Hence we see that structurally as well as topographically the branchial bars of Ammocœtes justify their claim to be considered as the origin of the vertebrate cartilaginous framework.
On the Structure of the Muco-cartilage in Ammocœtes.
We can, however, go further than this, and ask how this cartilage itself is formed in Ammocœtes? The answer is most definite, most instructive and suggestive, for in all cases this particular kind of cartilage is formed from, or at all events in, a peculiar fibrous tissue, which was called by Schneider "Schleim-Knorpel," or muco-cartilage, a tissue which is distinguishable from other connective tissues, not only by its structural peculiarities, but also by its strong affinity for all dyes which differentiate mucoid or chondro-mucoid substances.
This muco-cartilage is thus described by Schneider:—The perichondrium in Ammocœtes is not confined to the true cartilaginous structures, but extends itself in the form of thin plates in definite directions. Between these plates of perichondrium a peculiar tissue (Fig. [56])—the muco-cartilage—exists, consisting of fibrillæ, whose direction is mainly at right angles to the planes of the perichondrial plates, with star-shaped cells in among them, and with the spaces between the fibrillæ filled up with a semi-fluid mass.
From this tissue all the primitive cartilages which resemble the branchial bars are formed, either by the invasion of chondroblasts from the surrounding perichondrium, or by the proliferation and encapsulation of the cells of the muco-cartilage itself.
Fig. 56.—Section of Muco-cartilage from Dorsal Head-plate of Ammocœtes.
This very distinctive tissue—the muco-cartilage—is of very great importance in all questions of the origin of the skeletal tissues. In all descriptions of the skeletal tissues it has been practically disregarded until recent years when, besides my own observations, its distribution has been mapped out by Schaffer. Thus Parker, in his well-known description of the skeleton of the marsipobranch fishes, does not even mention its existence. Its importance is shown by its absolute disappearance at transformation and its non-occurrence in any of the higher vertebrates. It is entirely confined to the head-region, and its distribution there is most suggestive, for, as will be described fully later on, it forms a skeleton which both in structure and position resembles very closely the head-shields of cephalaspidian fishes. At the present part of my argument its more immediate interest lies in the method of tracing this tissue. For this purpose I made use of the micro-chemical reaction of thionin, a dye which, as shown by Hoyer, stains all mucin-containing substances a bright purple. Schaffer made use of a corresponding basophil stain, hæmalum. When stained with thionin, the matrix, or ground-substance of the branchial cartilages as well as the matrix or semi-fluid substance in which the fibrils of the muco-cartilaginous cells are embedded take on a deep purple colour, while the fibrous material of the cranial walls and other connective tissue strands, such as the perichondrium, are coloured light blue. Muco-cartilage, then, may be described as a peculiar form of connective tissue which differs from other connective tissue not only in its appearance but in 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 Ammocœtes.
The conclusions to which we are led by the study of the structure, position, and mode of origin of these primitive cartilages of Ammocœtes 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.