10 INTRODUCTION 



transpiring within. It is necessary, therefore, to study such embryos 

 after they have been preserved, sectioned, and stained. Nevertheless, 

 the emphasis in embryology is on the dynamic changes that occur, on 

 derivations, and on the end organs of the developmental processes. 

 One is not interested in a single frame of a moving picture nor is it 

 sufficient to describe all of the structures in a single section of an em- 

 bryo. The student of embryology is interested in the composite picture 

 presented by a succession of individual frames (sections) which are 

 re-assembled in his mind into a composite, three-dimensional whole. 

 It is necessary, in dynamic embryology, to reconstruct in the mind the 

 inner processes of development which proceed from the single-celled 

 egg to the multicellular organism functioning as a whole. The embryo 

 also must be regarded in the light of its future potentialities. While 

 parts of the embryo are isolated for detailed study, the student must of 

 necessity re-assemble those parts into a constantly changing three- 

 dimensional whole. The embryo is not static in any sense — it is 

 dynamic. 



Third: The student of embryology must have an intelligently directed 

 imagination, one based upon a foundation of scientific knowledge. He 

 must be rigidly loyal to demonstrable fact, but his mind must project 

 him beyond those facts. It is through men with intelligent imagina- 

 tion that there have been such remarkable advances in the super- 

 structure of embryology. Coupled with a healthy curiosity, such a 

 characteristic is causing men constantly to add facts, which withstand 

 critical investigation, to our ever-increasing body of knowledge. 



Why the Embryology of the Frog? 



It is the contention of the author that the student who understands 

 thoroughly the development of one species will have the foundation 

 for the understanding of the basic embryology of all species. This does 

 not imply similarity in development to the extent that there is no room 

 for comparative embryology of such forms as Amphioxus, fish, frog, 

 reptile, bird, and mammal. But the method of study, the language 

 used, and the fundamental processes are sufficiently alike so that a 

 thorough understanding of one form will aid materially in the under- 

 standing of the embryology of any other form. It is too much to cata- 

 pult a student into the midst of comparative or experimental em- 

 bryology and expect him to acquire any coherent conception of normal 

 development. 



