DEVELOPMENT OF THE URINARY AND GENERATIVE ORGANS 1205 



DEVELOPMENT OF THE URINARY AND GENERATIVE ORGANS. 



The 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 

 Miillerian 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 Mullerian 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 com- 

 municates 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. 



CStroma 



Genital] of ovary \ 



ridge j Primitive 'A, , 

 t ova * 



Mesentery "- 



^ Wolffian duct 



^.. Mullerian duct 



"~~5 WolfKan tubules 



Body-wall 



FIG. 1106. Section of the urogenital fold of a chick embryo of the fourth day. (Waldeyer.) 



The Mesonephros, Mullerian 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 develop- 

 ment 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, 



