When the Wolffian body has almost reached its greatest development a second longitudinal duct makes its appearance by the evagination of the ventral surface of the Wolffian body. These ducts lie in close proximity of the Wolffian ducts, along the dorsal aspect of the coelom, or body-cavity, and are known as the ducts of Müller, or the primitive female ducts. The function of these canals in lowly organized animals is that of receiving from the body-cavity the ova and of evacuating them from the body. The Müllerian ducts also open into the cloaca. At the lower end the Müllerian and Wolffian ducts run in close apposition and form the genital cord. At this stage of development the embryo is thus hermaphroditic like the worms. [Illustration: CUT III. _Fig. 1, horizontal cut through embryo; Fig. 2, vertical cut; schema of indifferent stage._ Wb, Wolffian body; Wd, Wolffian duct; Md, Müllerian duct; gg, genital gland; m, mesentery; i, intestine; sc, spinal cord; ch, chorda; ao, aorta; a, allantois; clo, cloaca; k, kidney.] The metanephros, or the true kidney, appears first as an epithelial or renal evagination of the Wolffian duct on the dorsal side of the latter and near its opening into the intestinal canal or cloaca. This bud grows forward, extends headward toward the position of the Wolffian body and becomes a long, narrow tube, the ureter. The blind end branches into different tubules, each having a sacculated end. They soon assume a tortuous and convoluted form and represent the permanent kidney. The Wolffian body is now replaced in its function by the true kidney, and enters into special relations with the sexual organs, by being transformed into the genital apparatus of the male. Before the Wolffian body has yet degenerated the mesothelial cells overlying the free surface of that body, at its upper part, and at the ventro-mesial side, assume a high columnar form and form an elongated swelling, known as the internal genital ridge. As the degeneration of the Wolffian body proceeds the genital ridge is differentiated into the indifferent sexual gland, by producing a projection upon the wall of the coelom or body cavity. This prominence is attached to the surface of the Wolffian body by a fold of the peritoneum. At this stage there is no distinction of sex. The sexual gland represents the indifferent type of the sexual apparatus. While this metamorphosis is going on at the head-end of the sexual ducts, the caudal ends undergo also a certain degree of development.