PREFACE 



A few words are needed to explain the scope of this book. The study 

 of the developmental processes of animals is an enormous field, of 

 which only a small fraction can be dealt with in a volume of this 

 size. The observational and comparative study of embryology falls 

 outside the boundaries of this series ; in any case, it has already been 

 treated in numerous authoritative works. Even on the experi- 

 mental and physiological side, however, there remains the difficulty 

 of selection from the vast mass of somewhat heterogeneous material 

 which many lines of research have provided for consideration and 

 synthesis. 



In the first place, development is not merely an affair of early 

 stages ; it continues, though usually at a diminishing rate, through- 

 out life. The processes of amphibian metamorphosis or of human 

 puberty ; the form-changes accompanying growth ; senescence and 

 natural death itself — these are all aspects of development ; and so, 

 of course, is regeneration. 



We feel that it is impossible to treat the whole life-cycle in a 

 single volume, and have accordingly set an arbitrary limit to our 

 material. We have deliberately restricted ourselves to the early 

 period of development, from the undifferentiated condition up to 

 the stage at which the main organs are laid down and their tissues 

 histologically differentiated — in other words, to Wilhelm Roux's 

 " prefunctional period ". Growth, absolute and relative ; the effects 

 of function on structure and on size ; the morphogenetic effects of 

 hormones — the details of these and of other related topics we have 

 deliberately omitted, and we have contented ourselves with the 

 addition of a final chapter in which the main peculiarities of the 

 functional period are contrasted with those of the pre-functional 

 period of primary differentiation. Any satisfactory treatment of the 

 latter portion of the developmental cycle would require a separate 

 volume. 



In the second place, within the period of early development, we 

 have exercised a further selection. In a new field of biology such as 



