646 PHOTOPERIODISM IN VERTEBRATES 



veloped in all vertebrates, and experiments by damage or by stimula- 

 tion have shown, among other things, that this region of the brain 

 powerfully influences temperature regulation, sugar and fat metabo- 

 lism, the states of sleeping and waking, and the sexual activities. 



An excellent review of the probable role of the hypothalamus in 

 reproduction is that of Donovan and Harris (1955). They note evi- 

 dence to show that experimental lesions of the anterior hypothalamus 

 may result in gonad atrophy, while bilateral lesions posterior to the 

 optic chiasma may result in constant oestrus. Electrical stimulation 

 may also induce ovulation, and, in passing, it may be wondered 

 whether the direct photostimulation demonstrated by Benoit (1955) 

 is in fact a similar artificial technique. 



Donovan and Harris discuss the route whereby the influence of the 

 hypothalamus may pass to the pituitary, and they suggest that this in- 

 fluence must be hormonal in nature and must pass with the blood 

 through the hypophyseal portal vessels which run alongside the pitui- 

 tary stalk. On good evidence they discount Thomson and Zuckerman's 

 (1954) theory that the reaction of the pituitary is not dependent on the 

 integrity of the portal vessels. Of particular importance in this connec- 

 tion is the observation of Harris and Jacobsohn (1952) that sexual 

 cycles are not reestablished in a hypophysectomized animal into which 

 a new pituitary has been grafted unless this new pituitary is placed be- 

 neath the hypothalamus and revascularized by the portal vessels. The 

 presence or absence of a nervous connection is evidently unimportant. 



In summing up, Donovan and Harris suggest "that the external en- 

 vironment affects the reproductive process by means of nervous reflexes 

 originating in a wide variety of sensory end-organs," that "such reflex 

 activity would appear to be co-ordinated and integrated in the hypo- 

 thalamus," and that the "hypothalamic nerve fibres liberate some 

 humoral agent" which "is carried by the vessels into the sinusoids of 

 the anterior pituitary gland." 



While discussing this subject, it is also tempting to compare the 

 wakefulness induced by the electrical stimulation of the hypothalamus 

 with the "Zugunruhe" of the sexually stimulated bird preparing to 

 migrate (Farner, 1955) ; to compare the obesity induced by lesions of 

 the hypothalamus (Long, 1957) with the fat deposition that occurs 



