pronephric glomerulus 



^rudimentary pronephric tubules 

 {5th or 6th somite) 



to gut 



C ADULT 



B 



35 mm 



Figure 10-18. Three stages in the development of the kidney of the 

 salmon. (After van den Broek et al., 1938) 



(Figure 10-18). In the salmon the pronephros is retained as 

 a lymphoid area containing suprarenal tissue (see Chapter 

 12). In a teleost such as the lantern fish Lampanyctiis. the 

 pronephros is developed much like the posterior part of the 

 kidney. There are four pairs of looped pronephric tubules, 

 each with a glomerulus, in this species. These tubules lie in 

 the anterior cardinal channel. In some teieosts the pronephros 

 degenerates and the adult kidney is an opisthonephros. 



The opisthonephros is generally a simple organ with pri- 

 mary, irregularly arranged, tubules and sometimes what 

 appear to be secondary and tertiary tubules. Where the 

 opisthonephros extends behind the vertical of the anus, an 

 independent duct or tube drains this part on each side. The 

 nephric ducts join above the area of the anus and the mid- 

 line sinus has a large, forward-extending outpocketing which 

 functions as a bladder. The urinary duct opens to the out- 

 side through a urinary papilla. 



The extremes of the group appear to be Lampanyctus, 

 with its holonephric kidney and lymphoid tissue; Cycloihone, 

 which has only a pair of pronephric tubules extending pos- 

 teriorly as the nephric ducts — undoubtedly a case of neoteny 

 in a fish which looks like a larva in many respects — and 

 several marine teieosts in which the tubules of the opistho- 

 nephros are aglomerular or reduced to small branching 

 diverticula of the nephric duct (Figure 10-19). Functionally 

 the kidney in teieosts is phagocytic, blood-cell producing 

 and excretory. In some fishes, the stickleback, posterior tu- 

 bules produce a secretion used as a glue in nest building; in 

 other species such tubules are modified into seminal vesicles, 

 producing a sperm-maintaining fluid. 



EMBRYOLOGICAL DEVELOPMENT In the Salmon the pro- 

 nephros develops in the 3rd to the 7th or 8th metotic seg- 

 ments. The nephrotomes (intermediate plate of mesoderm) 

 have nephrocoels which do not open into the coelom. The 

 roof of the nephrocoel invaginates upward forming a funnel 

 and the pronephric ridge or cord associated with these fun- 

 nels hollows out as the pronephric duct. The nephrocoels of 

 one or more pronephric tubules now form a single proneph- 

 ric chamber, not connected with the coelom, which leads 

 into the pronephric duct by a single large funnel, represent- 

 ing an original funnel or a fusion product of several of the 

 original funnels. Bilateral pronephric glomeruli develop and 

 complete the pair of pronephric elements. In Lampanyctiis 

 four pronephric tubules, each with a glomerulus, develop 

 and remain separate. 



The pronephric canal is extended posteriorly by forma- 

 tion of a crest followed by separation of a band of cells from 

 the outer layer of the intermediate plate, which is contin- 

 uous with the somatic mesodermal lining of the coelom. The 

 nephric duct eventually reaches and opens into the cloaca. 



In the salmon the pronephric area is largely lymphoid 

 and hemopoietic. It becomes completely so with the de- 

 generation of the pair of pronephric elements. This lymph- 

 oid area involves 12 to 13 somites, the more posterior 



THE EXCRETORY SYSTEM 



305 



