THE AMPHIBIA 97 



eye was primitively a light-sensitive device enabling the 

 ostracoderms to assume protective coloration. Long after 

 the pineal eye disappeared the pituitary continued to 

 secrete a chromatophore-expanding hormone— in the 

 elasmobranchs, teleosts. Amphibia, and reptiles, despite 

 the fact that in many of these higher forms a finer and 

 more rapid nervous control of the chromatophores is su- 

 perimposed on or completely replaces this older en- 

 docrine regulation. Paradoxically, in the birds and 

 mammals that have absolutely no chromatophores, the 

 pituitary continues to manufacture large quantities of 

 this chromatophore-expanding hormone, but it is con- 

 ceivable that this hormone now serves some other but 

 still unknown function in these classes. 



The control of protective coloration was, however, cer- 

 tainly not the only function of the primitive pituitary, 

 because even in the cyclostomes the gland is differenti- 

 ated into several distinctly different types of cells and 

 must have subserved several different functions. But here 

 we encounter a big hiatus in knowledge, and must rely 

 on inferences drawn from the function of the gland in 

 the higher animals. In addition to secreting several hor- 

 mones important in controlling the growth and metabolic 

 activity of other glands and tissues, the mammaUan 

 pituitary secretes two or more hormones important in 

 salt and water balance. The best known of these is the 

 'antidiuretic' hormone (familiarly known to renal phys- 

 iologists as ADH), which is secreted by that part of the 

 gland called the neural lobe or neurohypophysis. This 

 hormone derives its name from the fact that when ad- 

 ministered to man and other mammals in very minute 

 doses it prevents water diuresis by promoting the tubular 

 reabsorption of water from the glomerular filtrate; in 

 minimal, physiologically effective doses it has no effect 

 whatever on the filtration rate. 



In the Amphibia pituitary extracts not only enhance 

 the tubular reabsorption of water but in larger doses de- 

 crease the filtration rate by constricting the glomerular 



