THE LATER DEVELOPMENT OF THE FROG 



203 



or flap then fuses with the peritoneal wall forming a short 

 compressed tube open at both ends. This tube then extends 

 anteriorly and posteriorly : anteriorly it reaches to the anterior 

 end of the body cavity and then turns ventrally, stopping near 

 the base of the lung, where its open end forms the rudiment 

 of the ostium or infundibulum of the oviduct. Posteriorly the 

 formation of the shelf and its closure continue parallel with 

 and just to the outer side of the mesonephric duct, and entirely 



FIG. 75. Sections through the developing Miillerian duct of a 34 mm. tadpole 

 of R. sylvatica. After Hall. A. Section passing through the beginning of the 

 Miillerian evagination. B. Section posterior to A. Duct established but still 

 connected with peritoneum. C. Section still farther posterior, showing the 

 separation of the duct from the peritoneum. M. Miillerian duct; p, peritoneum ; 

 t, third pronephric tubule. 



independent of it, until it reaches the cloaca, with which it 

 connects some time after metamorphosis. 



In the male the development of the Miillerian duct ceases at 

 this stage, but in the female it continues to thicken and to 

 elongate, so that it becomes entirely free from the body wall, 

 though remaining suspended from it by a double fold of peri- 

 toneum; finally it acquires the characteristics described in the 

 beginning of the preceding chapter. 



B. THE GONADS 



The primitive germ cells are distinguishable quite early 

 (about 6 mm.) as a definite though slight ridge along the 



