Henry Gray (18251861). Anatomy of the Human Body. 1918.
a. Development of the Urinary and Generative OrgansThe urinary and generative organs are developed from the intermediate cell-mass which is situated between the primitive segments and the lateral plates of mesoderm. The permanent organs of the adult are preceded by a set of structures which are purely embryonic, and which with the exception of the ducts disappear almost entirely before the end of fetal life. These embryonic structures are on either side; the pronephros, the mesonephros, the metanephros, and the Wolffian and Müllerian ducts. The pronephros disappears very early; the structural elements of the mesonephros mostly degenerate, but in their place is developed the genital gland in association with which the Wolffian duct remains as the duct of the male genital gland, the Müllerian as that of the female; some of the tubules of the metanephros form part of the permanent kidney.
The Pronephros and Wolffian Duct.In the outer part of the intermediate cell-mass, immediately under the ectoderm, in the region from the fifth cervical to the third thoracic segments, a series of short evaginations from each segment grows dorsalward and extends caudalward, fusing successively from before backward to form the pronephric duct. This continues to grow caudalward until it opens into the ventral part of the cloaca; beyond the pronephros it is termed the Wolffian duct.
The original evaginations form a series of transverse tubules each of which communicates by means of a funnel-shaped ciliated opening with the celomic cavity, and in the course of each duct a glomerulus also is developed. A secondary glomerulus is formed ventral to each of these, and the complete group constitutes the pronephros. The pronephros undergoes rapid atrophy and disappears.
FIG. 1106 Section of the urogenital fold of a chick embryo of the fourth day. (Waldeyer.) (See enlarged image)
The Mesonephros, Müllerian Duct, and Genital Gland.On the medial side of the Wolffian duct, from the sixth cervical to the third lumbar segments, a series of tubules, the Wolffian tubules(Fig. 1106), is developed; at a later stage in development they increase in number by outgrowths from the original tubules. These tubules first appear as solid masses of cells, which later become hollowed in the center; one end grows toward and finally opens into the Wolffian duct, the other dilates and is invaginated by a tuft of capillary bloodvessels to form a glomerulus. The tubules collectively constitute the mesonephros or Wolffian body (Figs. 986,