Fig. 202.—A, a vertical section through a just-hatched larva of Petromyzon. a.v, Auditory vesicle; br.c, branchial cleft; h, heart; m, mouth; n, notochord; ol, olfactory pit; ph, pharynx; sp, septum or velum between the stomodaeum and the mesenteron; sp.c, spinal cord; th, thyroid outgrowth from the floor of the pharynx. (From Gegenbaur, after Calberla.) B, diagram illustrating the development of the thyroid, the thymus, and the accessory thyroids, and their relations to the branchial clefts. a.th, Accessory thyroids; g.p, gill-pouches; Ph, pharynx; t, thymus; th, median thyroid. (From Hertwig, after de Meuron.)
According to Dohrn the median thyroid is to be regarded as the vestige of a gill-cleft which primitively existed between the hyomandibular cartilage and the hyoidean arch. This conclusion seems, however, to be less in harmony with the facts of development than the view[[400]] that the organ is derived from the characteristic hypobranchial groove or "endostyle" of Ascidians and Amphioxus, which has undergone a change of function from a mucus-conveying groove to a blood-gland. On the other hand, the mode of origin of the paired thyroids certainly favours the suggestion that they represent a posterior pair of vestigial gill-clefts, a view which derives some support from the fact that in Notidanus, where additional branchial arches and clefts are present, the paired thyroids are absent.
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, two contiguous lobes of lymphoid tissue which apparently represent a thymus.
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.
In the Cyclostomata (Petromyzon) these bodies are represented by lobules of cells along the posterior cardinal veins, and also by masses of peculiar cells ("chromaffin cells") along the sides of the aorta and segmental arteries.[[406]] In Elasmobranchs there are two distinct structures, the paired supra-renals and the inter-renals (Fig. 203, A). The former are a series of pairs of segmentally arranged bodies, situated on the successive pairs of segmental arteries given off from the dorsal aorta. The two bodies which form the first pair are much larger than any of the others, and were formerly spoken of as "axillary hearts." The inter-renal is usually a thin elongated "ochre-yellow" body, from which one or two lobes may be detached in front, and extends for a variable distance in the median line between the two kidneys, or is unsymmetrically placed on the ventral surface of either kidney.[[407]] Sometimes (e.g. in Raia) the inter-renals are paired, in which case they are applied to the inner and hinder margins of the kidneys. In the Sturgeon (Acipenser sturio) the "supra-renals" appear as numerous "ochre-yellow" bodies, variable in size and distribution (Fig. 203, B). Some of them are visible on the surface of the kidneys, while others are scattered about in their substance, but on the whole are more anteriorly placed than in Teleosts. In the latter group the "supra-renals" are usually two in number (Fig. 203, C), but may be as many as five or reduced to one. They are disposed either on the ventral or the dorsal surface of the kidneys, generally near their hinder extremities, or more or less deeply embedded in their substance. Besides these bodies there are also chromaffin cells in the walls of the anterior cardinal veins.[[408]]
Histologically, the paired segmentally arranged bodies of Elasmobranchs differ considerably in structure from the inter-renal bodies, the former resembling the "medulla," while the inter-renals, as well as the so-called supra-renals of Acipenser, exhibit a striking resemblance to the alveolar "cortical" substance of the Mammalian supra-renals.[[409]] In Cyclostomes the cortex is apparently represented by the lobules of cells along the posterior cardinal veins and the medulla by the "chromaffin" cells, while in Teleosts the cortex and the medulla have their respective counterparts in the supra-renals and the "chromaffin" cells in the walls of the anterior cardinal veins. It may be concluded, therefore, that Elasmobranchs, Cyclostomes, and Teleosts possess anatomically distinct representatives of both the "medulla" and "cortex" of Mammalia, although the Sturgeon is at present only known to possess the equivalent of the "cortex." In Amphibia, Reptilia, and Aves both "cortex" and "medulla" are present, and in the varying intimacy of their relations offer a transition to the Mammalian arrangement of a central medulla closely invested by a sheath of cortical substance. A more or less intimate connexion exists between the paired supra-renals of Elasmobranchs and the sympathetic nervous system. The former are usually well supplied with sympathetic nerve fibres, and contain ganglion-cells in their substance.
The primitive origin of these organs is very obscure, and as regards their development there is much diversity of opinion. It seems certain, however, that the cortex and medulla of the higher Vertebrates, including their equivalents in the Elasmobranchs, have independent origins, and the balance of opinion seems to point to the derivation of the cortex from some portion of the germinal coelomic epithelium, while the medulla is derived from the embryonic nerve cells of the sympathetic ganglia.