RANA 



B 



BOMBINATOR 



vas deferens 



ampulla 



ALYTES 



Figure 10-1 5. Connections between the testis and the nephric duct in amphibians. (After von den Broek) 



EMBRYOLOGiCAL DEVELOPMENT In the frog there are three 

 pronephric funnels; the first is at the level of the sec- 

 ond myotome and the third visceral arch (Figure 10-16). 

 These funnels develop from the somatic layer of the inter- 

 mediate plate in the usual fashion. The nephric duct is con- 

 tinued posteriorly by segmental crest contributions until it 

 reaches and opens into the cloaca. The anterior nephrocoels 

 fuse and open widely into the coelom. In this process the 

 most anterior funnel is moved back into the next segment. 



These funnels are associated with a large medial glomer- 

 ulus which can be observed in the lO-mm frog. The con- 

 voluted tubules of the pronephros lie in the anterior cardinal 

 (venous) sinus. At the 10-mm stage there are no meso- 

 nephric elements, but the blastemas of these can be observed 

 dorsomedial to the nephric duct. The nephric duct has 

 already extended posteriorly to the cloaca. As develop- 

 ment progresses, the pronephric funnels may fuse into 

 a single unit. 



The "mesonephros" arises from about the 6th myotomic 

 segment back to the 1 1 th; it is doubtful that any metaneph- 

 ric materials are involved. The tubes of segments 9 and 10 

 are very advanced as compared with those in front or behind. 

 The number of units increases anteriorly and posteriorly up 

 to about a dozen, but these have no segmental correlation. 



The most anterior tubule or two loses its renal corpuscles 

 (glomerulus and Bowman's capsule), becomes connected 

 with the testis, and carries only sperm (Figure 10-13). The 

 next three or four primary tubules lose their corpuscles and 

 conduct sperm, but their secondary tubules are excretory. 

 More posteriorly the primary tubules degenerate to collect- 

 ing ducts serving many nephrons (excretory tubules). More 

 posteriorly these collecting ducts become complexly con- 

 voluted. In the most posterior part of the kidney some of the 

 orders of nephrons open into the nephric duct and cross 

 connections develop between the collecting ducts. New gen- 

 erations of nephrons continue to appear until full growth is 

 achieved, a matter of several years in some species. 



Peritoneal funnels develop in relation to the tubules of 

 the mesonephros, but they either lose connection with these 

 tubules or develop completely separate from them. These 

 peritoneal funnels, and second or higher funnels, open into 

 the venous sinuses of the kidney (Figure 10-17). The beat 

 of their cilia is toward the sinus; therefore we can assume 

 materials (coelomic fluid) pass into the sinus. 



The connection between testis and kidney develops by 

 outgrowth from the capsules of the primary tubules. These 

 outgrowing tubules unite distally to form a marginal canal. 

 Outgrowths from the central canal or network of the testis, 

 the efferent ducts, grow through the mesentery to reach the 

 marginal canal (Bidder's canal). The connection reflects the 

 common origin of the mesonephric blastema and the 

 medullary part of the gonad. At first, kidney and gonad are 

 broadly connected; later this is reduced to strands of tissue 

 which form the efferent ducts. 



In some anurans the anterior sexual part of the kidney is 

 reduced or lacking, and the testis is connected directly to 

 the nephric duct anterior to the definitive kidney. This re- 

 lationship is understandable in terms of the observation that 

 the anterior end of the indifferent gonad of the anuran is 

 modified into the fat body (Figure 10-40). The extension 

 connects by an efferent duct with an anterior tubule which 

 becomes isolated by degeneration of the enclosing kidney 

 and vascular tissues. 



Some male anurans develop a ureter by anastomosis of 

 the collecting tubules and extension of this common col- 

 lecting duct nearly to the cloaca by gradual separation 

 from the nephric duct. 



It has been suggested that the ampulla of the nephric 

 duct, which contains many tubules, represents a modification 

 of the most posterior part of the nephric tissue. An alter- 

 nate view is that it represents only a glandular development 

 of the terminal part of the duct. The suggestion that not all of 

 the nephric material is utilized in the opisthonephros finds 

 some support in the reduced number of segments seemingly 



302 • THE UROGENITAL SYSTEM 



