The conclusions to which we are led by the study of the structure, position, and mode of origin of these primitive cartilages of Ammocoetes 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 AMMOCOETES.
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 Ammocoetes, which consists of the trabeculæ, 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 composed of a foremost part, the trabeculæ (_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 Ammocoetes 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 { 133}brain can be clearly seen. In an earlier stage of Ammocoetes the two trabecular horns do not meet, but are separated by connective tissue, which afterwards becomes cartilaginous.