§1.53 MORPHOGENETIC HORMONES H 



ally in vertebrates, are secreted by endocrine glands and are sub- 

 ject to control by endocrinokinetic hormones. The nervous system 

 takes but a small share in their control, except in emergencies such 

 as haemorrhage or other forms of shock. More often the equili- 

 brium is maintained by a "feed-back" system, whereby the 

 accumulation of some product of the hormone's action inhibits 

 further secretion of the hormone until the accumulation is again 

 reduced. This applies directly to some metabolic hormones, and 

 indirectly to others, through its control of the endocrinokinetic 

 hormones that stimulate them (§ 5.5). 



1.53 MORPHOGENETIC HORMONES 



The morphogenetic hormones produce long-term changes that 

 involve cell division, growth and differentiation ; in contrast to the 

 quick-acting kinetic hormones, their effects can neither be reversed 

 nor repeated, at least for a considerable time. Formerly, these 

 hormones w^ere grouped under a more widely defined "metabolic" 

 heading. There was some justification for this, in that growth is 

 not possible without an adjustment of the metabolic processes to 

 provide the growing cells with the necessary building materials 

 for protein synthesis, and energy supplies in the form of glucose 

 (cf. §§ 5.2 and 5.5). Yet these and other metabolic processes con- 

 tinue throughout the life of the animal, and are not necessarily 

 linked to morphogenesis, which is often intermittent, as is easily 

 seen in moulting and metamorphosis and in the changes associated 

 with seasonal reproduction. 



Morphogenetic hormones affecting growth and regeneration 

 are present in several phyla, including Annelida and Mollusca, 

 from which no metabolic hormones have so far been reported, and 

 the means of controlling them is, in many cases, still unknown. It 

 is only in Crustacea, Insecta and Vertebrata that nearly all the 

 morphogenetic factors are secreted as vascular hormones and are 

 controlled by endocrinokinetic hormones, which link the processes 

 of growth, differentiation and reproduction indirectly to the 

 nervous system, and thence to seasonal changes in the environ- 

 ment (§ 4.232). 



Hormonal control of moulting and metamorphosis is now well 

 known in Crustacea and Insecta, in both of which ectodermal 



