ACKNOWLEDGMENTS 



For permission to reproduce illustrations acknow- 

 ledgment is made to the Cambridge University Press 

 (Engelbach, Endocrinology ; Behrens and Barr, Endo- 

 crinology; Huxley and de Beer, Experimental Embryo- 

 logy), Messrs. Longmans, Green & Co. (Quain, 

 Anatomy), Messrs. Macmillan & Co. (MacBride, 

 Invertebrate Embryology), the Railway Gazette (photo- 

 graph of Whitemoor Marshalling Yard, L.N.E.R.), 

 and to my own publishers (Dtirken, Experimental 

 Analysis of Development; Stockard, Physical Basis of 

 Personality) . 



