4 INTRODUCTION 



(ii) Neurosecretory cells with the swollen ends of their 

 original axons forming storage-and-release organs 

 ("neurohaemal organs" of Knowles and Carlisle, 

 1956), that make contact with blood vessels (§ 2.11). 

 (iii) Endocrine gland cellsj which secrete internally into 

 the blood and are formed from almost any tissue 

 of the body, including the nervous system (in which 

 case the distinction from neurosecretory cells is 

 only one of the degree of their histological modi- 

 fication). 

 The chemical activators to be considered as ''animal hormones" 

 in the present book are the circulatory activators, or vascular 

 hormones (2 b). Yet since there is really no logical point at which 

 some of them can be separated from other neurosecretions, or 

 even from the organisines which have actions so much like those 

 of morphogenetic hormones, reference to these will have to be 

 made in the relevant sections, as will chemicals involved in 

 nervous stimulation, where their functions overlap those of 

 hormones. 



The complex interaction of many of these chemical activators 

 is w^ell illustrated by considering the way in which genes initiate, 

 and hormones complete, the differentiation of the gonad rudiment 

 into a testis or an ovary within an embryo, the development of 

 which starts under the general control of an organizer, continues 

 by progressive chemcdifferentiation, and cannot be completed 

 without the combined action of the nervous system and yet other 

 hormones ! 



1.3 Mechanical activation 



The only other means made use of by animals for co-ordinating 

 their activities is purely mechanical. This can act in the absence of 

 nerves or of any chemical activators, and may well be a primitive 

 way of transmitting control. The action of a current of water 

 stimulating the sponge osculum to remain open appears to be 

 purely mechanical, but it is not certain that some chemical may 

 not be diffusing from cell to cell. The best example is in the 

 locomotion of the earthworm, where the muscles contracting in 

 one segment stretch those in the adjacent segment behind, and 



