THE UR1NOGENITAL ORGANS OF VERTEBRATES. 163 



series of ducts and Malpighian tufts which form the mass of the 

 Wolffian body 1 . 



The Miillerian 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 8oth and rooth hour of incubation the ducts 

 of the permanent kidneys begin to make their appearance. 

 Near its posterior extremity each Wolffian duct becomes ex- 

 panded, 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 im- 

 mediately after the formation of the permanent kidneys is 

 strikingly similar to its permanent condition in adult Sela- 

 chians. There is the Miillerian 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 perma- 

 nent kidneys in birds and Leydig's glands and the kidneys 

 in Selachians. 



The duct of the permanent kidney, which at first opens into 

 that of the Wolffian body, subsequently becomes further split 

 off from the Wolffian duct, and opens independently into the 

 cloaca. 



1 This account of the origin of the Wolffian body differs from that given by Wal- 

 deyer, and by Dr Foster and myself (Elements of Embryology, Foster and Balfonr), 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. 



I I 2 



