hall: mesonephros axd mullerian duct in amphibia. 35 



In the diagrammatic figures A-D it will be noticed that I have repre- 

 sented, in every case, the mesoraer as containing tissue from both 

 splanchnoderm and somatoderm, — thus making its cavity a true part of 

 the primitive coelom. This important conception "\ve owe to Adam 

 Sedgwick ('81). Having come to tlie conclusion that the middle plate 

 of the chick represented a portion of the splanchnoderm and somatoderm, 

 he proceeded to verify it in the elasmobranclis. 



The condition he described in these animals is briefly this : The 

 middle plate is in the form of a tube connecting the coelom of the lateral 

 mesoderm with the cavity of the myotome. This tube becomes cut off 

 from the myotome and its dorsal end curves downward and outward ^ to 

 join the pronephric (Wolffian) duct.''^ At a point near its connection with 

 the lateral plates, the dorsal wall of the tubule is invaginated to form a 

 glomerulus. The portion between the Malpighian body thus formed and 

 the lateral plates is the outer tubule ; that between the Malpighian body 

 and the duct is the inner tubule. The nephrostome is thus formed by 

 the persistence of an opening already present, and the cavities of the 

 outer and inner tubules, as well as that of the Malpighian body, are a 

 portion of the primitive body cavity. 



I believe Sedgwick's description is correct except in one important 

 detail ; instead of the inner tubule arising by a bending downward of the 

 blind pocket, which — as it contains both splanchnoderm and somato- 

 derm — would cause tlie lumen of the inner tubule to represent primary 

 coelom, the inner tubule is formed entirely by means of the evagination 

 of the somatic layer. This would make the lumen of the inner tubule, 

 in a way, a secondary one. Field ('91) lays stress on this point, and it 

 has been vaguely recognized by otlier authors. 



Sedgwick's conception, with more or less modification, has been found 

 to apply to most of the vertebrate groups which have been studied since 

 1881. In the Amphibia, however, two of the authors who have expressed 

 an opinion on the subject since that date have derived the mesonephric 

 elements from evaginations of the peritoneum.^ Hoffmann ('86) describes 

 their origin in Triton and states that the serial evaginations (in the form 

 of solid outgrowths) retain their connection with the peritoneum to form 

 the nephrostomes. Field ('91), in a work on the pronephros of Amblys- 

 toma, simply mentions that he could not be certain as to the mode of 



1 A glance at Figure C, page 83, may aid in understanding this process. 



2 No distinction will be made in this paper between pronephric and Wolffian 

 ducts. 



8 I leave out of account, for the present, that primitive amphibian, Ichtliyophis. 



