894 MR. W. WOODLAND ON THE [JSToV. 27, 



iuvaginable by structures adjacent to them ; e. g., in the Frog e\'eu 

 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 kidne3's arise in a position where space is extremely 

 limited, since dorsally and latei-alh' the adjacent space is filled 

 with closely-packed mesenchymatous tissue, which, as the "urinary 

 ridges " 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 neighboui-hood 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 in the facts just stated. It is imquestion- 

 able that the w^alls of the embryonic veins are exceedingly thin 

 and invaginable ; it is evident that the kidney during its develop- 

 ment 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 con- 

 tention that the encroachment of the renal tubules on the space 

 originally occupied by the venous sinus — the " reiial-porial system " 

 — is simply due to the expansion of the grotving 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 pi'oduct, 

 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 vaiious conditions of the 

 renal cardinal mesliAvork 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 vezious channels 

 (impossible in the case of Mammalia, e. g.). In a few instances 

 {Geratodus) 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 Cj^clostomes, many Teleosts, and some Elasmobranchs {Acanthias vul- 

 garis, Centrophorus 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, 

 invaginatiiig its wall as a reflection over themselves, so subdividing it into a network 

 of irregular channels," 



