§1.6 IDENTIFICATION 13 



secretion and enlarged nuclei. Once the secreting cells have been 

 located, it may be possible to make extracts of tissue containing 

 them, and to compare this with extracts of adjoining tissue 

 containing no such cells, in order to arrive at more definite results 

 than can be obtained from extracts of whole structures like the 

 crustacean eyestalk (§§3.12 and 3.223). 



A carefully planned experimental investigation is also essential 

 if the action of the hormone is to be fully established. This usually 

 falls into one of two categories, the pharmacological or the 

 physiological: in the first, it is shown that certain extracts, or 

 chemicals, have effects upon the animal, such as stimulating 

 muscle contraction, or raising the salt output in the urine ; in the 

 second, and usually more difficult category, an attempt is made to 

 prove that the chemical in question is used in the normal physi- 

 ology of the animal to control the same process. 



In general, it can be said that if only one hormone is concerned, 

 its control of a certain reaction can be sufficiently proved if 

 adequately controlled experiments show that: 



(i) removal of the source of the hormone is followed by loss 



of the reaction, 

 (ii) injection of an extract from the source can restore the 



reaction, 

 (iii) removal of any other structure does not cause loss of the 



reaction, 

 (iv) injection of any other extract does not restore the reaction. 



It is better if the reaction is shown by an organ with no nerve 

 connections, and if it can be interrupted by ligation of its blood 

 supply. If the reaction can be restored by injection or transfusion 

 of blood from another individual in which hormone secretion has 

 been stimulated by natural means (cf. Fig. 3-3), the proof that 

 this hormone plays a part in the natural physiology of the animal 

 is more convincing. To complete the identification of any parti- 

 cular hormone, it may be necessary to separate it from others 

 which can be extracted from the same source, and the problem is 

 never finally elucidated until the chemical constitution of the pure 

 hormone is known. 



If more than one hormone is involved in the control of a reaction. 



