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Show 894 MR. \V. WOODLAND ON THE [Nov. 27, im aginable by structures adjacent to them ; e. g., in the Frog even the ureter sinks into the lumen of the posterior cardinal so as nearly to be completely enclosed (Shore, 6). Now it is evident that the kidneys arise in a position where space is extremely limited, since dorsally and laterally the adjacent space is filled with closely-packed mesenchymatous tissue, which, as the " urinary riclges" show, must offer considerable resistance to displacement, and ventrally the space is bounded by the coelomic epithelium, the limited distensibility of which must also offer some resistance to the expansion of the kidney substance in this direction. Hence the developing kidney, especially if it attains to at all considerable dimensions *, is subject to pressure ; and the obvious inference is that if there exists in the near neighbourhood of the kidney a space which permits of invasion, then the kidney substance, i. e. the tubules, will tend to occupy this space, i. e. the venous sinus, as that offering the least resistance to expansion. The kidney develops in the position of the veins simply because there here exists more space for the growth of the renal substance than elsewhere. There is nothing new iii the facts just stated. It is unquestionable that the walls of the embryonic veins are exceedingly thin and invaginable ; it is evident that the kidney during its development must and does exert pressure on surrounding structures, and it is well established that the renal tubules do invaginate the walls of the large venous sinus, transforming the simple lumen into a large meshwork of sinusoids t. All that is new is my contention that the encroachment o f the renal tubules on the space originally occupied by the venous sinus- the u renal-portal system " - is simply due to the expansion of the growing kidney in the direction of least resistance, and is not in any way concerned, as already proved, with excretion. The renal cardinal meshwork is, on this hypothesis, a, purely mechanical and functionless product, and this supposition is congruous with all the known facts of the embryology and physiology of the vertebrate kidney. It is impossible to here review the various conditions of the renal cardinal meshwork to be found in the vertebrate series; I can merely state that in all cases a renal cardinal meshwork will be found to occur whenever the kidneys are well developed and are able to be situated in the vicinity of the venous channels (impossible in the case of Mammalia, e. g.). In a few instances (Cercitodus) the meshwork appears to be formed in connection rather with the iliac veins which join the posterior cardinals (renal-portals) than with these latter, but this is of course only an additional illustration of my contention. * In the Cyclostomes, many Teleosts, and some Elasmobvauchs (Acanthias vulgaris, Centropliorus calceus, and others), the kidney does not attain to considerable dimensions, and this is probably the reason for the absence of the renal cardinal meshwork in these forms. f As Shore (6 ) says, the kidney tubules invade the venous lumen adjacent to them, giving this a " sinus-like character, the nephridia having grown into it, invaginating its wall as a reflection over themselves, so subdividing it into a network of irregular channels." |
