10 AUTHOR'S PREFACE 



I am indebted to a number of authors and publishers 

 for permission to use illustrations and wish here to ex- 

 press my great appreciation of this courtesy. The indi- 

 vidual sources for these borrowed figures are in every 

 case indicated in the legends. To Dr. J. McKeen Cattell 

 I am especially grateful for allowing me the use of the 

 blocks from the magazine publication of this material in 

 the Scientific Monthly; to Dr. Alexis Carrel for permis- 

 sion to use unpublished photographs of his tissue cultures ; 

 and, finally, to Professor T. H. Morgan for critically 

 reading the manuscript and making many helpful 

 suggestions. 



E. P. 



BALTIMORE, 

 April 19, 1922. 



