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. As far, then, as the topography of this basi-cranial skeleton is concerned, the striking points are--the shape of the trabecular portion, diverging as it does around the infundibulum, and the presence on the parachordal portion of the two large auditory capsules. These two points indicate, on the hypothesis that infundibulum and oesophagus are convertible terms, that two supporting structures of a cartilaginous nature must have existed in the ancestor of the vertebrate, the first of which surrounded the oesophagus, and the second was in connection with its auditory apparatus. [Illustration: FIG. 57.--A, CARTILAGE OF TRABECULÆ OF AMMOCOETES, STAINED WITH HÆMATOXYLIN AND PICRIC ACID. B, NESTS OF CARTILAGE CELLS IN ENTOSTERNITE OF HYPOCTONUS, STAINED WITH HÆMATOXYLIN AND PICRIC ACID.] STRUCTURE OF THE HARD CARTILAGES.