nephric duct 



-^ medial 



^— LipC^nephric bud 



glomerulus 



^Bowman's capsule 



Figure 10-4. Five stages, (A to E), in the development of a meso- 

 nephric tubule. (After Ihle et al, 1927) 



end of the nephric duct (Figure 10-5). This outpocketing 

 enters the nephrogenic cord; it forms the ureter and from 

 it grow the calyces and several generations of collecting tubes. 

 The nephrons develop from the nephrogenic cord in a 

 sequential fashion as new generations of collecting tubes are 



produced (Figures 10-6, 10-7). Although the first few genera- 

 tions of nephrons are resorbed, at least 12 generations are 

 present at the time of birth. This means more than one mil- 

 lion nephrons are present in each kidney. In the later stages 

 of development, the kidneys move forward, or upward in 

 the case of man, beneath the peritoneum until they near 

 the diaphragm. The kidneys of monotremes remain in the 

 posterior part of the body cavity. 



The ducts of the kidney, the ureter, and the nephric 

 duct switch positions during development, so that the 

 ureters enter the bladder, while the nephric duct, which is 

 now the male vas deferens (degenerate in the female), en- 

 ters the urethra. The latter e.\tends from the bladder to the 

 outside. The bladder is formed from the allantois. 



At this point a few definitions appear to be in order, 

 based on structure. The pronephros is composed of tubules 

 formed segmentally at the anterior end of the body cavity. 

 These may open into the coelomic cavity by way of funnels. 

 The tubules unite distally to form a part of, or give rise to, 

 the entire nephric duct. The mesonephros can be defined as 

 having tubules formed from buds of nephrogenic tissue ly- 

 ing medial to the nephric duct. These tubules later estab- 

 lish entrance into the nephric duct. There may be several of 

 these tubules per segment; the two sides of the animal 

 are asymmetric. Peritoneal funnels may develop in relation 

 to these tubules. The metanephros is made up of a system 

 of collecting tubules which grow out from a single evagina- 

 tion from the posterior end of the nephric duct. Blastematic 

 buds of nephrogenic tissue form tubules which enter these 

 collecting ducts and thus form the functional metanephric 

 kidney. No peritoneal funnels develop in relation to metane- 

 phric tubules. 



In the mammal the pronephros is never functional. The 

 mesonephros is functional in the monotremes, marsupials, 

 and many placentals until the metanephros comes into action. 

 The mesonephros of some mammals is never functional, for 



hydrotid appendix of testis 

 mesoneptiros °'*''^'^ °* ^^ 



allantoic stalk 



cloaca 

 proctodeum 



metanephric 

 diverticulum 



bladd< 



urogenital sinus 



4 mm 



O 8 mm 



15 mm 



D 20. 



Figure 1 0-5. Four stages in the development of the human metanephros. (After Patten, 1 946) 



294 • THE UROGENITAL SYSTEM 



