CHAPTER I 



Introduction 



"Developmental physiology" is the term that will be applied 

 here to the branch of biology that studies the causal relation- 

 ships in animal development. About 1880, Wilhelm Roux 

 founded this branch of science on the strength of theoretical 

 considerations and a small number of specially designed expe- 

 riments. He called it "developmental mechanics". This name, 

 however, might well give rise to misconceptions as to its 

 nature. Roux applied the term "mechanics" in the wide sense of 

 "theory of causal connections" but its use may easily lead to the 

 idea that an attempt is being made to explain the developmental 

 phenomena on the basis of the laws of mechanics. For that 

 reason we prefer the name "developmental physiology", which 

 has the same scope. This means that the word "physiology" is 

 used here in its widest sense, viz. that of "causal science of 

 living organisms", so that it does not imply that we shall study 

 only those processes during development that are normally 

 included in "physiology" in its narrower sense, such as meta- 

 bolism, excitability, etc. The term "development" is meant to 

 mclude all irreversible changes that the organism goes through 

 m the course of its existence, from the moment of its origin 

 until the death of the individual. 



In broad outline, the development of all multicellular animals 

 proceeds along the same route. We shall give a short account 

 of this now, taking the fertilised egg as the starting point of 

 development. This arises from the fusion of a male and a female 

 germ cell (called sperm and egg respectively). (For the present 

 we shall disregard special cases such as that of vegetative 

 reproduction, in which the new individual originates from a cell 

 or cell group that separates itself from the body of an animal). 

 The first developmental process is a division of the fertilised 



Raven - Outline Physiologie . 



