§2.11 SECRETORY CELLS FROM THE NERVOUS SYSTEM 19 



pods gives rise to non-nervous cells, whereas that of insects gives 

 nervous cells. 



2.11 SECRETORY CELLS DERIVED FROM THE 

 NERVOUS SYSTEM 



A large number of hormones in many phyla are now known to 

 be secreted by nerve cells or their derivatives (Fig. 2-1). Some of 

 these have become so specialized for secretion that they have lost 

 almost all histological semblance of neurons, and their connection 

 with them is only apparent in their development or in the quality 

 of their secretion. Of such are the cells of the corpus cardiacum of 

 insects and the adrenal medulla of vertebrates (Fig. 2-1^ and/). On 

 the other hand, many secretory cells derived from neurons come 

 within the histological definition of neurosecretory cells ; these 

 have only recently been recognized as sources of hormones because 

 they are less easy to discern than the compactly aggregated endo- 

 crine glands, formed by most other cells of internal secretion. 

 They differ from neurons in secreting microscopically visible 

 quantities of granules or droplets, while retaining such characters 

 as Nissl bodies in the cytoplasm and axons with neurofibrillae ; 

 they may or may not have dendrites (Fig. 2-16, c and d). There 

 seems to be no real need to separate neurosecretory cells, which 

 secrete hormones into the blood, from any other endocrine cells 

 derived from either the nervous system or any other part of the 

 ectoderm, for their secretory activity is similar, although their 

 histological form is different. Since, however, much attention has 

 been focused recently upon neurosecretion, it will be well to 

 summarize some of the main points about it. 



Neurosecretory cells have been identified in animals represent- 

 ing most of the phyla with centralized nervous systems; but as 

 yet their production of vascular hormones has only been demon- 

 strated in a relatively small proportion of these phyla. In some of 

 these phyla, and notably in Annelida, they are only known to 

 yield morphogenetic hormones (Part II); but in the MoUusca, 

 Arthropoda and Vertebrata neurosecretory cells are known to 

 secrete vascular hormones with kinetic and metabolic actions. 



Neurosecretory cells can usually be recognized within the ner- 

 vous system by their large size (often 30 [i or more in diameter), 



