414 s. H ATTA : 



mentioned, wliicli are converted into the anterior section of the 

 segmental duct in these groups. It follows that the segmental 

 duct in this part of Peiromyzon and the two other groups, is 

 not the morphological equivalent of the duct of the same name, 

 but of the pronephros itself, in Selachia. 



In the second place, let us consider the mode of growth 

 of the segmental duct. Whichever view may he taken of its 

 origin, whether epiblastic or mesohlastic, this duct in Selachia 

 does not arise segmentally as in the otlier Craniota just referred 

 to. It is a backward growth produced either by delamination 

 from the epiblast, as Ruckert and van Wyhe affirm, or by 

 cell-multiplication within the structure of the mesohlastic collect- 

 ing duct itself, as Kabl states. Hence it can not be homologous 

 with the duct of the same name in Petroiiiyzon and the two 

 other groups, which is derived segmentally from the rudimentnry 

 pronephric tubules. The Selachian segmental duct is, in its 

 whole length, represented, as I believe, by the posterior small 

 section of the segmental duct in Peiromyzon and Amphibia. 



In Peiromyzon, the hind end of the duct comes into an 

 intimate connection with both the epiblast and the lateral diver- 

 ticula of the cloaca, filling up the space between them, and fusing 

 with both of them. We may suppose that the direct communication 

 of the duct with the exterior, if such truly existed in the an- 

 cestral history, may have been at this point of the epiblast. 

 This fused condition of the duct and the epiblast reminds us of 

 the early stages of the Selachian duct at the stage when it has 

 been produced only a little posteriorly from the pronephric region. 

 I believe that if the duct is to be compared in Peiromyzon and 

 Selachia, a stage such as the above ought to be taken. The 

 largest part of the Selachian duct is represented by a free hind- 



