360 F. M. BALFOUR. 



then it is clear that similar causes have produced a similar 

 modification in development (only with a difi'erent organ) 

 in Birds; while, at the same time, the primitive mode of 

 origin of the Oviduct (Miiller's duct) has been retained by 

 them. 



The Oviduct, then, may be considered as arising by an 

 involution from the pleuro-peritoneal cavity. 



The Wolffian duct arises by a series of such involutions, 

 all of which are behind (nearer the tail) the involution to 

 form the Oviduct. 



The natural interpretation of these facts is that in the place 

 of the Oviduct and Wolffian body there were primitively a 

 series of similar bodies (probably corresponding in number 

 with the vertebral segments), each arising by an involution 

 from the pleuro-peritoneal cavity ; and that the first of these 

 subsequently became modified to carry eggs, while the rest 

 coalesced to form the Wolffian duct. 



If we admit that the Wolffian duct is formed by the 

 coalescence of a series of similar organs, we shall only have 

 to extend the suggestion of Gegenbaur as to the homology 

 of the Wolffian body in order to see its true nature. Gegen- 

 baur looks upon the whole urino-genital system as homo- 

 logous with a pair of segmental organs. Accepting its 

 homology with the segmental organs, its development in 

 Elasmobranchii proves that it is not one pair, but a series of 

 pairs of segmental organs with which the urino-genital system 

 is homologous. The first of these have become modified so 

 as to form the Oviducts, and the remainder have coalesced to 

 form the Wolffian ducts. 



The part of a segmental organ which opens to the exterior 

 appears to be lost in the case of all but the last one, where 

 this part is still retained, and serves as the external opening 

 for all. 



Whether the external opening of the first segmental organ 

 (Oviduct) is retained or not is doubtful. Supposing it has 

 been lost, we must look upon the external opening for the 

 Wolffian body as serving also for the Oviduct. In the case of 

 all other vertebrates whose development has been investi- 

 gated (but the Elasmobranchii), the Wolffian duct arises by 

 a single involution, or, what is equivalent to it, the other 

 involutions having disappeared. This even appears to be 

 the case in the Marsipobranchii. In the adult Lamprey 

 the Wolffian duct terminates at its anterior end by a large 



200) are correct, then the ordinary view of the WolflBan duct arising in 

 Birds as a solid rod at the outer corner of the protoyertebrse will have to 

 be abandoned. 



