Chapter I 



HISTORICAL INTRODUCTION TO THE 

 PROBLEM OF DIFFERENTIATION 



The production of the adult Uving organism with all its complexity 

 out of a simple egg (or its equivalent in the terminology of the 

 ancients) is a phenomenon and a problem which has attracted the 

 attention of philosophers as well as of scientists for over two thou- 

 sand years. To give a brief account of the history of ideas relating 

 to this problem is no easy matter, but the task is fortunately facili- 

 tated by the fact that Dr E. S. Russell and Prof. F. J. Cole, F.R.S. 

 have recently devoted volumes to certain aspects of this subject, 

 and to the reader who desires to become better acquainted with it, 

 no better advice can be given than to refer him to The Interpretation 

 of Development and Heredity, and to Early Theories of Sexual 

 Generation. The historical section of Dr Needham's Chemical Em- 

 bryology, and various works of Dr Charles Singer also provide much 

 valuable information. 



Meanwhile, a brief attempt will be made in the following few 

 pages to outline the essential features of the chief schools of thought 

 concerning problems of development, in order to show how the 

 modern science of experimental embryology came into being, and 

 to present it in its proper historical setting. 



The kernel of the problem is the appearance during individual 

 development of complexity of form and of function where 

 previously no such complexity existed. In the past, there have 

 existed two sharply contrasted sets of theories to account for it. One 

 view accepts the phenomenon as essentially a genesis of diversity, 

 a new creation, and attempts to understand it as such. This coming 

 into existence of new complexity. of form and function during 

 development is styled epigeftesis. 



The difficuhies which other thinkers experienced in trying to 

 understand how epigenesis may be brought about led them to deny 

 that it exists: i.e. to say that there is no fresh creation of diversity 



HEE ^ 



