The Origin of Vertebrates

by Walter Holbrook Gaskell

The study of Ammocœtes gives yet another clue to the nature of the earliest skeleton, for these two marked groups of cartilage—the branchial and basi-cranial—are characterized by a difference in structure as well as a difference in topographical position. J. Müller was the first to point out that these two sets of cartilages differ in appearance and constitution, and he gave to them the name of yellow and grey cartilage. Parker has described them fully under the terms soft and hard cartilage, terms which Schaffer has also used, and I shall also make use of them here. The whole of the branchial cartilaginous skeleton is composed of soft cartilage, while the basi-cranial skeleton, consisting of trabeculæ, parachordals, and auditory capsule, is composed of hard cartilage, the only soft cartilage in this region being that which forms the nasal capsule, not represented in Fig. [53], B.

These two groups of cartilage arise independently, so that at first the basi-cranial system is quite separate from the branchial, and only late in the history of the animal is a junction effected between the branchial system and the trabeculæ and parachordals, an initial separation which is especially striking when we consider that in this animal all the cartilaginous structures of any one system are continuous: there is no sign of anything in the nature of joints.

Of these two main groups, the branchial cartilages are formed first in the embryo, a fact which suggests that they are the most primitive of the vertebrate cartilages, and that, therefore, the first true formation of cartilage in the invertebrate ancestor may be looked for in the shape of bars supporting the branchial mechanism. The evidence of the origin of the cartilaginous structures in Ammocœtes is given by Shipley in the following words:—

"The branchial bases are the first part of the skeleton to appear. They arise about the 24th day as straight bars of cartilage, lying external and slightly posterior to the branchial vessel.

"The first traces of the basi-cranial skeleton appear on the 30th day as two rods of cartilage—the trabeculæ."

Our attention must, in the first place, be directed to this branchial basket-work of Ammocœtes.

Underlying the skin of Ammocœtes in the branchial region is situated the sheet of longitudinal body-muscles, divided into a series of segments or myotomes, which forms the somatic muscles so characteristic of all fishes. This muscular sheet is depicted on the left-hand side of Fig. [54]. It does not extend over the lower lip or over that part in the middle line where the thyroid gland is situated. In these parts a sheet of peculiar tissue known by the name of muco-cartilage lies immediately under the skin, covering over the thyroid gland and lower lip. The somatic muscular sheet with the superjacent skin can be stripped off very easily owing to the vascularity and looseness of the tissue immediately underlying it. When this is done the branchial basket-work comes beautifully into view as is seen on the right-hand side of Fig. [54]. It forms a cage within which the branchiæ and their muscles lie entirely concealed.

This is the great characteristic of this most primitive form of the branchial cartilaginous bars and distinguishes it from the branchial bars of other higher fishes, in that it forms a system of cartilages which lie external to the branchiæ—an extra-branchial system.

This branchial basket-work is simpler in Ammocœtes than in Petromyzon, and its actual starting-point consists of a main transverse bar corresponding to each branchial segment; from this transverse bar the system of longitudinal bars by which the basket-work is formed has sprung. These transverse bars arise from a cartilaginous longitudinal rod, situated close against the notochord on each side. These rods may be called the subchordal cartilaginous bands (Fig. [53]), and, according to the observations of Schneider and others, each subchordal band does not form at first a continuous cartilaginous rod, but the cartilage is conspicuous only at the places where the transverse bars arise. In the youngest Ammocœtes examined by Schaffer, he could find no absolute discontinuity of the cartilage except between the first two transverse bars, but he says that the thinning between the transverse bars was so marked as to make it highly probable that at an earlier stage there was discontinuity. The whole system of branchial bars and subchordal rods is at first absolutely disconnected from the cranial system of trabeculæ and parachordals, and only later do the two systems join.