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Show 28 MR. A. D. IMMS ON THE GILL-RAKERS [May 3, In a gill-raker which has been decalcified in a 10 per cent* solution of acetic acid, and afterwards stained with Kleinenberg s haematoxylin and cut into longitudinal sections with the microtome, some additional features may be observed (PI. II. fig. 4). The ground-substance exhibits indications of being stratified and its layers show varied capabilities for absorbing s ta in in g -reagents. Traversing it are numerous canals, which are lined internally by a definite membrane and contain one or more blood-vessels and some loose connective tissue. In the region of the shaft the lacunae have protoplasmic contents which are readily stained, together with one or more deeply staining bodies which are apparently nuclei. Those lacunae which are situated nearest to the blood-channels can be distinctly seen to be in communication with them by means of their canaliculi. In the basal part of the gill-raker many of the lacunae are shrunken in their outlines and are very poor in stainable contents. The lower part of the central cavity, which is represented at c.c., appears to be nearly empty, containing only some nucleated tissue in which no definite cells are to be distinguished. The elastic fibres of the ligament, already mentioned, penetrate deeply into the substance of the base in much the same manner as the perforating fibres of Sharpey, which are composed partly of bundles of elastic fibres, pierce the circumferential lamellae in bone. In fig. 4 {e.f.), where these fibres are seen in section, they appear as variously shaped dots according to the angle through which they have been cut, and they are very evenly distributed through the peripheral ground-substance. The principal blood-vessel is seen to enter the gill-raker about the point of junction of the shaft with the basal part. The vessel then breaks up into several branches, which penetrate the ground-substance and reach the canals traversing it. The mucous membrane of the branchial arch (m. in figs. 4 & 5), accompanied by capillaries, is prolonged upwards as a complete and continuous investment to the outer surface of the gill-raker. In a decalcified gill-raker the presence of a membrane covering it is easy to make out without cutting sections, as it can be stripped off by using a fine needle under a dissecting microscope. The only fish which possesses gill-rakers at all comparable with those of Polyodon is Cetorhinus maximus. In this species the gill-rakers are of the same general form and, in proportion to the much greater size of the animal, they are correspondingly larger and stouter. The investigations of Hannover * and Turner t have shown, on histological grounds, that there is good reason for believing them to be very greatly elongated teeth. In common with those1 of Polyodon, they consist at their bases of a matrix permeated with anastomosing canals containing blood-vessels ; in the shaft they differ in that they contain but a single canal which runs straight up to the tip. There are no lacunae, but the matrix * Kong. Danske Vidensk.-Selskabs Skrifter, 1868, p. 485. A resume, is given in French in the Ann. Sri. Nat., Zool. t. ix. 1868, p. 373. f Journ. Anat. & Phys. xiv. 1879, pp. 273-286, pi. xii. |