PREFACE 



Vll 



his kindness in providing a photograph of an unpubHshed figure with per- 

 mission to reproduce and with full explanation, appearing as Figure 219, 

 C. For other figures from papers acknowledgment to the authors is made 

 in the legends with reference to the paper in each case. For redrawings of 

 certain figures and drawings or photographs of others the writer is deeply 

 indebted to Mr. Kenji Toda, artist of the Department of Zoology of the 

 University of Chicago, and to Mr. Dietrich Bodenstein, of the School of 

 Biological Sciences, Stanford University. 



A few citations of papers too recent to be included in the Bibliography 

 and of some others which should have been included are given in footnotes 

 or in the text. 



C. M. Child 



Stanford University 



