jrOEPHOLOGY OF CYCLOSTOMATA. oôo 



under the fiftli myotome ; both are of a triangular form nnd 

 contain a very spacious internal cavity of tlie same shape. On 

 the right side of fig. 53, the sixth myotome and the third tubule 

 are shown. The section next posterior to fig. Ô3 (fig. ô4) shows 

 the cross-section of the collecting duct (cd.) on the right side and 

 a slice of the hind wall of the second tubule on the left (^9^.2). 

 The cells composing the duct are closely set together, although 

 arranged more or less radially, acquiring a tubular form. As 

 has been repeatedly mentioned above, the epiblast is, as in 

 the foregoino' staoe, marked off from the mesoblast as well as from 

 the Anlage ; but at the ^^rcsent stage, the second tubule (figs. 54, 

 pt.2) pushes against the epiblast, probably in consequence of an 

 enormous multiplication of its component cells, so as to cause 

 the latter to be a little elevated externally. It must be remarked 

 here that the Anlagen, especially the first and the second, when 

 they first assume the tubular form, are brought into an intimate 

 relation with the epiblast, striking against it. In some of 

 my sections, a mitotic figure is seen at that point of the epiblast^' 

 (fig. 54, :c). This might lead some to assume a genetic connection 

 between the epiblast and the pronephros in Petronyzon ; but there 

 is, I believe, in reality no such relation. If the epiblast gives some 

 cells destined to build the pronephros or a part of it, cell-prolifera- 

 tion or some other mode of cell-production would necessarily be 

 observed in the epiblast in the preceding stages or at least, in 

 the stage here spoken of. In the foregoing stages, the epiblast 

 had, as has been repeatedly mentioned above, a sharp limit against 

 the structures inside it. At the present stages also, it is marked off 

 by the boundary-line of cells from the tissue .of the tubule, 



l)Tn the series of sections, from which fig. 54 is drawn, I ohserve mitotic fignres at 

 tliat point in several sections. 



