PROBLEMS AND MATERIAL 15 



environmental factors within the parent organism and in the external 

 world. In this reaction certain potentialities of the protoplasm are real- 

 ized as developmental order and pattern. All forms of development pro- 

 vide material for investigation of the roles of heredity and environment in 

 determining patterns. 



In short, embryonic development is not the only, and perhaps in many 

 respects not the most interesting and significant, material for study of 

 various developmental problems. An adequate general theory of develop- 

 ment must be based on the less specialized forms of development; it must 

 recognize and distinguish the factors common to different forms of devel- 

 opment from those characteristic of only a particular form; also, it must 

 attempt to interpret the more highly specialized embryonic form of de- 

 velopment as far as possible in terms of the common factors. 



In the following pages a consideration of the physiology of development 

 is undertaken with particular reference to the problems of the origin and 

 nature of developmental pattern. Data of observation and experiment 

 concerning different forms of development, the conclusions which have 

 been or may be drawn from them, and their bearing on the general prob- 

 lem of pattern and particularly on the question whether or to what extent 

 different forms of development may be fundamentally different or funda- 

 mentally similar in the essential features of pattern are discussed. Per- 

 haps this survey of developmental physiology on a somewhat broader 

 basis than has usually served— that is, in terms of a comparative analysis 

 of different forms of development with an attempt to discover common 

 factors of pattern — may have some value for the future because it endeav- 

 ors to show that the patterns of other forms of development are no less, or 

 even more, important than embryonic pattern in relation to the general 

 problem. Perhaps it may also serve as a stimulus or an irritant to further 

 investigation. 



