iv PREFACE 



tance or representing present tendencies in research, and of 

 papers containing extensive literature references, valuable 

 illustrations, or general summaries. As far as possible the lists 

 contain references to works presenting both, or several, sides 

 of mooted questions mentioned in the text. There will also be 

 found, in nearly every instance, the titles of papers from which 

 illustrations may have been taken. 



To a large extent the figures have been redrawn, from the 

 original sources, for this work: it is a pleasure to notice the 

 uniform courtesy with which authors have granted permission to 

 make this use of their illustrations. The following special debts 

 are gratefully acknowledged: to Prof. Edmund B. Wilson and 

 The Macmillan Company, for cliches and for permission to 

 copy a considerable number of illustrations in their "The Cell 

 in Development and Inheritance"; to Prof. Gary N. Calkins, 

 The Macmillan Company, and Lea and Febiger, for cliches and 

 for permission to copy certain illustrations in their "The 

 Protozoa" and "Protozoology"; to Prof. Ulric Dahlgren, Prof. 

 William A. Kepner, and The Macmillan Company, for permis- 

 sion to copy certain illustrations in their "Principles of Animal 

 Histology"; to Prof. J. W. Jenkinson and the Delegates and 

 Secretary of the Clarendon Press, for cliches from their " Experi- 

 mental Embryology"; and finally to Herr Gustav Fischer and 

 to the several authors, for cliches and for permission to copy or 

 otherwise make use of illustrations from Korschelt and Heider's 

 "Lehrbuch," Oscar Hertwig's "Handbuch," Doflein's "Proto- 

 zooenkunde," and Ziegler's "Lehrbuch." In every instance 

 specific reference, both to the immediate and the ultimate 

 sources of the figures borrowed, is made in the legends. I 

 desire also to acknowledge my indebtedness to the authorities 

 of The Johns Hopkins University, for the use of valuable 

 library facilities. 



W. E. K 



BALTIMORE, MD., 

 March, 1913. 



