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 Amnioccetes. 



The conclusions to which we are led by the study of the structure, 

 position, and mode of origin of these primitive cartilages of 

 Ammoccetes 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 Ammoccetes. 



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- 

 becular, 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 trabecular (Tr.), and of a hindermost 

 part, the parachordals (Pr.ch.), which are characterized by the 

 attachment on each side of the large auditory capsule {Au.). 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 {Inf.) of the 



