138 THE URINOriENITAL ORGANS OF VERTEBRATES. 



The duct is the primitive duct of the kidney 1 ; I shall call it 

 in future the segmental duct ; and the involutions are the com- 

 mencements of the segmental tubes which constitute the body 

 of the kidney. I shall call them in future segmental tubes 



Soon after their formation the segmental tubes become 

 convoluted, and their blind ends become connected with the 

 segmental duct of the kidney. At the same time, or rather 

 before this, the blind posterior termination of each of the seg- 

 mental ducts of the kidneys unites with and opens into one of 

 the horns of the cloaca. At this period the condition of affairs 

 is represented in fig. 2. 



FIG. i. DIAGRAM OF THE PRIMITIVE CONDITION OF THE KIDNEY IN A 

 SELACHIAN EMBRYO. 



pd. segmental duct. It opens at o into the body cavity and at its other extremity 

 into the cloaca ; x. line along which the division appears which separates the seg- 

 mental duct into the Wolffian duct above and the Miillerian duct below ; st. seg- 

 mental tubes. They open at one end into the body-cavity, and at the other into the 

 segmental duct. 



There is at pd, the segmental duct of the kidneys, opening 

 in front (p) into the body-cavity, and behind into the cloaca, and 

 there are a series of convoluted segmental tubes (st), each 

 opening at one end into the body-cavity, and at the other into 

 the duct (pd). 



The next important change which occurs is the longitudinal 

 division of the segmental duct of the kidneys into Miiller's duct, 

 or the oviduct, and the duct of the Wolffian bodies or Leydig's 

 duct. The splitting 2 is effected by the growth of a wall of cells 



1 This duct is often called either Miiller's duct, the oviduct, or the duct of the 

 primitive kidneys ' Urnierengang.' None of these terms are very suitable. A justifi- 

 cation of the name I have given it will appear from the facts given in the later parts 

 of this paper. In my previous paper I have always called it oviduct, a name which is 

 very inappropriate. 



2 This splitting was first of all discovered and an account of it published by 

 Semper ( Centralblatt f. Med. \Viss. 1875, No. 29). I had independently made it out 



