12 INTRODUCTION 



glands from the antennary or maxillary segments secrete moult- 

 promoting hormones. Their secretion is stimulated by an 

 endocrinokinetic hormone, prothoracotrophin, in Insecta (§ 4.211), 

 and probably by a similar hormone in Crustacea. Otherwise the 

 two classes differ in that, in the latter, moulting is restrained by a 

 moult-inhibiting hormone; this does not occur in Insecta, in 

 which a so-called juvenile hormone from the corpora allata 

 inhibits metamorphosis only. The initiation of metamorphosis in 

 Amphibia by thyroxine, with its dependence on the availability 

 of iodine, was an early discovery; its control by the endocrino- 

 kinetic thyrotrophin, TSH, w^as established later. 



The differentiation of the genital ducts and other sexual 

 characters has also been found to depend on hormones in a number 

 of invertebrates, as well as in vertebrates, where the pattern of 

 control differs in detail in different classes (§ 4.234 and Part II, 

 § 4). Only a few of the hormones producing these effects are 

 shown in Table 1 ; their detailed treatment is reserved for the 

 second Part of this work. 



The so-called "organisines", which stimulate regeneration in 

 Platyhelminthes, are not vascular hormones, since in the absence 

 of any circulation in these animals it must be assumed that the 

 substances diffuse through the tissues in a way that is reminiscent 

 of embryonic organizers, as their name suggests (Dubois and 

 Lender, 1956). 



1.6 Identification 



The technique of finding, testing and confirming the presence 

 and action of a hormone is exacting, and needs many controls if 

 the results are to be conclusive. It is difficult if only a single 

 hormone with a relatively clear-cut effect is under investigation; 

 for quite a long series of experiments is needed to elucidate the 

 situation. All too often some steps in the proof are missing, either 

 for technical reasons or because their importance was not fully 

 realized during the early stages of hormone investigation. 



Histological examination of tissues that are suspected of sec- 

 reting hormones is one side of the investigation, since cells capable 

 of this type of chemical activity often have a recognizable cytologi- 

 cal appearance (Figs. 2-2 and 4-7), with granular precursors of the 



