224 G. C. PRICE, 



work on the excretory system of IchthjopUs, showed that this animal 

 has a much more extensive pronephros than any other at that time 

 known, and now in Bdellostoma we have a case where tubules, which 

 in development are homologous with pronephric tubules, are practically 

 co-extensive with the segmental duct, so that it now seems that the 

 third point in Rückert's theory has been demonstrated. 



The theory that primitively the excretory system consisted 

 of segmental tubules, which opened independently of one another along 

 the side of the body, and that the segmental duct has been derived 

 from these tubules, and consequently has arisen in the Vertebrate 

 phylum, seems to have very strong anatomical and embryological 

 evidence in its favor. 



Bdellostoma gives no evidence in favor of the fourth point in 

 Rückert's theory, namely, that the mesonephric tubules represent a 

 second generation of pronephric tubules ; nor, as it seems to me, does 

 it give any evidence in favor of Boveri's theory, that the segmental 

 duct has been derived from the peribranchial chamber of Amphioxus, 

 and the mesonephric tubules from the gonadial pouches. 



München, July 25, 1896. 



