246 



THr. INTFRNAL ENVIRON MFNT OF THF. BODY 



Part III 



'tloping m*$ontphr0t 

 •QMphric duct 



'ervecord 



Notochord 

 -MyotOTM 



lotneTuluS 



Sephrostome 



Cardau^ v*in} 



Fig. 14.3. The kidneys of vertebrates, a succession of types, pronephros, mes- 

 onephros, metanephros, all of them paired tubes associated with the blood. Upper 

 left, The pronephros of the amphibian embryo shown after the body wall and 

 viscera are removed. Each pronephridium opens into the body cavity by a ciliated 

 funnel as in the earthworm but the other end connects with the pronephric duct 

 leading via the cloacal chamber to the external opening. The partly developed 

 mesonephros is visible, a similar series of tubules that join the pronephric duct. 

 The pronephros degenerates and is succeeded by the mesonephros whose important 

 advance is the association of the blood vessels of the glomerulus and the kidney 

 tubule. Upper right, Diagram of a cross section of the dogfish embryo showing 

 that the kidney tubules and the capillaries of the glomerulus are independent. 

 Lower left. Diagram of a cross section of an amphibian embryo in which the 

 capillaries of the glomerulus are in the clasping cup of the tubule. Lower right, 

 Embryo of man showing the beginnings of the metanephros, the final kidney. 

 (Courtesy, Little: Structure of the Vertebrates. New York, Long and Smith, 1932.) 



in coelomic fluid; their outer ends are joined and form the pronephric ducts, 

 one on each side of the vertebrae extending backward to a single opening near 

 the anus (Fig. 14.3). Pronephric kidneys occur in the adults of only a few of 

 the most primitive fishes. They develop, however, and are present a short time, 

 often only as rudiments, in the embryo of every vertebrate including man. 

 They exist for a time as the functional kidneys of young tadpoles. The meso- 

 nephros is the kidney of the majority of adult fishes and of amphibians and, as 

 the follower of the pronephros, is present and functions for a time in the 

 embryos of reptiles, birds, and mammals. 



Most kidney tubules end in a saclike enlargement, the renal capsule, that 

 holds a tangle of capillaries, the glomerulus (L., a little ball). The capsule and 

 capillaries together constitute a working unit of the kidney, called a renal or 

 Malpighian body (Fig. 14.3). In every such unit, water and other products 



