128 COMPARATIVE PHYSIOLOGY 



only to the Craniata, and the existence of its homologue in 

 Amphioxus and Tunicates is doubtful (de Beer). 



The physiological activity of extracts of the posterior lobe 

 of the pituitary was, like that of extracts of suprarenal medulla, 

 first revealed by the classical researches of Oliver and Schafer 

 (1895). Extracts of the pituitary of mammals, birds, reptiles, 

 and fishes (teleostean and elasmobranch) have a powerful 

 excitatory action on the mammalian uterus and upon mammary 

 secretion (Herring, 1913 ; Hogben and de Beer, 1925). 

 Extracts of the pituitary of all classes of Amniota, Amphibia, 

 and Teleostei exert a pressor- diuretic action on the mammal. 

 This has not so far been obtained from extracts of the elasmo- 

 branch pituitary ; such extracts, however, are of a much 

 lower order of activity than equivalent extracts (by weight of 

 gland substance) prepared from the teleost pituitary, when 

 tested on the virgin uterus (Hogben and de Beer). Extracts 

 of the pituitary of fishes, amphibia, and amniota have also a 

 specific depressor action on the circulation of the bird. While 

 Krogh (1922) has given evidence for the conclusion that 

 pituitary secretion contributes to the maintenance of capillary 

 tone in the frog, it is an interesting fact that none of the above 

 responses have any established physiological as opposed to 

 pharmacological significance. In fact, the posterior lobe of 

 the mammal, while a storehouse of probably several substances 

 of prodigious activity and not a little interest to the phar- 

 macologist, has not yet been proved conclusively to have any 

 functional significance. There is, on the other hand, the clearest 

 evidence for regarding pituitary secretion as the main factor 

 in co-ordinating the pigmentary responses of amphibia to 

 the changing conditions of its environment. 



The anatomical and bionomic aspects of Amphibian colour- 

 response may now be summarised briefly as follows. The 

 effector organs concerned with colour-response in Amphibia 

 are the dermal and epidermal melanophores and the dermal 

 xantholeucophores. It seems probable that in Amphibia 

 the activity of the pigmentary effector depends on migration 

 of pigment-granules rather than the movement of the cell as 

 a whole. The rhythm of colour-response in Amphibia depends 



