MORPHOLOGY OF CYCL0^JT03IATA. 417 



was the tertiary stage of changes in the system. Such a course 

 of the phylogenetic development of tlie system is, Jiowever, no 

 other than that advanced by Kückeiit {'SS, p. 200). 



In the present paper, the historical comparison will he 

 limited to the groups of Yertebrata stated above ; the review of 

 Amuiota and some other theoretical considerations will be reserved 

 to a future paper, in which I propose to deal v\'ith the further 

 fate of the pronephros and the development of the mesonephros 

 in Petromyzon. 



Having compared the results arrived at in the present work, 

 with those in difterent classes of Anamnia, I may be justified 

 in drawing the following conclusions. 



In Petromyzoii, the first indications of the pronephros be- 

 comes apparent at a stage earlier than those hitherto regarded 

 as the starting point, that is, at a stage in which the mesoblast 

 in the anterior rcL'ion has undergone the metameric segmentation 

 but the lateral plate is not yet cut ofi' from the somite. 



The tissue giving rise to the pronephros is the parietal layer 

 of a small section of the mesoblast, which forms the distal 

 (ventral) half of the mesoblastic somite. This section of the 

 mesoblast exactly corresponds to the "Nephrotom" of Kuckert 

 in Selachia. 



The Anlage of the pronephros in all the groups of A^ertebrata 

 above referred to is produced by the evagination of the parietal 

 layer of the nephrotome which theoretically ought to contain a jwrt 

 of the cœlomic cavity. Tn Cyclostomata, such a cavity is 



