File:Keibel Mall 2 622.jpg

Fig. 622. Transverse section of the urogenital fold of a human embryo of 11 mm. greatest length and 9 mm. head-foot length. (Embryo P. 1; slide 13, row 3, section 5; from the collection of Professor Hochstetter, Vienna.) The section passes through the middle of the third thoracic foramen intervertebrale. The mesonephros is undergoing degeneration in its cranial portion, and the urogenital fold has therefore diminished in size and appears to be triangular; its covering of coelomic epithelium is greatly split on the ventral surface. At the apex of the triangle the coelomic epithelium of the urogenital fold is greatly thickened (funnel area) and invaginated. This funnel-like invagination is the anlage of the ostium abdominale of the Mullerian duct. The funnel is placed frontally, and one can therefore distinguish a dorsal and a ventral lip. The latter has grown out and is curved into a hook to close the opening of the funnel. Into the primary excretory duct the degenerating tubulus collectivus of the second mesonephric tubule opens, beside it and medially is the tubulus secretorius of the third tubule, then the third Malpighian corpuscle, and, finally, the second Malpighian corpuscle.