120 COMPARATIVE PHYSIOLOGY 



glandular structure (endocrine organ or ductless gland) 

 specifically concerned with its manufacture. In defining the 

 criteria for ascribing to any organ an endocrine function, 

 attention need only be directed for our present purpose to 

 the regulation of specific responses in effector organs. If it 

 is known that an organ contains a substance which evokes a 

 specific local response in some effector unit {e.g, action of 

 adrenaline on the pupil or of pituitary extract on frog melano- 

 phores) its endocrine function may be estabHshed by one (or 

 both) of two methods. In the first place, it may be shown 

 that when responses which can be specifically evoked in isolated 

 effectors by the presence of its active material occur 

 spontaneously in the intact animal, they are associated with 

 the Hberation into the blood-stream of a substance having the 

 same properties as its extract, and in amount significantly 

 greater from that which is normally present in the blood. 

 One may formulate the alternative as follows. Given the 

 fact that an organ contains a specific constituent which evokes 

 response in an isolated effector unit, it is legitimate to conclude 

 that such an organ is of endocrine function, when the con- 

 sequences of its removal upon the given effector system may 

 be compensated by introducing the active material of its 

 extract into the circulation. 



The study of endocrine mechanisms has been prompted 

 to a very large extent by clinical interests which lie outside 

 the scope of the present discussion. Examples of the action 

 of hormones will be found in other chapters dealing with 

 developmental and metabolic processes. The pages which 

 follow will be concerned with the part played by internal 

 secretions in regulating response of a type which is not met 

 with in the mammalia, and to consideration of such evidence 

 as suggests the presence of hormones in the lower animals. 

 Both examples selected for this purpose deal with the regula- 

 tion of colour-response in cold-blooded vertebrates. 



In fishes the controlling mechanism has been shown by 

 Pouchet , V . Frisch , and others to be nervous . The melanophores 

 of fishes are directly supplied with nerve-fibres, and the effects 

 of local section and stimulation of nerve-trunks conclusively 



