MORPHOLOGY OF CYCLOSTOMATA. 383 



omic epithelium, which extends through seven section?, 



the thickening has not been caused by a proliferation of cells, 

 but certain cells having assumed the form of columnar epithe- 

 lium, while the adjoining cells retained the form of flat epithelium. 



later an evagination will liere take place, to form a 



segmental tubule " (p. 209). These evaginations are connected 

 with one another " by a streak of columnar epithelium, which 

 in transverse section lesembles the first tubule anläge, except that 

 there is no concavity on the lower surface ; this is the segmental 

 (collecting) duct." " The union between the duct and tubules is," 

 in another place he says, '' primary and not secondary " 

 [loco cit., p. 210). 



The pronephros in Bdellostoma comprises, according to Price, 

 69 segments (spinal ganglions). As it begins at the transverse 

 plane opposite the eleventh spinal ganglion, it is inferred 

 that the pronephros in Bdellostoma is extended over the whole 

 length of the branchial region. But " the excretory system 

 disappears through the greater part of this region before the 

 gills are formed " {loco cit., p. 217). 



The segmental duct (in .^. .s/r.) is, according to the author, 

 brought about by the rudiments of the hinder 20 degenerated 

 tubules (in his Stage C) ; the number of the declining tubules 

 increases by stages : in Stage A, there are two ; in Stage B, 

 nineteen ; and in Stage C, twenty. 



This account is thus in close agreement with that given in 

 the present work, excepting a slight difference as to the origin 

 of the collecting duct and as to the number of the tubules. In 

 Bdellostoma, the collecting duct develops out of the Anlagen 

 independent from that of the tubules, while in Petromyzon, as 

 stated in the foregoing description, the Anlage in a mesoblastic 



