EXCRETORY ORGANS. 



569 



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

 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 ' 

 mesonephric duct, while the other is known as the Milllerian duct. 



('?>) The kidney proper or metanephros. This organ is only/ 

 found in a complete 1}' differentiated form in the anmiotic V^ertebrata. > 

 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 Elasmobranchii 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 remarkable 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 miikes its appeai^ance 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 



spn 



Fio. 385. Two SECTiOKs of a Piustiurus embhyo with three visceral clefts. 



The sections illustrate the development of the segmental duet (pd) or primitive duct 

 of the pronephros. In A (the anterior of the two sections) this appears as a solid knob 

 (pd) projecting towards the epiblast. In B is seen a section of the column which has 

 grown backwards from the knob in A. 



spn. rudiment of a spinal nerve; mc. medullary canal; ch. notochord; A', sub- 

 notochordal rod; vip. muscle-plate; mp'. specially developed portion of muscle-plate; 

 ao. dorsal aorta; j)d. segmental duct; so. somatojoleure ; sp. splanchnopleure; pp. 

 body cavity; ep, epiblast; al. alimentary canal. 



