Comparative Animal Physiology 



54 



order: Necturus vtaculatus <:Rana clamitans <Rana pipiens <Bufo 

 americanus. '"' This order is the same as the order of decreasing depend- 

 ence on a water medium. Toads are unaffected by pituitary injections until 

 the end of metamorphosis, when they become terrestrial. ^'"^ The hormone 

 increases skin permeability to water in amphibians but probably not in other 

 classes of vertebrates; it is found, however, in the pituitary of other verte- 

 brates. '"" The amphibian water balance principle has not been proved to 

 decrease urine output and is different from the antidiuretic hormone of 

 mammals. Permeability of the skin to water is greatly increased by the 

 principle. 



A frog, like a fish, can never excrete a salt-free urine. The salt is partly 

 made good by food, but the ability for selective salt absorption is as well 

 developed as in fresh-water hsh. Isolated frog skin bathed by Ringer solu- 

 tion on each side transports chloride from the outside in. ^^'^ Oxidative 

 energy is necessary for this process. Frogs kept in distilled water which is 

 changed daily lose chloride by way of both skin and kidneys. The loss in 



TABLE 3. VITAL LIMIT OF WATER LOSS 



urine is relatively constant (Cl~ in urine 0.5 to 1 millimolar), whereas loss 

 from the skin starts at a high rate and declines after 3 days to a very low 

 level. '^"'' '^•' When the frogs are replaced in tap water or dilute Ringer 

 solution an active uptake of salt occurs. In tap water or 0.01 Ringer solu- 

 tion, chloride was taken up at 0.05 /xM/hr./cm.- of body surface.^^^''' ^^-' ^^'^ 

 This ability actively to absorb sodium and chloride from dilute solutions 

 must be important in replenishing the body salts during periods when food 

 is not taken. Axolotls absorbed salts similarly. 



In addition to salt replacement by food and selective absorption, Am- 

 phibia have a well developed ability to retain salt by their kidneys. The 

 functioning of the amphibian (frog and Nectunis^ kidney has been ex- 

 tensively studied by Richards^"' ■ -•*"• --*"• -*-^ and his associates, and by Hober 

 (see summary in Chapter 35 of Physical Chemistry of Cells and Tissue^*^^')- 

 Necturiis is very suitable because of its small number of large (308 fi 

 diameter) units. The glomeruli lie in a layer close to the mesial border of 

 the kidney. Beyond each glomerulus is a ciliated neck, then the thick-walled 

 proximal tubule, then a narrow intermediate tubule, and finally the distal 

 tubule which empties into a collecting tube (Fig. 20). Richards' group 

 collected fluid from various regions of a kidney unit by means of a mercury- 



