CHAPTER VII 



APPENDIX: RECENT RESEARCH 



I. INTRODUCTION 



RESEARCH into the details of the physiological 

 mechanisms controlling the vertebrate reproductive 

 cycles stopped almost completely during the war. Since 

 then, and since the first edition of this book was written, 

 new details have been discovered and described, although 

 our overall picture of these mechanisms has not signi- 

 ficantly altered. 



It has been confirmed that certain species may respond 

 by gonad maturation to increasing daylength in spring 

 (see p. 25) or to decreasing daylength in autumn (see 

 p. 31), or in the case of such amphibians as Rana tem- 

 poraria to changing temperature (van Oordt and van 

 Oordt^^^), or in certain reptiles to what has been called 

 'behavioural thermoregulation' (Bartholomew^^*), or 

 in certain tropical birds to the onset of the rainy season 

 (Marshall and Disney-^'). It has therefore become 

 increasingly obvious that the external environmental 

 factor which is primarily responsible for the onset of 

 sexual maturation may be quite different in different 

 species and consequently that the effects of the environ- 

 ment must be felt through different sense organs in 

 different species. A general conclusion is that in all cases 

 the external stimulus must be translated into nervous 

 messages passing to the brain, and that therefore the 

 brain may form an important link in the reaction. 



Modern research thus falls into two main categories: 

 that concerned with further and more detailed in- 

 vestigations into those external factors which control 

 the timing of the various vertebrate breeding cycles, 

 and that concerned with the role of the brain in the 

 chain of physiological events which lead ultimately to 

 gonad maturation. 



