106 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



mouths to form a single large funnel. This union continuing inward, a 

 single large duct is formed, which then grows in length cephalad by a 

 fusion of the lips of tlie opening. In this manner the ostium is carried 

 forward about 100 niicra. In early stages, when the duct is repre- 

 sented by a cord which reaches buck only about halfway along the 

 luesonephros, the Wolffian duct contributes cells towards its formation. 

 In older stages, when the cord has nearly reached the urino-genital sinus, 

 it takes far fewer cells from tlie Wolffian duct and finally grows backward 

 independent of that duct. Hence in Tupaia the Mulleriau duct is derived 

 from two entirely diffijrent sources : 



(1) The anterior end — that is, the ostium and the adjoining portion 

 of the duct — is formed from the peritoneal epithelium. 



(2) The remainder of the duct is formed largely from the Wolffian 

 duct. Anteriorly, however, it takes a larger percentage of cells from 

 the Wolffian duct than posteriorly. 



Erinaceus : There are two evagiuatioiis. As the Wolffian duct reaches 

 much farther cephalad than in Tupaia, the two cords from the evagina- 

 tions reach it before they have fused. As a consequence the Wolffian duct 

 must form two coi*ds which fuse comparatively late. The anterior evagi- 

 nation often loses its connection irith the peritoneum, so that the Miiilerian 

 duct appears branched anteriorly. The posterior portion of the duct 

 takes no cells from the Wolffian duct. 



Mouse: The "vertical" portion of the duct arises entirely independent 

 of the ostium, for stages are found in which a considerable extent of tlie 

 vertical portion is present unconnected with the ostium by the horizontal 

 portion, which develops later. Kip suggests an explanation of this condi- 

 tion which consists in the assumption that a rudimentary evagination, 

 homologous with one of those found in Erinaceus, gives rise to the 

 vertical portion of the duct and then atrophies, leaving the anterior end 

 of the duct to end free, — just as one branch occasionally does in Erina- 

 ceus. This free end, he suggests, then becomes connected with an ostium 

 •which arises later, this ostium not being homologous with any found 

 in the Insectivora.^ 



In rodents the connection between Miiilerian and Wolffian ducts is far 

 less intimate than in Insectivora. 



Kip states his belief that the development of the Miiilerian duct has 



1 Kip found a rudimentary ostium in the rabbit posterior to tlie final ostium. 

 Tlie latter appears only after the degeneration of the rudimentary posterior one. 



