Fig.216. Transverse section of the lancelet, in the fore half. Fig. 216—Transverse section of the lancelet, in the fore half. (From _Ralph._) The outer covering is the simple cell-layer of the epidermis (_E_). Under this is the thin corium, the subcutaneous tissue of which is thickened; it sends connective-tissue partitions between the muscles (_M_1) and to the chorda-sheath. (_N_ medullary tube, _Ch_ chorda, _Lh_ body-cavity, _A_ atrium, _L_ upper wall of same, _E_1 inner wall, _E_2 outer wall, _Lh_1 ventral remnant of same, _Kst_ gill-reds, _M_ ventral muscles, _R_ seam of the joining of the ventral folds (gill-covers), _G_ sexual glands. Above the sexual glands, at the dorsal angle of the atrium, we find the kidneys. These important excretory organs could not be found in the Amphioxus for a long time, on account of their remote position and their smallness; they were discovered in 1890 by Theodor Boveri (Fig. 217 _x_). They are short segmented canals; corresponding to the primitive kidneys of the other vertebrates (Fig. 218 _B_). Their internal aperture (Fig. 217 _B_) opens into the body-cavity; their outer aperture into the atrium (_C_). The prorenal canals lie in the middle of the line of the head, outwards from the uppermost section of the gill-arches, and have important relations to the branchial vessels (_H_). For this reason, and in their whole arrangement, the primitive kidneys of the Amphioxus show clearly that they are equivalent to the prorenal canals of the Craniotes (Fig. 218 _B_). The prorenal duct of the latter (Fig. 218 _C_) corresponds to the branchial cavity or atrium of the former (Fig. 217 _C_). Fig.217. Transverse section through the middle of the Amphioxus. Fig. 218. Transverse section of a primitive fish embryo. Fig. 217—Transverse section through the middle of the Amphioxus. (From _Boveri._) On the left a gill-rod has been struck, and on the right a gill-cleft; consequently on the left we see the whole of a prorenal canal (_x_), on the right only the section of its fore-leg. _A_ genital chamber (ventral section of the gonocœl), _x_ pronephridium, _B_ its cœlom-aperture, _C_ atrium, _D_ body-cavity, _E_ visceral cavity, _F_ subintestinal vein, _G_ aorta (the left branch connected by a branchial vessel with the subintestinal vein), _H_ renal vessel. Fig. 218—Transverse section of a primitive fish embryo (Selachii-embryo, from _Boveri._). To the left pronephridia (_B_), the right primitive kidneys (_A_). The dotted lines on the right indicate the later opening of the primitive kidney canals (_A_) into the prorenal duct (_C_). _ D_ body-cavity, _E_ visceral cavity, _F_ subintestinal vein, _G_ aorta, _H_ renal vessel.