PEEFACE 



THE problem which, since the time of Aristotle, has stood 

 first in interest and importance among the great questions of 

 Biology is that of the causes which direct the development of 

 the individual that marvellous process by which the germ 

 is built up into the complex organism, by which the embryo 

 clothes itself with the characters peculiar to its species, by 

 which even minute individual traits of form and action are 

 exactly reproduced in the offspring from its parents. 



The burden of clearing up this problem has fallen, naturally 

 enough, upon the shoulders of students of morphology. For 

 since morphologists deal with form, they are properly especially 

 concerned with the interpretation of form they may well be 

 asked to account for it. Thus the problem of development is 

 an acknowledged morphological problem. 



Several distinct steps can be recognized in the progress 

 which has been made in the interpretation of form. The 

 earlier studies were concerned chiefly with answering the ques- 

 tion. What are the differences between the various adult forms ? 

 The results of observations and reflections relating to this 

 question constitute the sciences of descriptive and comparative 

 anatomy. Xext, a more fundamental inquiry was entered 

 upon : Sow are these forms produced or developed ? The 

 results of observations and reflections upon this subject con- 

 stitute the science of comparative embryology. Finally, in 

 these later days a still more fundamental question has come to 

 the front : Why does an organism develop as it does ? What 

 is that which directs the path of its differentiation? This is 

 the problem which the new school of " Entwicklungsmecha- 

 nik " has set for itself it is likewise the problem with which 

 this book is concerned. 



