6 INTRODUCTION 



in a rapid increase in the chemodifferentiation of the embryo. 

 Here a leading role may sometimes be played by particular 

 cell groups (organisers), distinguished perhaps by a higher 

 content of certain determining substances (Ch. VIII). In the 

 following period, a complicated system of topogenetic and in- 

 ductive processes transforms the embryo into a mosaic of 

 organ primordia. Each of these has different physicochemical 

 properties, and consequently different developmental potencies. 

 This stage is succeeded by the period of tissue differentia- 

 tion, during which the cells of the primordia develop into 

 definite cell- and tissue structures, as determined by their 

 previous chemodifferentiation (Ch. IX). Later the structure of 

 organs and tissues may be further perfected under the influence 

 of their functions (functional adaptation) (Ch. X). Finally, 

 some sort of an equilibrium is achieved. From then on, the 

 importance of developmental processes is very small only. In 

 many animals, however, a disturbance of this equilibrium by 

 the removal of part of the body leads to renewed developmental 

 processes which result in a more or less complete regeneration 

 of the lost part (Ch. XI). 



