PREFACE TO THE FOURTH EDITION 



In the present edition the plan of the book has been modified in certain respects. 

 The chapter on the cell has been omitted because in the opinion of the authors the 

 previous training of the student who commences the study, of the embryology of 

 vertebrates has been sufficient to bring to his attention the salient features of cell 

 organization. In former editions the early processes of development, viz: cleav- 

 age, gastrulation, and mesoderm formation, were treated as topics in separate 

 chapters. The present plan comprises the treatment of the early stages in succes- 

 sion in a given animal form; individual chapters are devoted to Amphioxus, the 

 frog, the chick, and the mammal. This change has been made because it is our 

 opinion gained from experience in teaching that the student acquires a better 

 understanding of the development of the germ layers by following the processes as 

 a continuous series in a given animal. A number of old illustrations have been 

 replaced by new figures the sources of which have been duly credited. 



Apart from the insertion of the chapter on fcetal membranes the second part 

 of the book, comprising organogeny, has been revised only in so far as the results 

 of recent investigation have modified the ideas expressed in the previous edition. 



We wish to express our appreciation of the helpful criticisms of our colleagues 

 and other friends. 



THE AUTHORS. 

 JULY, 1921. 



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