278 BULLETIN OF THE 



the pronephric structures is traversed by a series of glomeruli which re- 

 semble most closely those of the niesonephros. Siemerling calls them 

 transitional glomeruli. The pronephros of our scheme would be repre- 

 sented in this account by the region in front of the fifth protovei'tebra ; 

 the intermediate region would correspond to the space occupied by the 

 transitional glomeruli, and also, as I believe, to that previously occupied 

 by the primary cords ; the mesonephros would form the rest of the 

 organ. 



According to Sedgwick's ('81) account of the development in the 

 chick, the Wolffian duct, in separating from the proliferation in which 

 it arises (region between the 7th and 11th proto vertebra;), remains 

 connected with the peritoneal epithelium by short cords of cells. Be- 

 tween the 8th and 15th protovertebrse, the duct, as it grows freely back- 

 wards, comes secondarily into contact with such a cord of cells in each 

 somite. Behind the 15th somite, the fundaments of the tubules (inter- 

 mediate cell mass) do not join the duct until their differentiation is 

 somewhat advanced. The cords of cells in the region between the 7th 

 and 11th protovertebr£e acquire lumens which may be continued even 

 into the duct ; but both cords and duct soon entirely disappear. Al- 

 though no glomus is described, this region probably represents the 

 pronephros. Between the 12th and 15th protovertebrse typical nephro- 

 stomal funnels are formed, in which transitional glomeruli develop. This 

 portion of the organ would then correspond to the intermediate region of 

 the general scheme ; behind this region comes the typical mesonephros. 

 Sedgwick regarded the first mentioned region as pronephric ; but he 

 hoped to be able to harmonize such a view with the position (cf. page 276) 

 formerly taken by himself and Balfour (Balfour and Sedgwick, '79).^ 



In the foregoing description I have assumed that the most anterior 

 portion of the Wolffian duct and the accompanying transverse canals 

 observed by Sedgwick corresponded to the pronephric region as de- 

 scribed by Siemerling. This interpretation seems to me in all probabil- 

 ity correct ; yet it should be recalled that the pronephros described by 

 Siemerling lies in front of the 5th somite, and is anterior to the region 

 in which the early proliferation to form the duct took place ; whereas, 



1 Mihalkovics's statement, that Sedgwick abandoned his former view, is incor- 

 rect, as will be seen by referring to the closing paragrapli of his article (Sedgwick, 

 '81, p. 468). 



In the second edition of Foster and Balfour's ('83, p. 218) Elements of Embry- 

 ology, revised by Sedgwick and Heape, the anterior end of the Miillerian duct is the 

 only homologue of the Amphibian pronepliros suggested. 



