bends away from the middle line and curves round until it meets 
its fellow on the opposite side, thus forming, as it were, the head of 
a racquet of which the notochord forms the splice in the handle. 
The strings of the racquet are represented by a thin membrane, in 
the centre of which the position of the infundibulum (Jnf.) of the 
