550 THE EYE IN EVOLUTION 



It is obvious that to become effective to a multicellular organism, a 

 hormone must be distributed through the circulation ; specialized endocrine 

 organs are therefore found in Annelids, Molluscs, Arthropods and Chordates. 

 It is not surprising that with a function akin to that of nervous tissue, these 

 chemical messengers — or at any rate those which have been investigated — 

 are initially in great part, both phylogenetically and embryologically, 

 associated with the nervous system. Other origins, however, are common 

 particularly in Vertebrates. In these, neuro-endocrine organs are found in 

 the diencephalon — the hyjjothalamus, the pituitary and pineal glands — and 

 in the medulla of the adrenal, a tissue of autonomic nervous origin which 

 has migrated outside the central organization ; but in addition, from the 

 endoderm there arise such hormones as the principles of the anterior lobe 

 of the pituitary, thyroxin, and insulin, and from the mesoderm, the sex 

 hormones and the steroids of the adrenal cortex. 



Those hormones which mediate the migration of the retinal pigment are 

 concerned with the sensory aspects of vision ; several others have associa- 

 tions with the action of light upon organisms and are therefore of interest 

 from our immediate point of view ; to these we shall mainly confine ourselves, 

 and since some of their reactions have already been discussed, a relatively 

 short note is all that is called for at this stage. All of these concern the 

 products of the neuro-endocrine system. We have already seen that certain 

 cells of the central nervous system, particularly those of the cerebral ganglion 

 in Invertebrates and of the diencephalon in Vertebrates, show a considerable 

 degree of light-sensitivity ^ ; the dual function of the pineal body,^ some- 

 times optical, sometimes endocrine, is an example of the same association. 

 It is not surprising therefore that nerve cells in these regions should some- 

 times respond by the secretion of hormones to the direct stimulus of light 

 and at other times to indirect stimulation through the eyes. 



In general terms the neuro-endocrine system exercises a controlling 

 influence over («) the integumentary pigmentary system and the ocular 

 pigments, (h) growth, differentiation and metamorphosis, (c) the development 

 of the gonads and the regulation of the reproductive cycle, and {d) a number 

 of processes in intermediate metabolism, principally affecting water, salts, 

 oxygen and carbohydrates. A noteworthy feature of many of its activities 

 is the rh3^thmic variation in several of these activities, either as a divirnal 

 rhytlim as is seen in the control of pigment migration and in some metabolic 

 processes, or as cycles of longer duration such as are exemplified in moulting 

 or the sexual rhythms. 



NEURO -SECRETORY CELLS, that is, nerve cells ivhich also have the characteristics of 

 glandular cells in that they show cyiological evidence of secretory activity,^ were first 



^ pp. 520, 52.5, 537. " p. 711. 



' All uorve cells " secrete " active substances (e.g., acetylcholine) : this may be termed 

 neurohumo i activity. Neurosecretion is a term best reserved for the activities of nerve cells 

 which also ; sess the cytological attributes of glandular cells. For a complete discussion, see 

 Convegno siil: yieurosecrezione. Pub. della Stazione Zoologica di Napoli, 24, Supp. (1954). 



