Development of the Excretory Organs of IMellostoma stouti Lockington. 221 



have an example of the pliylogenctic development being repeated in 

 the ontogeny. For me, it is easier to believe this, than that the great 

 similarity in structure between the tubules in Bdellostoma and the 

 mesonei)hric tubules in other Vertebrates is simply accidental; and 

 after having studied their development, it is impossible for me to 

 doubt that they are formed in the same way as pronei)hric tubules. 



The idea of an organ being a pronephros in one stages of its 

 development, and a mesonephros in another, seems to involve a posi- 

 tive contradiction, but, as I believe, it is a contradiction in terras and 

 not in facts. 



If the view above expressed be correct, the mesonephros of the 

 Myxiuoids is primitive in a much more fundamental sense than has 

 hitherto been supposed, and the natural inference would be that it 

 represents the ancestral condition, from which the mesonephros of 

 other vertebrates has been derived. If so, one ought to be able to 

 show how this has been effected; but this I shall not attempt to do, 

 and I fully grant that it would be a difficult task. I should like to 

 say, however, that embryology teaches that the excretory system of 

 the Myxinoids is not only more primitive, but much more primitive, 

 than that of anyother known Vertebrate except Amphioxus, and it is 

 not to be wondered at, if in some points there should be a marked 

 ditierence between the development of the system here and in other 

 forms. 



I do not wish it to be thought that I am claiming originality for 

 the idea that the pronephros and mesonephros are intimately related 

 structures. Various investigators, from work on different animals, 

 have been led to the conclusion that the two structures are homo- 

 logous ; but while some see complete serial homology between the two, 

 others see only partial homology, and believe that formerly the pro- 

 nephros extended throughout the region where the mesonephros now 

 exists ; and that in some indirect way the latter has been derived 

 from the former. For a discussion of the first view, together with a 

 review of the literature upon the subject, the reader is referred to 

 the excellent paper by Field ('91), while Hückert ('88) and Semon ('91) 

 may be cited as giving strong arguments in favor of the latter view. 



If the interpretation of the excretory organs of Bdellostoma above 

 expressed be correct, some addition to the nomenclature will be 

 rendered necessary; for it would lead to confusion to speak of an 

 organ as pronephros at one period of its development, and as meso- 

 nephros at another, and further, it would not be known, when the 



Zool. Jahrb. X. Abth. f. Morph. J^5 



