THE REPRODUCTIVE SYSTEM 383 



on each side in the cervical region. These masses, which are probably derived 

 from the mesothelial lining of the body cavity, may or may not become hollow, 

 but do not connect with the pronephric duct, and soon disappear. The pro- 

 nephric duct appears about the same time as a derivative of the mesoderm just 

 lateral to the primitive segments. It extends from the cervical region to the 

 caudal region of the embryo where it bends mesially and opens into the gut. 

 These ducts persist and become the ducts of the mesonephroi. 



The mesonephroi begin to develop almost as soon as the pronephroi and just 

 caudal to them. Condensations appear in the mesodermal tissue lateral to the 

 primitive segments and become more or less tortuous. Lamina appear in these 

 condensations, thus forming tubules which then connect at one end with the 

 pronephric duct (now the mesonephric or Wolffian duct). The tubules develop 

 progressively from before backward and finally form a series extending from the 

 cervical region to the pelvic region of the embryo. At the distal end of each 

 tubule a glomerulus, containing branches from the aorta, develops. The 

 tubules increase in length and number and come to form a pair of large struc- 

 tures which project into the dorsal part of the body cavity. These are often 

 spoken of as the Wolffian bodies. They reach the height of their develop- 

 ment during the fifth or sixth week. 



During the period of their existence in the embryo in higher animals, the 

 mesonephroi functionate as urinary organs, not only through the agency of the 

 glomeruli but also by means of the epithehum of the tubules themselves, among 

 which numerous branches of the posterior cardinal veins ramify. 



From the sixth week on, and coincident with the development of the meta- 

 nephroi or kidneys, the mesonephroi atrophy, leaving finally only certain parts 

 which differ in the two sexes. In the male some of the tubules in the cephalic 

 portion persist as the vasa efferentia, while a few in the caudal portion remain 

 as the paradidymis and vasa aberrantia; the duct persists as the vas epididy- 

 midis, vas deferens, and ejaculatory duct. In the female the mesonephric tu- 

 bules disappear for the most part, only a few remaining to form the epoopho- 

 ron and paroophoron, while the duct persists in part as Gartner's canal. 



Each kidney begins in embryos of 5 to 6 mm. as a hollow bud on the dorsal 

 side of each mesonephric duct near its opening into the gut. This bud grows 

 dorsally, and then turns cranially in the mesoderm between the vertebral 

 column and the mesonephros. The proximal portion remains more slender as 

 the ureter, while the distal end becomes dilated to form the primitive renal 

 pelvis. From this dilated end a number of secondary evaginations grow out 

 to form the papillary ducts and straight collecting tubules. 



The mesodermal tissue surrounding the outgrowths from the primitive renal 

 pelvis becomes condensed in places and gives rise to the convoluted portions of the 

 uriniferous tubules and to Henle's loops. A glomerulus develops in connection 

 with the distal end of each convoluted tubule. The portion of each tubule 

 derived from the mesodermal tissue unites secondarily at the arched (junctional) 

 tubule with the portion derived from the renal pelvis. Thus the kidney tubules 

 are derived in part directly from undifferentiated mesoderm (convoluted 

 tubules and Henle's loops) and in part from an outgrowth from the mesonephric 

 duct (straight collecting tubules and papillary ducts). 



