690 ELASMOBRANCHII. 



kidney, though it may perhaps be more suitably called, adopting 

 Lankester's nomenclature, the pronephros. Its duct, which forms 

 the basis for the generative and urinary ducts, will be called the 

 segmental duct. 



(2) The Wolffian body, which may be also called the 

 mesonephros. It consists of a series of, at first, segmentally 

 (with a few exceptions) arranged glandular canals (segmental 

 tubes] primitively opening at one extremity by funnel-shaped 

 apertures into the body cavity, and at the other into the 

 segmental duct. This duct becomes in many forms divided 

 longitudinally into two parts, one of which then remains 

 attached to the segmental tubes and forms the Wolffian or 

 incsoncpliric duct, while the other is known as the Miillerian 

 duct. 



(3) The kidney proper or metanephros. This organ is only 

 found in a completely differentiated form in the amniotic Verte- 

 brata. Its duct is an outgrowth from the Wolffian duct. 



The above parts do not coexist in full activity in any living 

 adult member of the Vertebrata, though all of them are found 

 together in certain embryos. They are so intimately connected 

 that they cannot be satisfactorily dealt with separately. 



Elasmobranchii. The excretory system of the Elasmo- 

 branchii is by no means the most primitive known, but at the 

 same time it forms a convenient starting point for studying the 

 modifications of the system in other groups. The most re- 

 markable peculiarity it presents is the absence of a pronephros. 

 The development of the Elasmobranch excretory system has 

 been mainly studied by Semper and myself. 



The first trace of the system makes its appearance as a knob 

 of mesoblast, springing from the intermediate cell-mass near the 

 level of the hind end of the heart (fig. 385 A, pd). This knob is 

 the rudiment of the abdominal opening of the segmental duct, 

 and from it there grows backwards to the level of the anus a 

 solid column of cells, which constitutes the rudiment of the 

 segmental duct itself (fig. 385 B, pd}. The knob projects 

 towards the epiblast, and the column connected with it lies 

 between the mesoblast and epiblast. The knob and column do 

 not long remain solid, but the former acquires an opening into 

 the body cavity (fig. 421, sd} continuous with a lumen, which 



