592 AM N IOTA 



volutions of the peritoneal epithelium, connected together by more 

 or less well-defined ridge-like thickenings of the epithelium. It 

 takes its origin from the layer of thickened peritoneal epithelium 

 situated near the dorsal angle of the body cavity, and is situated 

 some considerable distance behind the front end of the Wolffian 

 duct. 



In a slightly later stage the ridges connecting the grooves be- 

 come partially constricted off from the peritoneal epithelium, and 

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

 is illustrated by fig. 40 i, representing three transverse sections 

 through two grooves, and through the ridge connecting them. 



The pronephros may in fact now be described as a slightly con- 

 voluted 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 90th and 120th 

 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 Miillerian duct consists merely of a very small rudiment, con- 

 tinuous with the hindermost of the three peritoneal openings, and 

 its solid extremity appears to unite with the walls of the Wolffian 

 duct. 



After the atrophy of the pronephros, the Miillerian duct com- 

 mences to gnjw rapidly, and for the first part of its course it 

 appears to be split off as a solid rod from the outer or ventral wall 

 of the Wolffian duct (fig. 406). Into this rod the lumen, present in 

 its front part, subsequently extends. Its mode of development in 

 front is thus precisely similar to that of the Miillerian duct tn 

 Elasmobranchii and Amphibia. 



This mode of development only occurs however in the anterior 

 part of the duct. In the posterior part of its course its growing point 

 lies in a bay formed by the outer walls of the Wolffian duct, but 

 does not become definitely attached to that duct. It seems however 

 possible that, although not actually split off from the walls of 

 the Wolffian duct, it may grow backwards from cells derived from 

 that duct. 



