Footnote 45: This account of the origin of the Wolffian body differs from that given by Waldeyer, and by Dr Foster and myself (_Elements of Embryology_, Foster and Balfour), but I have been led to alter my view from an inspection of Mr Sedgwick's preparations, and I hope to shew that theoretical considerations lead to the expectation that the Wolffian body would develop independently of the duct.
[Illustration: FIG. 8. TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN EMBRYO FOWL OF 45 H. TO SHEW THE MODE OF FORMATION OF THE WOLFFIAN DUCT.
_A._ epiblast; _B._ mesoblast; _C._ hypoblast; _M.c._ medullary canal; _Pv._ Protovertebræ; _W.d._ Wolffian duct; _So._ Somatopleure; _Sp._ Splanchnopleure; _pp._ pleuro-peritoneal cavity; _ch._ notochord; _ao._ dorsal aorta; _v._ blood-vessels.]
The Müllerian duct arises in the form of an involution, whether at first solid or hollow, of the germinal epithelium, and, as I am satisfied, quite independently of the Wolffian duct. It is important to notice that its posterior end soon unites with the Wolffian duct, from which however it not long after becomes separated and opens independently into the cloaca. The upper end remains permanently open to the body-cavity, and is situated nearly opposite the extreme front end of the Wolffian body.
Between the 80th and 100th hour of incubation the ducts of the permanent kidneys begin to make their appearance. Near its posterior extremity each Wolffian duct becomes expanded, and from the dorsal side of this portion a diverticulum is constricted off, the blind end of which points forwards. This is the duct of the permanent kidneys, and around its end the kidneys are found. It is usually stated that the tubules of the permanent kidneys arise as outgrowths from the duct, but this requires to be worked over again.
The condition of the urinogenital system in birds immediately after the formation of the permanent kidneys is strikingly similar to its permanent condition in adult Selachians. There is the Müllerian duct in both opening in front into the body-cavity and behind into the cloaca. In both the kidneys consist of two parts--an anterior and posterior--which have been called respectively Wolffian bodies and permanent kidneys in birds and Leydig's glands and the kidneys in Selachians.