54 WILLIAM MORTON WHEELER, 



do not reach the collecting duct till it has grown past them, and that 

 in Ichthyophis these tubules fail to reach the duct at all. In these 

 cases, therefore, we should have no traces of the Acraniote or Am- 

 phioxus-like pronephros in the trunk behind the shortened Craniote 

 pronephros, with the exception of a few abortive tubules. The duct 

 would have grown into the trunk from the cephalic end instead of 

 being an autochthonous structure. One leaves the perusal of Rückert's 

 résumé with the impression that this mode of duct formation is the 

 usual one, and such it may be in Selachians and Amniota — but I 

 deem it more expedient to return to his original utterances above 

 quoted, since they furnish a basis for the understanding of the Cyclo- 

 stomes at least. 



In Petromyzon^ as I have attempted to show, there are distinct 

 traces of tubules uniting with one another to form the pronephric 

 duct even in the mid-trunk region several segments behind the pro- 

 nephros proper, which consists of only five or six tubules. It is, indeed, 

 possible to recognize even the funnels of these backward directed 

 vestigial tubules and the way in which the blind end of each bends 

 back and unites with the next posterior tubules. With the cutting 

 off of the duct from the somatopleure the openings into the coelome 

 (nephrocœle), corresponding to nephrostomal openings of the pronephros, 

 close and disappear, as do also the individual tubules as such, when 

 they become only parts of the continuous pronephric duct. These 

 vestigial tubules are really so short that they correspond in great 

 part only to the portions which in the pronephros fuse to form the 

 collecting duct. That this stage in the formation of the duct is of 

 short duration and not apparent in all embryos is no argument against 

 its important bearings, for nothing that occurs in the embryo is to 

 be regarded as insignificant in the present stage of our knowledge. 

 The observations on the formation of the duct in Petromyzon are the 

 very evidence needed to support Rückert's hypothesis. The extreme 

 posterior end of the duct when it enters the cloaca, it is true, is not 

 accounted for, but as yet no one has accounted for this satisfactorily 

 on any hypothesis. It is, perhaps, easier to understand how certain 

 pronephric tubules in the posterior trunk region could depart from 

 the conditions shown by their more anterior fellows, and open into 

 the cloaca instead of onto the surface of the body or into one another, 

 than to conceive how the duct arose from the very first by growing 

 back through the trunk to the cloaca. 



Another set of observations which we owe to Price and Maas 



