402 THE DEVELOPMENT OF THE CHICK 



behind the external glomeruli of the pronephros. The lips 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 tubce 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 day. 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, 

 has not been supported by subsequent investigators. A short 

 distance in front of the growing point the Miillerian duct receives 

 a lumen, and mesenchyme presses in from above and below, 

 and forms a tunic of concentrically arranged cells around it 

 (Fig. 221). 



The Miillerian duct thus begins to project above the surface 

 of the Wolffian body, and, as it does so, the thickened epithelium 

 of the tubal ridge becomes flat and similar to the adjacent peri- 

 toneum; whether it is used up in the formation of the mesen- 

 chymatous tunic of the epithelial Miillerian duct is not known. 

 Up to this time the development is similar in both sexes and on 

 both sides of the body. 



In the male development of these ducts ceases on the eighth 



