MUSEUM OF COMPARATIVE ZOOLOGY. 289 



consideration of the development of the segmental duct. As is well 

 known, observers up to a very recent date have been almost unanimous 

 in ascribing a mesodermal origin to this structure. In regard to the 

 details of the process, however, they have been less at one ; and I shall 

 accordingly treat of their accounts under three heads, which seem to me 

 to represent fairly well marked phases of opinion. 



According to one view, the duct arises as an evagination of somatopleure, 

 its lumen being therefore a detached portion of the bod// cavity. Such a 

 mode of origin was advocated by Rosenberg ('07, pp. 42 et seq.) for 

 Teleosts ; and this feature of his account has gained almost universal 

 acceptance both for Teleosts and for Amphibia, having been recently 

 entirely confirmed by Hoffmann ('8G) and Henneguy ('88, '80). Ac- 

 cording to Wilh. Miiller (T5) and Furbringer, the duct arises in this way 

 also in Petromyzon, and a similar claim has been made for Ganoids by 

 Kowalewsky, Owsjaunikow, and Wagner (TO), and by Balfour and Parker 

 ('82). In Selachians, however, the weight of the evidence is distinctly 

 opposed to this view, and I am not aware of its having been advocated 

 by any one besides Schultz (To). 



In Amniotes also such an account of the early development has not 

 received general acceptance ; it was first claimed in this class by Romiti 

 (T4), and was adopted, with some modification it is true, by R, Kowa- 

 lewsky (To), and by Dansky und Kostenitsch ('80, p. 24). Very re- 

 cently such a mode of origin has been reasserted by Fleischmann ('87) 

 for Carnivores and the Duck. 



My own observations on Amphibia indicate that in this group the 

 duct does not arise as a fold ; and I am of opinion that, in both Cyclo- 

 stomes and Ganoids, the evidence that the duct arises by evagination is 

 at present unsatisfactory. It seems to me probable, on the contrary, 

 that the method of origin which is usually recognized as characteristic of 

 all the Anamnia with the exception of Selachii exists, if at all, only in 

 Teleosts. In view of the peculiar obstacles which Teleostean material 

 presents for embryological study, one should be cautious in affirming for 

 this group a mode of development which, in my opinion, is not proved to 

 exist in any other class of Vertebrates. 



A second view of the origin of the duct is, that it arises from a solid 

 proliferation of somatopleure. According to Furbringer (T8 a ), Spoof ('83, 

 p. 84), and the earlier writers (Remak, '55, Kolliker, '61, Bornhaupt, 

 '67, Waldeyer, TO, and Foster and Balfour, T4), the duct arises in the 

 chick by a proliferation in situ of the subjacent mesoderm, and a similar 

 origin is maintained for Petromyzon by Scott ('82). The more recent 



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