from the fourth pair of glomeruli back. The first pair 

 of glomeruH is associated with small tubules. The glomeruli 

 increase in size until the third or fourth pair, when internal 

 glomeruli begin to be present; from here back the external 

 glomeruli dwindle, over a distance of one or two segments, 

 and the internal ones increase in size and number. 



The pronephric elements intergrade with those of the 

 mesonephric area (Figure 10-10). Each mesonephric tubule 

 has an internal glomerulus but no peritoneal funnel. From 

 the 16th to the 22nd somite there are about 12 primary 

 asymmetric tubules. With the appearance of new tubules 

 each segment comes to have 2 to 5, usually 4, units in it. 

 These lie in a lateral to dorsal-medial progression and each 

 unit opens separately into the nephric duct. The nephric 

 duct evaginates to meet the later tubules and these out- 

 growths become collect ducts. 



In the metamorphosing bird, the fourth Or fifth day or 

 later, depending on the species, the pronephros is nearly 

 regressed. The mesonephros is a distinct body stretching 

 from about the 1 2th to the 24th somite; its nephric duct enters 

 the cloaca in the sacral region at about the 29th or 30tli 

 somite. Degeneration of the anterior end of the mesoneph- 

 ros continues, while the more complex posterior part com- 

 pletes development of all its units and becomes functional 

 (there is evidence of nitrogen excretion on the fifth day of 

 development). 



The metanephros appears at the time of metamorphosis 

 as an evagination from the nephric duct at the level of the 

 29th somite. This evagination enters the nephric blastema 

 and grows rapidly forward, dorsolateral to the nephric duct. 

 This evagination induces tubule formation in the cap of 

 nephrogenic tissue lying dorsal to the posterior cardinal. 

 Three distinct lobes form, and in these the collecting tubules 

 extend outward from the metanephric duct (ureter). The 

 organ now extends from the 20th somite to the 28th. The 

 ureter opens separately, in front of the opening of the 

 nephric duct into the cloaca, at the 37th or 38th somite. 



The mesonephros continues to function till the time of 

 hatching, when the metanephros is well differentiated. The 

 anterior part has degenerated back to about the 20th seg- 

 ment. At hatching, only the tubes involved with the testis 

 are retained, and their glomeruli are lost. This area of 

 tubules is transformed into the epididymis. 



The Miillerian duct, the female sexual duct, appears on 

 the lateral aspect of the anterior end of the mesonephros 

 next to the nephric duct. It appears first as a thickening of 

 the epidermis, then as a groove which closes over to form a 

 tubule with an anterior ostial opening (ostium). The tubule 

 extends posteriorly until it reaches the cloaca. The develop- 

 ment of this tubule in a bird or in the reptile is the same as 

 in the mammal. 



Amphibians 



Salamander In Necturus or Cryplnhranchus the opisthoneph- 

 ric kidney is long and thin. An opisthonephros is a kidney 



which is assumed to incorporate both mesonephric and 

 metanephric materials. It can also be defined as an adult 

 kidney — not a developmental stage — which is primarily, if 

 not entirely, of mesonephric origin. The anterior half is 

 partly modified in the male into an epididymis which con- 

 nects with the elongate testis (Figure 10-11). The nephric 



Mijllerian duct 



(anterior end offset to right). 



^midline glomerulus 



i pronephric funnels 



glomerulus^ 

 '^accessory funnel 



nephrostome, 



-nephric duct (vos deferens) 



> blastema of mesonephros 



degenerating glomerulus- 



ostium- 



I 



ctoQcal 



A 7.9 mm TRITON ALPESTRIS 



vas deferens separated from 

 urinary part of system- 



i« 



cloaca.. 



D 67 mm ADULT MALE 

 TRITON PUNCTATUS 



Figure 10-11. Two stages in the development of the kidney of the 

 salamander Triton, A, 7.9-mm larva; B, 67-mm (adult) male. (After 

 Kindahl, 1938) 



THE EXCRETORY SYSTEM • 299 



