§ 2.114 SECRETORY CELLS FROM THE NERVOUS SYSTEM 35 



mammals (Fig. 2-10; Scharrer and Scharrer, 1954a). Unlike most 

 neurosecretory cells of the arthropods, those of the hypothalamus 

 of vertebrates possess dendrites (Fig. 2-1^). 



In fish and aquatic Urodela their axons are not well developed 

 and their function is uncertain ; but in all terrestrial vertebrates 

 from the terrestrial Urodela and Anura upwards the axons pass 

 down the infundibular stalk to end in an enlarged neural lobe 

 (pars nervosa). This becomes distinct from the median eminence 

 at the base of the infundibulum and is not present in the lower 

 forms (Fig. 2-11). In the course of evolution, the neural lobe has 

 acquired an independent blood supply from the internal carotid 

 arteries, forming a relatively rich vascular network (Fig. 2-12) 

 with which the axons of the neurosecretory cells make contact by 

 their swollen endings, called Herring bodies, similar in appearance 

 to those of the comparable cells in the crustacean sinus gland. 

 They can be shown by appropriate staining to be filled with neuro- 

 secretory granules. In mammals the secretory granules first become 

 visible in the embryo, where they appear to be carriers for 

 the actual hormones released from the neurohypophysis. The 

 relation between secretion and hormone formation has now been 

 as clearly shown here as anywhere, although the proofs were 

 obtained later than in the invertebrates. In any vertebrate from 

 fish to mammals, section of the axons in the infundibular stalk 

 results in accumulation of the secretion in the parts proximal to 

 the cut and its depletion beyond ; and the possibility of obtaining 

 active extracts from the different parts of the system follows the 

 same pattern. It has also been possible to grow^ cells from the 

 supraoptic nucleus of a dog in tissue culture and to observe (and 

 even to make a cine film of) the secretory granules passing from 

 the cell body down the axon. 



The neurohypophysis therefore acts as a storage-and-releasc 

 organ for the hypothalamic secretion or secretions. Two distinct 

 hormones have been recognized, the antidiuretic hormone and 

 oxytocin, but there does not seem to be any constant arrangement 

 of the neurosecretory cells which produce them. In the dog, for 

 instance, both substances can be obtained from both supraoptic 

 and paraventricular nuclei, although the proportion of oxytocin to 

 the antidiuretic fraction is always small; but in the camel, oxytocin 



