In a slightly later stage the ridges connecting the grooves become partially constricted off from the peritoneal epithelium, 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 Müllerian 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 Müllerian duct.
The pronephros is an extremely transitory structure, and its development and atrophy are completed between the 90th and 120th hours of incubation.
Fig. 406. Two sections shewing the junction of the terminal solid portion of the Müllerian 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. Müllerian duct; Wd. Wolffian duct.
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 Müllerian duct consists merely of a very small rudiment, continuous 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 Müllerian duct commences to grow 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 Müllerian duct in Elasmobranchii and Amphibia.