DEVELOPMENT OF THE ELASMOBRANCH FISHES. 103 



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 coa- 

 lescence 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 urine-genital system as homolo- 

 gous 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 Ovi- 

 ducts, 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 investigated (but the 

 Elasmobranchii), the Wolffian duct arises by a single invo- 

 lution, or, what is equivalent to it, the other involutions having 

 disappeared. This even appears to be the case in the Mar- 

 sipobranchii. In the adult Lamprey the Wolffian duct ter- 

 minates at its anterior end by a large ciliated opening into 

 the pleuro-peritoneal cavity. It will, perhaps, be found, when 

 the development of the Marsipobranchii is more carefully 

 studied, that there are primitively a number of such open- 

 ings 1 . The Oviduct, when present, arises in other vertebrates 



1 While correcting the proofs of this paper I have come across a memoir of W. 

 Muller (" Ueber die Persistenz der Urniere bei Myxine Glutinosa, " Jcnaische Zeit- 

 schrift, Vol. VII. 1873), in which he mentions that in Myxine the upper end of the 



