32 WILLIAM MORTON WHEELER, 



The formation of the pronephros of the embryo and larval Petro- 

 myzon as described in the foregoing paragraphs, differs, in certain 

 important particulars, from the accounts of nearly all previous in- 

 vestigators. The older writers may be passed over and only those 

 recent writers need be considered, who have undertaken to describe 

 the embryonic formation of the pronephros. These are Scott (1880, 

 1881 and 1887), v. Kupffer (1888), Shipley (1888), Goette (1890) 

 and Semon (1890). 



Scott in his third paper (1887) declared his previous view that 

 the pronephric funnels are differentiated from the duct, to be er- 

 roneous. I infer, therefore, that at his latest writing, he believed 

 that the pronephric tubules are formed first and the duct somewhat 

 later, as I have shown. Scott's contentions that the duct is split 

 off in situ from the somatic mesodernj and does not grow backward 

 independently, and that the pronephric tubules are originally meta- 

 meric, are also in accord with my observations. 



I have seen nothing to support v. Kupffer's statement that the 

 pronephric duct arises from the ectoderuL Probably v. Kupffer has 

 long since abandoned this view, which must have taken form at a 

 period when several investigators of other Vertebrates fancied they 

 could derive the pronephric duct from the outer germ-layer, v. Kupffer's 

 description of the pronephric tubules originating as successive conical 

 projections from the somatic mesoderm is based on sound observation. 

 He erred only in not finding the definitive number of tubules. 



I find, on the other hand , nothing to support Shipley's view 

 that the whole pronephros arises as a fold of the somatopleure, the 

 orifice of which closes in such a way as to leave four or five openings 

 persisting as the nephrostomes. Such an interpretation could only 

 have arisen from a rather careless perusal of sections. The remainder 

 of Shipley's description shows plainly, that he did not see the earlier 

 and more important stages in the development of the pronephros. 



Shipley's erroneous view is repeated and illustrated by Goette. 

 He not only derives the pronephros from a continuous fold of the 

 somatopleure, but the pronephric duct as well. The observation that 

 the duct is found at certain points arising from hollow diverticula of 

 the somatopleure, at others as a solid cord, was regarded by Goette 

 as unimportant. It becomes important, however, in connection with 

 my own observations on the origin of the duct from a fused series of 

 abortive pronephric tubules. Goette, like Shipley, did not study the 



