372 EMBRYOLOGY. 



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

 mesonephric duct. Such a double origin BALFOUR AND SEDGWICK 

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

 The part produced by invagination of the peritoneum (fig. 210 c) 

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

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

 consist of a single invagination 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 yr 2, yr 3, r 2). From this ridge is 

 formed a slightly curved, 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 BALFOUR AND 

 SEDCW.CK. 



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



gr 2, 3, Second and third furrows ; r 2, second ridge ; v;d, Wolffian duct. 



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

 pronephros, must have undergone a 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, SEDGWICK maintains that it arises by being split off from the 

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

 the pronephric part of the Miillerian duct in union at its posterior 

 end with the ventral wall of the mesonephric duct. He maintains 

 that it is enlarged at the expense of the latter in somewhat the same 

 manner as the mesonephric duct grows from in front backwards by 

 a proliferation of the outer germ-layer, The cross sections A and J3 



