EXCRETORY ORGANS. 719 



and develop a lumen. The condition of the structure at this 

 stage is illustrated by fig. 404, representing three transverse 

 sections through two grooves, and through the ridge connecting 

 them. 



The pronephros may in fact now be described as a slightly 

 convoluted duct, opening into the body cavity by three groove- 

 like apertures, and continuous behind with the rudiment of the 

 true Miillerian duct. 



The stage just described is that of the fullest development 

 of the pronephros. In it, as in all the previous stages, there 

 appear to be only three main openings into the body cavity ; but 

 in some sections there are indications of the possible presence of 

 one or two additional rudimentary grooves. 



In an embryo not very much older than the one last 

 described the pronephros atrophies as such, its two posterior 

 openings vanishing, and its anterior opening remaining as the 

 permanent opening of the Miillerian duct. 



The pronephros is an extremely transitory structure, and its 

 development and atrophy are completed between the QOth and 

 i2Oth hours of incubation. 



The position of the pronephros in relation to the Wolffian 

 body is shewn in fig. 405, which probably passes through a 

 region between two of the peritoneal openings. As long as the 

 pronephros persists, the Mullerian duct consists merely of a very 



FlG. 406. TWO SECTIONS SHEWING THE JUNCTION OF THE TERMINAL SOLID 

 PORTION OF THE MtJLLERIAN DUCT WITH THE WOLFFIAN DUCT. (After Balfour 



and Sedgwick.) 



In A the terminal portion of the duct is quite distinct ; in B it has united with the 

 walls of the Wolffian duct. 



md. Mullerian duct ; Wd. Wolffian duct. 



