106 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



mouths to form a siugle 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 the opening. In this manner the ostium is carried 

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

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

 mesonephros, 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 the Wolffian duct and finally grows backward 

 independent of that duct. Hence in Tupaia the Miillerian duct is derived, 

 from two entirely diffisrent 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 evaginations. 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 cords which fuse comparative!}' late. The anterior evagi- 

 nation often loses its connection icith the peritoneum, so that the Mtillerian 

 duct appears branched anteriorlj''. 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 ai-e found in which a considerable extent of the 

 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 Miillerian and Wolffian ducts is far 

 less intimate than in Insectivora. 



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



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

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



