Secondary and tertiary segmental tubules are formed in the Chick, on the dorsal side of the primary tubules, as direct differentiations of the mesoblast. They open independently into the Wolffian duct.
In Mammalia the segmental tubules (Egli) are formed as solid masses in the same situation as in Birds and Reptiles. It is not known whether they are united with the peritoneal epithelium. They soon become oval vesicles, which develop into complete tubules in the manner already indicated.
Fig. 404. Sections shewing two of the peritoneal invaginations which give rise to the anterior part of the Müllerian duct (pronephros). (After Balfour and Sedgwick.)
A is the 11th section of the series.
B is the 15th section of the series.
C is the 18th section of the series.
gr2. second groove; gr3. third groove; r2. second ridge; wd. Wolffian duct.
After the establishment of the Wolffian body there is formed in both sexes in all the Amniota a duct, which in the female becomes the oviduct, but which is functionless and disappears more or less completely in the male. This duct, in spite of certain peculiarities in its development, is without doubt homologous with the Müllerian duct of the Ichthyopsida. In connection with its anterior extremity certain structures have been found in the Fowl, which are probably, on grounds to be hereafter stated, homologous with the pronephros (Balfour and Sedgwick).
The pronephros, as I shall call it, consists of a slightly convoluted longitudinal canal with three or more peritoneal openings. In the earliest condition, it consists of three successive open involutions 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.
Fig. 405. Section of the Wolffian body developing pronephros and genital gland of the fourth day. (After Waldeyer.) Magnified 160 times.
m. mesentery; L. somatopleure; a´. portion of the germinal epithelium from which the involution (z) to form the pronephros (anterior part of Müllerian duct) takes place; a. thickened portion of the germinal epithelium in which the primitive germinal cells C and o are lying; E. modified mesoblast which will form the stroma of the ovary; WK. Wolffian body; y. Wolffian duct.