KIDNEYS 85 



instance, the renal capsule and glomerulus are only developed 

 in the three groups just mentioned and the Teleostei. Gener- 

 ally the pronephros consists merely of coiled ciliated tubules 

 leading from the coelom to the connecting duct. In some 

 cases the renal chambers may fuse into a longitudinal pronephric 

 chamber, and the glomeruli may also combine into a single 

 ' glomus.' 



A second set of blood-vessels may project into the coelom, 

 near the coelomostome ; these constitute the ' outer glomerulus ' 

 found in Actinopterygii, Dipnoi, most Amphibia, and Amniota. 

 The region of the coelom surrounding the outer glomerulus may 

 become incompletely separated off from the abdominal coelom, 

 forming an ' outer chamber,' as in Ganoids, Anura, and Urodela, 

 and to some extent in the Amniota. On the contrary, in others 

 the coelomostome may, apparently, open out so that the renal 

 chamber merges with the general coelom (Elasmobranch ?, 

 Petromyzon 1 ). 



It is important to notice that at first the tubules are strictly 

 segmental, although subsequently they may shift and become 

 closely packed together (Myxinoid), or even fuse to a single opening 

 (Elasmobranch). 



In the anterior region the longitudinal canal, which lies outside 

 the somatopleure, is formed by the fusion of the distal extremities 

 of the pronephric tubules (Fig. 56). Further back a region of 

 varying length may be developed by a direct folding off of a 

 longitudinal ridge of the somatopleure. Lastly, the terminal 

 portion of the duct, which opens into the cloaca, is usually formed 

 by the growing backwards of the free posterior extremity of the 

 duct. Here, again, we meet with great variation. While in the 

 Gymnophiona [45] the pronephric duct extends backwards almost 

 entirely by free growth, in the Anura and Teleostei it develops 

 to a great extent as a folding off of the somatopleure. Further, in 

 the Cyclostomes there is evidence of the whole duct being really 

 formed by the fusion of segmental rudiments representing tubules, 

 much reduced in Petromyzon [486], but reaching fuller development 

 in Bdellostoma [335]. 



The number of distinct tubules forming the head kidney, or 

 pronephros, varies considerably. In the higher vertebrates it is 

 never large — from two to six — but the number of rudiments is 

 always larger. The Gymnophiona may have as many as ten [45], 

 and the Myxinoids about twenty tubules. 



The pronephros soon loses its importance as an excretory 

 organ, and its function is assumed by the mesonephros (Wolffian 

 body). With few exceptions (Cyclostomes, p. 43 ; and Teleostei, 

 p. 364), the pronephric tubules disappear almost completely in the 

 adult, or contribute to the formation of the funnel of the Mullerian 



