176 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



hormones. They seem to form intermediate links in the whole chain 

 of events connecting the environment with the act of oviposition, but 

 not the end links. The environmental factors considered in the last 

 chapter must surely act on the nervous system of the animals. These 

 nervous stimuli perhaps act on the pituitary and induce a state of 

 oestrus. This itself does not produce normal oviposition, but in the 

 presence of a male and certain other requirements, some of which may 

 have been absent from Noble and Aronson's experiments (for they 

 were not quite natural observations, after all), eggs are laid, and not 

 merely extruded. This distinction must again be nervous in origin. 



The circle of the frog's hfe, with which I began this book, is now 

 complete, for in passing from stone to stone in our progress, we have 

 come again to eggs in a pond. If in travelling with me round the circle, 

 the reader has had as much pleasure as I have had in making the 

 observations, reading about the work of others, and writing the 

 results of my thoughts about it all, then I shall not have laboured in vain. 



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