41G s. HAïïA : 



and the mesoblast. The latter view seems probable to me. The 

 figures ('80, figs. 5 a-c) given by van Wyhe to illustrate his view 

 of the epiblastic origin of the duct, are from the vertical j^lane 

 of the eighth Eumpfsegment of a Scyllium embryo with 37 

 somites, which corresponds to the twelfth Gesammtsegment of 

 Kabl. The figures ('96, figs. 9a, Ob, 10a, and 10b) given by Rabl 

 to the negation of van Wyiie's view, a]-e from the vertical plane 

 of the twenty-second Gesammtsegment of an embryo with the 

 ()3 somites. These two cases are, I suppose, the two ends of the 

 same duct in different stages ; the anterior end, being the 

 equivalent of the hind end of the segmental duct in Pdromyzon, 

 actually receives cells out of the epiblast, as the figures by \'ax 

 Wyhe show ; the other end, which is seen in Rabl's figures, is 

 the point of mere contact with the epiblast, along which it is 

 shifting backwards. 



If the above comparimn he eorrect, the segmental duct in 

 Selaehia is, except the anterior very ><ni all section xvlùcli is formed 

 directly of the abortive tulnilex, not hoiiioloyous with the duet of 

 the same name in Petromyzon, but is a dructure secondarily acquired. 



From the above account, it may be safely concluded that in 

 its primary phylogenetic stage, the pronephric system of the 

 Craniota above referred to consisted of a number of segmentally 

 arranged tubules, which were directly formed, in each mesoblastic 

 segment, from the distal (ventral) portion of the mesoblastic 

 somite, and opened independently to the exterior ; that the lateral 

 extremities of these tubules were afterwards secondarily united 

 with one another, thus constructing the collecting and segmenta^ 

 duct, the hind end of which opened directly to the exterior ; and 

 tliat the acquisition of an opening of the duct into tlie cloaca 



