dorsal aorta 



posfcovo 

 mesentery of gut, 



marginal canal 

 spermatocyte with follicle ce 



nephron 



glomerulus 



renal portal channel 



nephric duct 



Venous sinus 

 nephrostome 

 follicle cell 



Sertoli cell with sperm 

 residual spermatogonium 



interstitial tissue 



follicle filled with sperm'' 



^^ '^^"^''^^follicle cell 



Figure 10-17. Cross section of an opisthonephros of the frog showing relationships with the blood 



supply and the testis. Nephrostomes opening into the venous sinuses are shown. (After Witschi, 1956) 



appears to be only the posterior half of the larval structure. 

 Within the kidney the efferent ducts connect with some of 

 the capsules of the nephric tubules. The arrangement is 

 similar to that of the sturgeon or Amia and differs from the 

 frog in that the glomeruli of the sperm-carrying tubules are 

 retained. The nephric ducts unite posteriorly and the uri- 

 nary sinus thus formed opens into the cloaca along with the 

 Miillerian ducts and the gut. A cloacal bladder extends for- 

 ward above the rectum. 



In Protopterus and Lepidosiren the central net is extended 

 from each testis to the midline where it fuses with that of the 

 other side; the median canal continues posteriorly nearly to 

 the posterior end of the kidney. In Protopterus a few efferent 

 ducts enter the kidney and connect through a large number 

 of glomerulus-retaining tubules of the last two segments of 

 that structure; the efferent ducts connect with tubules of the 

 last few (5 to 6) segments in Lepidosiren. The nephric ducts 

 extend back to the cloaca, whose roof they enter just poster- 

 olateral to the midline tubercle of the female genital ducts. 

 In the male oi Protopterus the nephric ducts unite and open 

 at the tip of a distinct midline urinary papilla which retains 

 bilateral openings into the cloaca. The cloaca extends for- 

 ward above the gut as a midline urinary bladder. 



EMBRYOLOGICAL DEVELOPMENT The pronephros involves 

 about eight segments, of which two anterior ones produce 

 funnels. The funnels lie in metotic (postotic) somites 4 and 5 

 in Protopterus and Lepidosiren, 5 and 6 in Neoceralodus. The 

 nephric duct arises as a ridge from the segmental nephro- 

 tomes. This ridge separates from the lateral plate and lies 

 dorsolaterally just below the epidermis. It is extended by 

 segmental contributions, which lie just below the epidermis, 

 to the cloaca (somite 42). 



The pronephric funnels open into the pericardial coelom. 



dorsolateral to a large medial glomus. Occasionally accessory 

 funnels are present. The pronephros is later reduced to a 

 single funnel, the posterior one or a fusion product of both 

 original funnels. The Miillerian duct arises in connection 

 with this funnel in Neoceratodus and grows posteriorly, just 

 medial to the nephric duct. 



The mesonephros begins to develop relatively late in larval 

 life. The tubules appear far back from the pronephros, in 

 somites 23 to 38 (the cloaca lies in about the 42nd somite). 

 The most anterior tubules are somewhat behind those 

 further back in their stage of development. The primary 

 tubules are neither symmetrically nor segmentally arranged. 

 As development proceeds, the number of tubules increases. 

 In the anterior part of the kidney, at 52 mm, only the pri- 

 mary tubules are present, whereas in the posterior two- 

 thirds, there are primary, secondary, and tertiary tubules, in 

 a ventrodorsal sequence. The glomeruli are clumped to- 

 gether, while the groups of peritoneal funnels are joined by a 

 groove. The primary tubules oi Neoceratodus and Protopterus 

 may be interconnected at different points. Later generations 

 of tubules arise from balls of cells budded off from the primary 

 tubules. The several generations of tubules have renal cor- 

 puscles and peritoneal funnels. 



In Protopterus up to six orders of tubules have been de- 

 scribed for the hind part of the opisthonephros, each draining 

 into the order preceding it. A glomerulus may be shared by 

 two or three nephrons. In this genus, and Lepidosiren, the 

 urinary bladder is said to arise as an anterior diverticulum 

 from the urinary sinus, but such a bladder is not present 

 in a 200-mm larva. The bladder is probably of cloacal origin. 



Actinopterygians 



fe/eosf The teleost kidney is sometimes a holonephros, 

 that is, one made up of pronephros and opisthonephros 



304 . THE UROGENITAL SYSTEM 



