appendix: recent research 103 



pituitary is revascularised by the portal vessels. The 

 presence or absence of a nervous connexion between the 

 hypothalamus and the pituitary is evidently unimportant. 



Finally, it has been questioned whether the wake- 

 fulness that can be induced by experimental stimulation 

 of the hypothalamus may be in any way comparable with 

 the restlessness of the sexually stimulated bird about to 

 migrate (Farner;^*^^ and see p. 93); whether the obesity 

 that may be induced by lesions of the hypothalamus 

 (Long^^^) may be basically similar to the fat deposition 

 that is also seen in birds about to migrate (Farner;^*^® 

 Koch and de Bout^^*); and whether indeed the hypo- 

 thalamus may prove to be the site of the seasonal 

 migratory urge and of the post-breeding refractory 

 period. 



Experiments involving the hypothalamus are especially 

 difficult to devise and to carry out, but it is becoming 

 increasingly evident that a thorough knowledge of this 

 region of the brain will be necessary before a full explana- 

 tion can be given of the mechanism of control of verte- 

 brate reproductive cycles, whether these are linked to the 

 external environment or whether they are entirely 

 internal. 



4. CONCLUSIONS 



Evidently the external world may exert its influence on 

 the reproductive cycles of the vertebrates through the 

 nervous reactions of a wide variety of sensory end- 

 organs, and the effects of all this nervous activity may 

 be co-ordinated within the hypothalamus. However, the 

 nervous impulses arise not only from the initial reaction 

 to a single external factor, such as increasing daylength, 

 but also from an appreciation of such environmental 

 factors as the presence of a mate and of a suitable breeding 

 place, of courtship behaviour, and of copulation itself 

 (see p. 76). Thus the main stimulus to gonad maturation 

 may result from an initial but prolonged stimulus to one 



