viii PREFACE 



the most important results with the comparatively imperfect 

 methods of research then at their disposal, and stimulated 

 and guided the work of those who followed them, whose 

 facilities with perfected modern methods have been very 

 much greater ; results which might never have been obtained 

 had the way not been pointed out by their predecessors in 

 the same field of knowledge. 



I desire to most gratefully acknowledge my indebtedness 

 to friends who have given me assistance in carrying out this 

 work ; especially to my friend Mr. Montagu Hopson, to whom 

 I owe many valuable suggestions, and whose kind help in 

 reading the proof-sheets and assisting in the compilation of 

 the index has been of the greatest value. 



I also wish to acknowledge my indebtedness to Professor 

 Symington, F.R.S., for his kindness in supplying me with 

 valuable material and permission to reproduce a photograph 

 from the Atlas of Skiagrams ; to Sir E. Sharpey Schafer, 

 F.R.S., for the use of an illustration from the Microscopic 

 Anatomy ; to Sir E. Ray Lankester, F.R.S., for permission 

 to reproduce a figure from the Quarterly Journal of Micro- 

 scopical Science ; to Professor Osborn for the use of two 

 figures from his work on The Evolution of the Mammalian 

 Molar Teeth ; and to the publishers of Leduc's Mechanism 

 of Life for permission to reproduce an illustration from that 

 work. 



To Mr. John Humphreys of Birmingham I am indebted 

 for the loan of several important preparations from his 

 valuable collection. 



The illustrations, when not otherwise stated, are from my 

 own photographs and preparations. 



J. HOWARD MUMMERY. 



70 ALBERT BRIDGE ROAD, S.W. 



