[Illustration: FIG. 55.--A, BRANCHIAL CARTILAGE OF AMMOCOETES, STAINED WITH THIONIN. B, BRANCHIAL CARTILAGE OF LIMULUS, STAINED WITH THIONIN.] Hence we see that structurally as well as topographically the branchial bars of Ammocoetes justify their claim to be considered as the origin of the vertebrate cartilaginous framework. ON THE STRUCTURE OF THE MUCO-CARTILAGE IN AMMOCOETES. We can, however, go further than this, and ask how this cartilage itself is formed in Ammocoetes? 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 Ammocoetes 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. {131}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. [Illustration: FIG. 56.--SECTION OF MUCO-CARTILAGE FROM DORSAL HEAD-PLATE OF AMMOCOETES.]