56 WILLIAM MORTON WHEELER, 



duct, like the one shown in Maas' fig. 3, have an epithelium of a 

 very dififerent appearance from that of the duct, so that we can hardly 

 suppose even the anterior mesonephric tubules to have originated as 

 Price would have them. If we take some such view as this of Price's 

 work, we have cleared the way for understanding Maas' observations. 

 Maas bases his views, which are in perfect accord with Rückert's 

 hypothesis, mainly on the structure of the glomeruli and their relations 

 to the pro- and mesonephric tubules and to the pronephric duct. These 

 observations, which are of the greatest value, I shall consider later. 

 Suffice it to say in this place that they fully confirm the view that 

 the pronephric duct is really a portion of the original extensive pro- 

 nephros, that it consists of fused abortive tubules that have not grown 

 back from the pronephros sensu stricto et auctorum but have arisen from 

 the mesoderm of the trunk where they remain, and, furthermore, that 

 the mesonephros consists of metameric tubules which open into the 

 duct, i. e. into the aborted and modified pronephric tubules of the 

 corresponding segments. It follows, of course, that there can be no 

 serial homology of meso- with pronephric tubules, since both structures 

 actually coexist and function in the same segments and must arise from 

 dififerent regions of the mesoderm ^). 



Further indirect evidence of the truth of Rückert's hypothesis 

 are the observations of Boveri (1892 a) and Maas that the pronephric 

 duct of Myxine is glandular in structure. I have made the same 

 claim for the pronephric duct of Petromyzon. Indeed there would be 

 little use for the capillary net-work which invests the duct in both 

 these forms if the duct were not capable of performing some function 

 similar to that of the tubules. I have also given reasons for sup- 

 posing that the glandular cells of the duct are replaced periodically 

 by "Ersatzzellen" in the same way as the cells of the tubules. 



1) It may be urged that there is one way of escape from the 

 cogency of these facts for investigators who, like Field (1891), believe 

 in the serial homology of the pro- and mesonephros. This is to sup- 

 pose that each of the original Amphioxus-like pronephridia is at present 

 represented in the ontogeny of the Craniote by two rudiments, which 

 make their appearance at different times, the distal end appearing first 

 and uniting with the distal ends of other tubules to form the duct, while 

 the proximal ends appear later as the mesonephric tubules. There is, 

 of course, no valid a priori objection to such a view, since apparently 

 single organs, like the Vertebrate hypophysis, are known to arise from 

 discontinuous centers, but in the present instance such a method of devel- 

 opment appears strained and difficult to prove. 



