§4.322 CONTROL OF THE SECRETION OF KINETIC HORMONES 159 



the prothoracic glands, and possibly by the corpora allata, arc 

 probably also under nervous control. The action of climatic and 

 other factors upon the brain certainly determines whether or not 

 these activating hormones are released; but this is best seen in 

 relation to the morphogenetic actions of the moulting hormones. 

 There the brain hormone, prothoracotrophin, determines the 

 time of larval moults by stimulating simultaneous secretion from 

 both the prothoracic glands and the corpora allata; but it deter- 

 mines the change-over to metamorphosis by activating the pro- 

 thoracic glands alone (Part II, § 3). 



In the crustacean brain little is as yet known of the activation 

 of the neurosecretory cells ending in Hanstrom's sensory pore 

 organ, but they may be assumed to be influenced by the nervous 

 system like other cells of the same sort. Their endocrinokinctic 

 activity is not yet fully established (§ 4.211). 



In vertebrates nervous stimulation of secretory cells of nervous 

 origin is well authenticated for the adrenal medulla, which 

 maintains its connection Vv^ith the sympathetic nervous system, 

 from which its cells are derived (§ 2.114). The situation in the 

 neurohypophysis (§ 2.114), which derives its secretion from the 

 neurosecretory cells in the brain, is less clear. There is good 

 presumptive evidence for nervous control of the secretion of 

 OXYTOCIN to cause contraction of myoepithelial cells (Fig. 3-7). 

 Moreover, it is clear that aff"erent nerve pathways lead through 

 the brain to the neurosecretory cells in the supraoptic and para- 

 ventricular nuclei in the hypothalamus. These cells release 

 oxytocin, which is then stored in the neurohypophysis; but the 

 action of the nervous system on the neurosecretory cells is natur- 

 ally difficult to investigate. Section of the neurohypophysial stalk 

 provides no more answer to the problem than removing the 

 neurohypophysis altogether, since it merely separates the store of 

 the secretion from its source and leaves the source in contact with 

 the rest of the brain. It has already been observed that after removal 

 of the neurohypophysis, sufficient oxytocin can be released from 

 the hypothalamus to ensure parturition (§ 3.113). An investigation 

 of VASOPRESSIN, ADH, from the same nuclei in the hypothalamus 

 would suffer from the same difficulties; but the action of ADH in 

 contracting blood vessels may well be accidental, and the means 



