Further, in fully-grown specimens of Ammocœtes, 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 Ammocœtes. 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.
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 region, where in the Ammocœtes 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 Ammocœtes 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.
Fig. 133.—A, Muco-Cartilage of Velum; B, Embryonic Muco-Cartilage of Tentacular Bar.
I conclude, therefore, that originally the branchial or mesosomatic region was covered with a dorsal plate of muco-cartilage, which carried on its under surface the dorsal part of the branchial basket-work, and sprang from the central core of skeletogenous tissue around the notochord; this plate was separated from the plate which covered this region ventrally by the lateral grove in which the gill-slits are situated. The ventral plate carried on its under surface the ventral part of the branchial basket-work, and was originally continuous with the plate over the thyroid gland.
Fig. 134.—Skeleton of Head-Region of Ammocœtes. A, Lateral View; B, Ventral View; C, Dorsal View.
Muco-cartilage, red; soft cartilage, blue; hard cartilage, purple. sk1, sk2, sk3, skeletal bars; c.e., position of pineal eye; na. cart., nasal cartilage; ped., pedicle; cr., cranium; nc., notochord.