iv MULLEKIAN DUCT 245 



A survey of the phenomena as described for the various sub- 

 divisions of the Vertebrata shows that as a rule they may be fitted 

 without difficulty into one or other of these types of modification. 



Thus in the Amphibia some, especially, of the older observers, 

 described the extension of the Miillerian duct as taking place by 

 splitting off from the archinephric duct as in Elasmobranchs, others 

 as being due to independent intrinsic growth, still others as a 

 process of folding or splitting off from the splanchnocoelic epithelium. 

 One of the most careful modern accounts (H. Eabl, 1904) based upon 

 the phenomena observed in the relatively primitive Urodela 

 (Salaviandra) states that the funnel is the persisting " second " 

 nephrostome of the pronephros and that the portion of duct behind 

 this arises as a thickening of the coelomic epithelium — the cells first 

 assuming a columnar shape, then becoming arranged in several 

 layers to form a ridge projecting into the subjacent connective tissue, 

 and finally becoming split off as a solid rod. Only the anterior 

 portion of the Miillerian duet is formed in this way, the rod-like 

 rudiment so formed proceeding to grow back independently to form 

 the hinder part of the duct. 



In Eeptiles and Birds the ostium is described as originating as 

 a pit in the coelomic epithelium, which we may look on as a delayed 

 and modified nephrostome, and the extension backwards as taking 

 place by independent growth. The evagination of the epithelium 

 to form the pit is, as is usual in such cases, preceded by the 

 epithelium becoming somewhat thickened. 



The mode of origin of the Miillerian duct has not yet been 

 worked out in detail in the Ganoids and Lung-fishes. In ordinary 

 fishes (Teleostei) the conditions are peculiar and will be dealt with 

 along with the development of the ovary. 



As regards the further development of the Miillerian duct, it 

 should be noted that its completion and opening into the cloaca is 

 commonly delayed till a comparatively late stage — often till a period 

 but shortly before sexual maturity. 



Though primarily retroperitoneal the Miillerian duct comes, with 

 increasing growth, to bulge into the splanchnocoele, pushing inwards 

 the peritoneal lining which comes to surround it as a sheath con- 

 taining muscles, blood-vessels, etc. Its lining epithelium becomes 

 glandular and specialized to minister to the nutritive and protective 

 needs of the egg in ways which differ in the different groups. 



Various modifications make their appearance in later stages. 

 Very frequently the coelomic opening becomes shifted by the 

 addition to the tube of a secondary extension formed from the 

 peritoneal lining. In Elasmobranchs this shifting is towards the 

 mesial plane and except in a few species leads to complete fusion so 

 as to form a single median opening for the two oviducts. Again the 

 hinder ends of the Miillerian ducts are in many cases approximated 

 and they too may fuse to form a terminal unpaired portion. 



In the case of the Birds the right oviduct lags behind in 



