442 SUMMARY 



Further, the epigenetic analysis of development is pointing the way 

 to a large extension of the field of heredity, in the shape of physio- 

 logical genetics. It is only through a study of development that it 

 will be possible to understand what the term ''genetic characters" 

 really stands for — in other words, what are the basic processes in- 

 volved in the action of a particular Mendelian gene. 



Experimental embryology as a separate branch of science was 

 initiated by Roux; in its next phase, in which Driesch, Boveri, 

 Wilson, Herbst, Morgan, Brachet and Jenkinson are outstanding 

 names, a large body of facts was amassed, and the experimental 

 proof of epigenesis provided; in the third phase, Spemann and 

 Harrison are the outstanding figures within the sub-science, while the 

 theories of Child have not only linked the facts of regeneration with 

 those of embryonic differentiation, but have provided a scientific 

 basis for a field hypothesis for early development, thus filling a large 

 gap in the theoretical aspect of the subject. Meanwhile, experi- 

 mental embryology has been making fruitful contacts with physio- 

 logy, notably in the field of hormone action, with genetics, and with 

 growth studies. 



The fourth stage is now beginning, in which this framework of 

 general principle will be filled in through intensive research, and 

 the whole science deepened by a search for the physico-chemical 

 bases of the empirical biological principles which have been dis- 

 covered in its earlier stages. 



