EXCRETORY SYSTEM 565 



In Reptiles, Birds, and Mammals, traces of gill-clefts 

 occur in the embryos, but without lamellae or respiratory 

 function. In the embryo the blood is purified, as will be 

 explained afterwards, by aid of the foetal sac known as the 

 allantois ; and after birth the animals breathe by lungs. 

 All adult Amphibians also have lungs, to which the lung or 

 swim-bladder of Dipnoi is physiologically equivalent. 



The gill-clefts arise as outgrowths of the endodermic gut 

 which meet the ectoderm and open. The ventral paired 

 lungs arise from an outgrowth of the gut, as does also 

 the swim-bladder of many Fishes, though it usually lies 

 on the dorsal surface, has rarely more than a hydrostatic 

 function, and usually has a blood supply different from 

 that of the lungs. In Dipnoi and some " Ganoids " it is 

 supplied by a pulmonary artery arising from the sixth 

 aortic arch. There is probably a homology between lung 

 and swim-bladder. 



Excretory system. — The development of this is always compli- 

 cated. In the embryos of Vertebrates at an early stage there are always 

 traces of a pronephros, or so-called head-kidney. This is perhaps seen 

 in its most primitive condition in Amphioxus, where, as already de- 

 scribed, there is a series of tubules, segmentall}^ arranged, opening on 

 the one side into the body cavity by several flame-cells, and on the 

 other into the atrial chamber, i.e. the exterior. On the surface of 

 each tubule a vessel connecting the sub-intestinal vein with the dorsal 

 aorta forms a vascular plexus — the so-called glomus. Such a con- 

 dition of parts is never in its entirety found in the Craniata. There 

 the tubules open not directly to the exterior, but into a longitudinal 

 pronephric or segmental duct, and they are usually few in number ; 

 but in their segmental arrangement, as shown by the blood supply, 

 and in the presence of glomera, they agree entirely with those of Am- 

 phioxus. In connection with the glomera, it may be noted that while 

 the blood supply usually comes directly from the dorsal aorta, it has been 

 shown by Paul Mayer and Riickert that in the embryos of Selachians 

 connecting vessels occur between the dorsal aorta and the sub-intestinal 

 vein, which form rudimentary networks on tlie tubules of the pronephros. 

 This shows a very striking correspondence with the conditions seen in 

 Amphioxus. 



The pronephros develops from the parietal mesoderm at the junction 

 of the muscle segments and the unsegmented body cavity (see Fig. 321) 

 in the anterior region, and varies greatly in its degree of development. 

 In Myxine and Bdellostoma it persists in adult life, though apparently, 

 at least in part, in a degenerate condition, and is said to be the functional 

 excretory organ of the little (degenerate ?) fish Fierasfer and some other 

 Bony Fishes. In most Bony Fishes, and in Amphibia, it is merely a 

 larval organ, but is then large and important. In Elasmobranchs and 



