THE EXCRETORY ORGANS 337 



though they occur in most ichthyopsida. In the pronephros the 

 Malpighian corpuscle is rudimentary or lacking at alLstages, while 

 there is no differentiation of convoluted tubules and Henle's loop. 



The function of the various parts of the nephridial tubule is in 

 outline as follows: Theoretically it would appear that in the primi- 

 tive condition the nitrogenous waste, which is elaborated in the liver, 

 collected in the coelom and, together with the coelomic fluid, was 

 passed outward through the nephrostomes and the tubules, which 

 acted merely as ducts. Higher in the scale the parts become more 

 differentiated and specialized. The renal corpuscles form a filtering 

 apparatus by which water is passed from the blood-vessels of the 

 glomerulus into the tubules near their beginning, and this serves to 

 carry out the urea, uric acid, etc., secreted by the glandular portions 

 of the walls of the tubules (convoluted tubules, ascending hmb of 

 Henle's loop). 



In development there may be two or three successive series of 

 nephridial structures, the higher number occurring only in the amni- 

 otes. These are known as the pronephros (head kidney), meso- 

 nephros (Wolffian body), and the metanephros (permanent kidney 

 of the amniotes). All three are closely related in development and 

 structure, but are distinguished by differences in origin and in the 

 final details. 



Three views are held as to their relations one to another. According to one 

 they are parts of an originally continuous excretory organ (holonephros) which 

 extended the length of the body cavity. This has become broken up into the 

 separate parts which differ merely in time of development and function, with 

 minor modifications in details. A second view is that they are t hree separat e 

 oxgauftr while a third regards them as superimposed structures which occasipn- 

 alljLpverlap (birds, gymnophiona) and thus aTe not, strictly speaking, homolo- 

 gous but rather homodynamous. The first view has the most in its support, 

 but for convenience the three structures are kept distinct here. 



All three neiaj iridia arise from the m esomeric somiteS-(p. i6) or 

 from the Wolffian ridge which appears on either side of the median 

 line where the mesomeres separate from the rest of the wall of the 

 body cavity, the mesomeric cells furnishing the nephrogenous tissue 

 from which the definitive organs develop. 



Pronephros. — The pronephros is the first to appear in develop- 

 ment. As will be recalled (p. 15) the mesomere, like the epimere, 

 becomes segmented; and later, when the epimere separates to form 

 the myotome, the dorsal end of each mesomere becomes closed, the 



