4 INTRODUCTION 



characterised as a transition from intensive to extensive 

 multiplicity. 



At this point we can briefly indicate how we can imagine 

 such a development, in which multiplicity changes from in- 

 tensive to extensive, to take place. This will at the same time 

 illustrate the line of thought underlying this book. Before we 

 start, however, two points must be made clear. First, it must 

 be pointed out that development follows very different lines 

 in different animal groups. It is only in broad outline that all 

 these different modes of development are governed by the same 

 laws. Only these general features can be sketched here, and 

 naturally we shall often have to resort to broad generalization 

 in so doing. In the second place, it must be remembered that 

 developmental physiology is a young branch of biology, and 

 that it has by no means yet constructed a generally accepted 

 and balanced system of ideas. Therefore, the best attempt at 

 a well rounded picture of development we can now make will 

 still be of a very subjective and preliminary nature, and will 

 be open to many future changes and improvements. The need 

 for generalization entails the danger of partiality. Moreover, 

 it will be necessary now and then to draw conclusions from 

 factual data that are at present insufficient, and to bridge gaps 

 in our knowledge by means of hypothetical constructions. We 

 can base such opinions only upon the facts that have so far 

 become known to science. Therefore, the reader must take care 

 not to regard the picture of animal development given in this 

 book as definitive and irrefutable. We have only tried to place 

 the results of the several experiments in such a context that 

 a wider circle of readers may appreciate the importance of 

 the phenomena discovered. 



The fusion of sperm and egg starts a number of processes 

 which inaugurate development (Ch. II). The structure of the 

 fertilised egg is still very simple; the various components of 

 the cytoplasm are more or less evenly distributed so that all 

 parts of the egg are still approximately equivalent (Ch. III). 

 Yet the main axes of the egg are already fixed; it has a 

 polarity, and often a bilateral symmetry. This polarity and 



