bladder, which it enters. The bladder stores urine, which 

 passes from here to the outside through the urethra. 



Embryological de ve/opment The development of the mam- 

 malian kidney can be described as a progressive event begin- 

 ning with some small tubules at the anterior end of the body 

 cavity and extending from here in a posterior direction. The 

 stages as generally described are: the "first kidney" or pro- 

 nephros, then a "middle kidney" or mesonephros, and fin- 

 ally the "last kidney," the metanephros. The pronephros 

 and mesonephros undergo degeneration in turn, leaving 

 only the metanephros. 



The pronephros appears very early (2.5-mm embryo). Its 

 first evidence is a series of thickenings of the lateral plate 

 mesoderm in the si.xth to twelfth metotic or postotic (be- 

 hind the otic capsule) segments of the body (Figure 10-2). 

 These structures appear lateral to the myotomes as knots of 

 cells extending outward from the intermediate plate and ly- 

 ing just under the epidermis. These rudiments connect to- 

 gether to form a strand separated from the coelomic cavity 

 by the somatic mesoderm. As the coelom extends medially, 

 into the intermediate plate, the cord of cells is separated 

 from the somatic mesoderm and contacts it only at seg- 

 mental intervals. The solid rudiments now begin to hollow 

 out and form funnels opening into the coelom at points of 

 contact with the somatic mesoderm (Figure 10-3). The an- 

 terior rudiments begin to degenerate as the more posterior 

 ones reach full development. The latter appear to be dis- 

 placed backward as they form, so that a large number of 

 them will lie within three or four somites (8 in a Echidna 

 associated with spinal ganglia 4 to 6; 14 to 16 in the mar- 

 supial Tmhosurus associated with spinal ganglia 6 to 8). The 

 elements of somites 9 to 12, from the 5th cervical posteriorly, 

 produce rudiments from which a nephric duct develops and 

 grows posteriorly (Figure 10-3). The nephric duct is variously 

 referred to as the pronephric duct, mesonephric duct, and 

 wolffian duct. The nephric duct extends posteriorly by multi- 



intermediate mesoderm 

 (pronephric blastema) 



dorsal aorta 



somatic 

 mesoderm 



right umbilical vein 



f 

 visceral mesoderm 



coelom 



itoderm-^"" 



Figure 10-3. The pronephric and early mesonephric rudiments of the 

 human. A, general appearance and relationships of kidney rudiments in a 

 2 5-mm embryo; B, pronephric area enlarged. (B after Ihle et al. 1 927) 



Figure 10-2. Cross section of a 1 0-somite stage human showing the 

 relationship of the pronephric blastema to the rest of the mesoderm. 

 (After Torrey, 1954) 



plication oft he cells of its tip , not by segmental contribu- 

 tions. It re^diw the clo^l area in the 4.3-mm embryo. The 

 mesonephros extends from the 8th cervical to the sacral 

 region in the 5.4-mm embryo. 



The tubules of the mesonephros arise from clumps 

 of cells formed in the nephrogenic cord lying along the 

 nephric duct (Figure 10-4). The nephrogenic cord of cells 

 arises from the intermediate plate mesoderm in much the 

 same way as the pronephric end of the nephric duct (Figure 

 10-3). It derives its name from the fact that it "gives rise" to 

 tubules. A few tubules of the mesonephros develop anterior 

 to the 14th somite, thus perhaps overlapping the posterior 

 part of the pronephros. About 30 to 40 tubular units in all 

 develop, two or three per segment. These are not symmetri- 

 cally placed. In marsupials and monotremes, this kidney 

 becomes functional. The mesonephros ofthe human is histo- 

 logically in between the functional type of the hoofed mam- 

 mals and the nonfunctional type of the rat, mouse, or dog 

 which never develop glomeruli. Many ofthe mesonephric 

 tubules of the human develop peritoneal funnels, which are 

 rudimentary or lacking in the mouse or dog. 



The posterior end of the mesonephros is fully developed 

 at about 15 mm (head-rump length of embryo). Mean- 

 while, the anterior part has started to degenerate. Regres- 

 sion then begins posteriorly, as well as continuing from the 

 anterior end, until only a small middle region persists. This 

 is connected with the male reproductive system and is modi- 

 fied into the epididymis or retained as a nonfunctional 

 vestige, the epoophoron, in the female. 



The' metanephros of man begins to develop at about the 

 5.4-mm stage, as a small diverticulum from the posterior 





THE EXCRETORY SYSTEM • 293 



