Development of the Excretory Orgmis of Rdfillostoma stouti Lotkingtoii. 223 



that in Anipbioxus there is a series of excretory tubules leading from 

 the coelom, and opening independently of one another, not to the 

 exterior, it is true, but into the peribranchial chamber, and therefore 

 upon a surface which in the embryo was a part of the external sur- 

 face of the body. Boveri gave such arguments in favor of the homo- 

 logy of these tubules with pronephric tubules, that it amounted almost 

 to demonstration. In one important particular, however, the excretory 

 system of Amphioxus differs from the excretory system of all other 

 Vertebrates ; it occurs only in the region of the gills, while in all other 

 Vertebrates it occurs only back of the gills. But in Bdellostoma, as 

 we have just seen, the excretory system in the embryo occurs in both 

 regions. This is a welcome connecting link, and seems to remove 

 all doubts, if any existed, as to the homology of the excretory tubules 

 of Amphioxus with the pronephric tubules of other Vertebrates. On 

 the other hand the presence of excretory tubules in the gill region 

 of Amphioxus is a strong argument in favor of the primitive nature 

 of the excretory system in Bdellostoma. 



In Amphioxus the excretory tubules extend as far forwards as the 

 anterior gill slits. In Bdellostoma this is not the case, but the dege- 

 nerate nature of the anterior end of the system, and the fact that in 

 one embryo it extends farther forwards on one side than on the other, 

 shows that the system is here undergoing a process of reduction, and 

 it is altogether probable that it once extended farther forwards than at 

 present, perhai)S as far as is now the case in Amphioxus. 



In Bdellostoma no case has been observed where the segmental 

 tubules come into contact with the external epiblast, and it is quite 

 probable that this does not take place even in the younger embryos; 

 but at the anterior end there are independent tubule anlages, and 

 with the strong reasons we have for thinking the conditions in Am- 

 phioxus represent a phylogenetic stage in the excretory system of the 

 Vertebrates, it is no rash statement to say that formerly these in- 

 dependent anlages had an independent opening to the exterior. 



The segmental duct in Bdellostoma is not formed by the distal 

 ends of the tubules bending backwards and uniting with the tubules 

 just posterior, but at the anterior end , at least , the tubule anlages 

 are first formed, and the duct anläge afterwards, and this is really 

 the important thing. 



At the time of the publication of Rückert's theory there was no 

 case known where the pronephric tubules were present in more than 

 a very few of tlie anterior segments. Semon ("91), in his interesting 



15* 



