CHAPTER 2 



SOURCES OF KINETIC AND 

 METABOLIC HORMONES 



Before describing the actions of the various kinetic and metaboUc 

 hormones, an account will be given for reference of the sources 

 from which they are derived and of some of the ways in which 

 they reach the blood stream. 



The cells in the animal body which are able to secrete hormones 

 into the blood can be conveniently grouped by their embryological 

 origins. Invertebrate examples will be given first. It will then be 

 noted that the sources of those kinetic and metabolic hormones 

 that are so far known from invertebrates all come from the 

 ectoderm (§ 2.1) and that it is only in the vertebrates that the 

 endoderm (§ 2.2) and the mesoderm (§ 2.3) also provide sources 

 for these kinds of hormones. The sources of morphogenetic 

 hormones, which affect growth, differentiation and reproduction, 

 include the gonads of both invertebrates and vertebrates, as well 

 as the ectodermal glands which secrete moulting hormones in the 

 Arthropoda, namely, the Y-organ in Crustacea and the prothoracic 

 glands and their homologues in Insecta. Passing references will be 

 made to some of these morphogenetic hormones in the chapters 

 that follow, but their main actions and details of their sources will 

 be described in Part II. 



2.1 Ectodermal sources 



The hormone-secreting, or endocrine, cells which are formed 

 from the embryonic ectoderm can be divided into those which 

 arise from the nervous system (§2.11) and those which arise from 

 non-nervous epithelium (§ 2.12); but the distinction may be 

 rather arbitrary, since the stomodaeal epithelium of the cephalo- 



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