372 EMBRYOLOGY. 



anil a posterior, which is formed by being split off from the 

 mesonephiic duct. Such a double origin BALFOUR AND SEDGWICK 

 endeavor to establish for the Miillerian duct in the Chick also. 

 The part produced by imagination of the peritoneum (fig. 210 z) 

 they interpret as pronephrcs. A similarity with the latter they find 

 in the fact that this part does not, according to their investigations, 

 consist of a single imagination of the peritoneal epithelium, but of 

 three open imaginations lying one behind the other, which are 

 joined together by ridge- like epithelial thickenings which after- 

 wards become hollow (fig. 211 gr 2, gr 3, r 2). From this ridge is 

 formed a slightly curvrd. short duct, which communicates with the 

 body-cavity through three openings. 



If this explanation is right, the most anterior fundament of the 



Fig. 211. Cross sections through two peritoneal invaginations out of which is formed the 

 anterior region of the Miillerian duct (the pronephros) of the Chick, after BA.LFOUK AND 

 SEIX . i 



A is the llth, B the 15th, C the ISth section of the whole series. 



grt, 3, Second anil third furrows ; t'2, second ridge ; ir<(. Wolffian duct. 



excretory system of the Chick, which was described on page 356 as 

 pronephros, must have undergone p change in position, and, with the 

 appearance of the Wolffian body, have slipped backward somewhat 

 along this organ. As long as this alteration of position is not 

 demonstrated by the study of intermediate stages, the interpretation, 

 however probable it may seem to us, still lacks actual proof. 



As far as regards the posterior, longer region of the Miillerian 

 duct. K maintains that it arises by being split off from the 



mesonephric duct. One always finds, according to his researches, 

 tin- |iriitit|ihrk> part of the Miillerinn duct in union at its posterior 

 end with tin- \ntral wall of the meson* phrir duct. He maintains 

 that it Ls enlarged at the expense of the latt r in somewhat the same 

 manner its the mesonephiic duct grows from in front backwards by 

 a proliferation of th- out, r germ-layer. The cross sections A and B 



