MORPHOLOGY OF CYCLOSÏOMATA. 40*J 



The " Nierencanälclien " of Amphioxu><, according to Boveki 

 ('92), extends over and is limited to, the whole extent of the 

 branchial region, the posterior larger part of which covers the 

 hepatic cœcum. The pronephros of Cyclostomata extends 

 from the anterior body-somite to the cloaca. The anterior section 

 of the system constitutes afterwards the glandular part represented 

 by the pronephric tubules and is found in front of, and over, 

 the Anlage of the liver, or in the region of the fore-gut ; a certain 

 number (two in Fetromijzon, twenty in Bdellostonia) of the anterior 

 nephric segments are found in the branchial region, and the 

 posterior one or two segments of the glandular part {Petromyzon) 

 cover the liver-Anlage. It follows that the six^^ to twenty 

 or more pronephric tubules correspond to as many " Nieren- 

 canälchen " in about the middle one tliird'-^ of the branchial resiun 

 0Î AmpJiioxu><, and that the " Nierencauiilchen " lying posterior to 

 this point are represented, in Cyclostomata, by a number of the 

 rudimentary tubules which are converted into the segmental duct. 



The '' Nierencanälchen " are not put in communication with 

 one another by the collecting duct, as in the pronephros of Cyclo- 

 stomata, but open to the exterior segmeutally. I have stated in 

 the descriptive part (p. 333) that the free extremities of the 

 pronephric tubules in Pciroiiiyzon are brought into close contact 

 with the epiblast, so that the latter is pressed out by the enor- 

 mous growth of the tubule and that this is especially the case in 

 the first and second tubules. This fact throws light upon the 

 homology of the pronephric tubules in Pelroniyzon with the 

 " Nierencanälchen " of Ainphioxus : in other words, the condition 



l)TIie glandular part nf the pronephros in Fetromyzon, are represented by the six 

 pronephric tubules. 



2) The branehiomeres in the posterior section of the gill-basket of Amphioxus are after- 

 wards added (see pp. 41U-4I1). 



