8 INTRODUCTION 



altering the physical and chemical conditions relating to the embryo, 

 the normal mechanism of development would be understood better 

 through an analysis of the embryonic adjustments to these altera- 

 tions. Before the turn of the century, the earlier workers in this field 

 were His, Roux, Weismann, Born, Driesch, and the Hertwigs (Oscar, 

 Richard, and Paula). Then came Morgan, Spemann, and Jacques 

 Loeb. Finally during the last several decades there has developed a 

 host of experimental embryologists, many of whom were inspired by 

 association with the above workers. Reference should be made briefly 

 to Adelmann, Baltzer, Bataillon, Bautzmann, Boell, Brachet, 

 Child, Conklin, Copenhaver, Dalcq, de Beer, Detwiler, Ekman, Fank- 

 hauser, Goerttler, Guyenot, Hadorn, Hamburger, Harrison, Herbst, 

 Holtfreter, Just, Korschelt, Lehmann, the Lewises, the Lillies, Man- 

 gold, Nicholas, Oppenheimer, Parmenter, Pasteels, Patten, Penners, 

 Rawles, Rotmann, Rudnick, Schleip, Schultz, Spratt, Swingle, Twitty, 

 Vogt, Weiss, Willier, E. B. Wilson, and a host of others. 



A further refinement of this approach is in the direction of chemi- 

 cal embryology or a study of the chemistry of the developing embryo 

 and the raw materials from which it is formed. As Needham ( 1942) 

 says: "Today the interest has been shifted to the analysis of the funda- 

 mental morphogenetic stimuli which operate in embryonic life." Such 

 stimuli as these may well be of an ultra-chemical or ultra-physical 

 nature. 



Embryology as a division of science has gone through a period of 

 its own development from the purely descriptive phase to the present- 

 day biochemical and biophysical analysis of development. However, 

 each generation must recapitulate this sequence; therefore each 

 student must begin with the foundation of basic, morphological em- 

 bryology before he can expect to comprehend the possibilities in the 

 superstructure of the experimental approach. 



It is the function of this particular book to provide the student with 

 the foundational information relative to one genus, namely the com- 

 mon frog. This will introduce him to the major aspects of embryonic 

 development and at the same time give him a factual foundation upon 

 which he may later make his contribution in one of the fields of 

 embryology. Where it will not confuse, but might clarify the develop- 

 mental process, reference will be made to specific findings in the field 

 of experimental embryology. 



