{333}In Fig. 132, B, I give a specimen of this tissue stained by osmic acid; in Fig. 132, A, I give a drawing of ordinary muco-cartilage taken from the plate of the lower lip; and in Fig. 133, A, a modification of the muco-cartilage taken from the velum, which shows the formation of a tissue intermediate between ordinary muco-cartilage and this branchial fat-tissue. Further, in fully-grown specimens of Ammocoetes, in the region of undoubted muco-cartilage, a fatty degeneration of the cells frequently appears, together with an increase in the blood spaces,--the precursor, in fact, of the great change which overtakes this tissue soon afterwards, at the time of transformation, when it is invaded by blood, and swept away, except in those places where new cartilage is formed. I conclude, then, that the tissue of this vascular space was originally muco-cartilage, which has degenerated during the life of the Ammocoetes. The fact that in most cases undoubted muco-cartilage is to be found here and there in this space, is strong confirmation of the truth of this conclusion. [Illustration: FIG. 132.--A, MUCO-CARTILAGE OF LOWER LIP (_Mc._); _m.ph._, muscle of lower lip; _m.sm._, somatic muscle; _Cor._, laminated layer of skin. B, DEGENERATED MUCO-CARTILAGE OF BRANCHIAL REGION. _F._, fat layer; _P._, pigment; _Bl._, blood-space; _N._, somatic nerve; _m.br._, branchial muscle; _m.sm._, somatic muscle.] If this conclusion is correct, we may expect that it would be confirmed by the embryological history of the tissue, and we ought to find that in much younger stages a homogeneous tissue of the same nature as muco-cartilage fills up the spaces in the branchial {334}region, where in the Ammocoetes only blood and fat-containing cells are present. For this purpose Shipley kindly allowed me to examine his series of sections through the embryo at various ages. These specimens are very instructive, especially those stained by osmic acid, which preserves the natural thickness of this space better than other staining methods. At an age when the branchial cartilages are seen to be formed, when no fat-cells are present, a distinctive tissue (Fig. 133, B) is plainly visible in the velum and at the base of the tentacles, in the very position where in the more advanced Ammocoetes muco-cartilage exists. Taking, then, this tissue as our guide, the specimens show that the space between the skin and the visceral muscles in which the cartilaginous basket-work lies is filled with a similar material. At this stage a sheet of embryonic tissue occupies the position where, later on, blood-spaces and fat-cells are found, and this tissue resembles that seen in the velum and other places where muco-cartilage is afterwards found. [Illustration: FIG. 133.--A, MUCO-CARTILAGE OF VELUM; B, EMBRYONIC MUCO-CARTILAGE OF TENTACULAR BAR.]