2 INTRODUCTION 



The first indication of any hormone in an invertebrate was that 

 postulated by Kopec (1922) as carrying the brain stimulus for 

 moulting in Lymantria. Then Roller (1927) found a blood-borne 

 factor controlling the colour changes of certain shrimps, and 

 Perkins (1928) located its source in the eyestalk. The discovery of 

 other hormones has followed, mainly in crustaceans and insects, 

 where they have almost as many actions as those carried out by 

 the better-known hormones of vertebrates. 



1.2 Chemical activators 



During this period, when hormones were being discovered in 

 ever-increasing numbers, different kinds of chemical activators 

 were being found in other fields of biology. Substances akin to 

 hormones were found in plants ; nerve transmission in vertebrates 

 and some invertebrates was found in many unrelated species to 

 be due to release of either acetylcholine or adrenaline, at the point 

 of contact between one neuron and the next, or between the motor 

 axon and its effector. The control of the pattern of development 

 in early embryos of Amphibia was found to be due to the diffusion 

 from cell to cell of particular chemical substances or organizers ; 

 these substances were not specific in that they were capable of 

 producing similar effects in a wide range of genera (Spemann and 

 Mangold, 1924). 



Some order was brought into the variety and diversity of these 

 and other chemical activators by Huxley (1935) in an important 

 scheme of classification. Its main weakness was that it did not 

 include neurosecretory cells derived from nerve cells and capable 

 of yielding hormones. These cells had been recognized histologi- 

 cally in vertebrates by Dahlgren (1914), and in some invertebrates 

 by Hanstrom (1931); but their action in releasing hormone-like 

 substances into the blood was first established by the Scharrers 

 (1937). They are now well known in Annelida, Arthropoda and 

 some other invertebrates as well as in vertebrates. 



Huxley's (1935) classification of chemical activators may 

 therefore be modified as follows, to include neurosecretion: 



A. Para-Activators. By-products of normal and pathological 

 metabolism with effects on correlation or differentiation, e.g. 

 carbon dioxide in its effect on the respiratory centre. 



