. HALL: MESONEPHROS AXD MULLERIAN DUCT IN AMPHIBIA. 107 



become independent of the AVolffian duct in reptiles, many birds, and 

 possibly some mammals, by tlie extension cephalad of the method of 

 formation seen in the posterior portion in insectivores. The ostia in 

 mammals are homologous with the pronephric nephrostomies of elasmo- 

 branchs, but the ostia of insectivores cannot be homologous with the 

 same nephrostomes as the ostia of rodents. 



From the above abstract, it appears that a remarkable agreement exists 

 between the development of the MiiUerian duct in Insectivora and Am- 

 blystoma. In tlie I'odents there would seem to be an additional process 

 consisting in the formation of a second ostium, anterior to, and later 

 than, the first. This condition might easily be derived from that in 

 Amphibia by supposing tliat the lengthening of the duct, cephalad, was 

 brought about not by a progressive infolding and fusion of the thickened 

 band, beginning at the ostium, but in the following manner : The infold- 

 ing and fusion begins at the ante)'ior end of the band and progresses back 

 to the ostium. The final result would be the same in both cases. 



That the position of the evaginations in the Insectivora is as far back 

 as the mesonephros could be explained by assuming that that portion of 

 the tissue of the thickened band which gives rise to the evaginations 

 migrates caudad before instead of after the proliferations have taken 

 place. 



For comparison I have briefly summarized below the observations of 

 the various authors and my own. 



Urodela. 



Fiirbringer: The Miillerian duct is formed from the Wolffian by a 

 longitudinal splitting. The anterior end fuses secondarily with the thick- 

 ened peritoneum to form the ostium. No cephalic migration of the 

 ostial opening noted. 



Hoffmann : The ^Miillerian duct is formed as described by Furbringer, 

 except tliat in the male only the more anterior portion of the duct arises 

 from the Wolffian duct. 



Wilson : The ostium and anterior end of the duct are formed by an 

 evagination of tlie peritoneal epithelium. The rest of the duct arises 

 from the peritoneal cells of the oviducal welt.' He seems to have seen, 

 without realizing the fact, two separate evaginations and a cephalic 

 migration of the ostial opening. 



