The Renal Organs

The higher Vertebrata are all provided with two sets of renal organs, the one existing only during the early foetal state, the other persisting throughout life.

The former are the Wolffian bodies, the latter the true Kidneys.

The Wolffian bodies make their appearance very early, on each side of the ventral aspect of the spinal region of the embryo, as small transversely-disposed tubuli, opening into a duct which lies upon their outer side, and enters, posteriorly, into the base of the allantois, and thence into the primitive cloaca with which that structure is connected. The Wolffian duct is one of the first - formed structures in the embryo, and precedes the tubuli.

The Kidneys appear behind the Wolffian bodies, and, apparently, independently of them; their ducts, the ureters, are also distinct, but likewise terminate in the pelvic part of the allantois. Thus the urinary secretion passes into the allantois, and it is that portion of this organ which lies within the abdomen, and becomes shut off from the rest by the constriction and obliteration of the cavity of an intermediate part, and its conversion into the urachus, that gives rise to the urinary bladder. The ultimate secreting tubuli of both the Wolffian body and the kidney, are alike remarkable for ending in dilatations which embrace convoluted capillaries - the so-called Malpighian tufts. Neither Wolffian bodies nor kidneys have been observed in Amphioxus. It is doubtful whether true kidneys are developed in Ichthyopsida, or whether the so-called kidneys of these animals are not, rather, persistent Wolffian bodies.