THE URIXOGEXITAL SYSTEM 401 



left only remaining as the functional oviduct. We have now to 

 consider, therefore, (1) the origin of the ducts during the in- 

 different stage, and (2) their subsequent history in the male 

 and in the female. 



The origin of the IMlillerian duct is preceded by the formation 

 of a strip of thickened peritoneum on the lateral and superior 

 face of the Wolffian body extending all the way to the cloaca 

 (cf. Fig. 220). This strip, which may be called the tubal ridge, 

 appears first at the anterior end of the Wolffian body on the 

 fourth da}", and rapidly differentiates backwards; it lies imme- 

 diately external to the Wolffian duct. The anterior part of the 

 Miillerian duct arises as a groove-like invagination of the tubal 

 ridge at the cephalic end of the Wolffian body immediately 

 behind the external glomeruli of the pronephros. The hps of 

 this groove then approach and fuse on the fifth day, so as to form 

 a tube which soon separates from the ridge. This process, how- 

 ever, takes place in such a way as to leave the anterior end of 

 the tube open and this constitutes the coelomic aperture of the 

 oviduct, or ostium tuh(£ abdominale. Moreover, the closure of 

 the groove does not take place uniformly, and one or two open- 

 ings into the Miillerian duct usually occur near the ostium on 

 the fifth clay. Typically, however, these soon close up, though 

 persistence of one of them may lead, as a rather rare abnormality, 

 to the occurrence of two ostia in the adult. There is no ground 

 for the view (see Balfour and Sedgwick) that the two or three 

 openings into the anterior end of the Miillerian duct correspond 

 to nephrostomes of the pronephros; they are situated too far 

 posteriorly and laterally to bear such an interpretation. 



The anterior part of the Miillerian duct is thus formed by 

 folding from the epithelium of the tubal ridge; it constitutes a 

 short epithelial tube situated between the Wolffian duct and the 

 tubal ridge, ending blindly behind. The part thus formed is rela- 

 tively short; the major portion is formed by elongation of the 

 anterior part, which slowly grows backwards between the Wolffian 

 duct and the tubal ridge, reaching the cloaca on the seventh day. 

 The growing point is solid and appears to act like a wedge sepa- 

 rating the Wolffian duct and the tubal ridge, being thus closely 

 pressed against both, but apparently without receiving cells from 

 either. Balfour's view, that it grows by splitting off from the 

 Wolffian duct or at the expense of cells contributed by the latter, 



