THE THYMUS.—In the embryo Elasmobranch and Teleost[401] the thymus has a multiple origin, being derived from a series of distinct epithelial thickenings, one of which is developed at the dorsal extremity of each of the gill-clefts except of the spiracle. These rudiments subsequently detach themselves from the epithelial surface and sink inwards, eventually fusing together on each side to form a single independent structure. Later, the epithelial mass thus formed becomes invaded by connective tissue, and by leucocytes which form lymph follicles, and the thymus gradually assumes the structure of a lymphoid organ. From its mode of development it has been suggested that the thymus owes its evolution to the metamorphosis and ingrowth of branchial filaments,[402] but it is also noteworthy that each embryonic rudiment of the organ closely resembles, both in position and origin, one of the developing branchial tongue-bars of Amphioxus.[403] The abundance of leucocytes which it contains has also prompted the further suggestion that the origin of the thymus may be due to the necessity of providing for the phagocytic protection of the gills themselves from the ravages of harmful micro-organisms, fungoid spores, etc., as well as to aid in the removal of such portions of the gills as may have been injured.[404] A thymus is probably present in all Fishes, if not in the adult at all events in the embryo, but is always relatively small in size. In Elasmobranchs the organ lies on each side above the branchial arches and beneath the dorsal musculature; and in Teleostomi at the dorsal extremity of the last branchial arch, in close proximity to the mucous membrane of the branchial cavity. In a similar position in the Dipnoi (e.g. _Protopterus_)[405] there are, on each side, {346}two contiguous lobes of lymphoid tissue which apparently represent a thymus. [Illustration: FIG. 203.—Supra-renal and inter-renal bodies of Fishes. A, of _Scyllium catulus_; B, of _Acipenser sturio_; C, of _Pagellus centrodontus_. _d.a_, Dorsal aorta; _tr_, inter-renal body; _l.m_, lymphoid portion of the mesonephros; _m_, mesonephros; _mtn_, metanephros; _oes_, oesophagus; _sg.a_, segmental arteries; _sr_, supra-renal bodies; _sy.n_, sympathetic nerves. (From Swale Vincent.)] THE SUPRA-RENAL BODIES.—The supra-renal bodies are organs of problematic function, which are present in the Cyclostomata, and probably in all Fishes, and situated in close proximity to the kidneys.