PREFACE 



IN WRITING this book I have had three aims in mind. I have tried to ex- 

 pound a picture of embryology, which has been formed during a quarter 

 of a century's work, and to do so in a form sufficiently factual and system- 

 atic to be useful as a textbook for students specialising in that subject, 

 or in the allied fields of genetics or experimental zoology. At the same time, 

 I have attempted to meet the needs of research workers in other branches 

 of biology who wish to fmd out what is going on in the study of develop- 

 ment at the present time. 



Embryology grew up as a branch of comparative anatomy; and when 

 the science is referred to without qualification, even to-day most biologists 

 probably think first of a descriptive account of developmental changes in 

 anatomy and histology. But there is, of course, by now a very large body 

 of data relating to the causal analysis of development. This is often re- 

 garded as a separate corpus of knowledge, referred to not as 'embryology' 

 but as 'experimental embryology'. A few decades ago, phylogeny and the 

 evolutionary aspects of comparative anatomy constituted the core of 

 animal biology, and it was not unjustified for the descriptive approach to 

 development to be accorded the title 'embryology' tout simple. But now 

 the situation seems to me to be different. The part of our subject which is 

 of prime interest as a facet of general biology is that which deals with 

 causal analysis, and if anyone claims to have studied embryology, this is 

 the part which we ought to expect him to know about. I have therefore 

 distributed the weight in this book in a manner quite different to that 

 usual in textbooks of embryology, with more emphasis on the experi- 

 mental and less on the descriptive approach, hi fact, of the latter I have 

 provided only the bare minimum which suffices to make the experimental 

 work comprehensible. This book is, however, not intended to be for most 

 students their first contact with embryology, but rather to serve the needs 

 of their later university years; and it is to be expected that most users of 

 it will have made some preliminary acquaintance with the anatomical 

 facts, either in practical class work or through one of the many elementary 

 texts which exist. Perhaps the ideal previous reading would be Barth's 

 excellent Embryology, which has the advantage of providing not only a 

 fuller descriptive account, but also a very stimulating introduction to the 

 experimental analysis. 



In surveying such a wide field as embryology, within a compass that 

 can be used as a text by students, a considerable amount of selection has 

 to be exercised. It is natural, and indeed probably desirable, that an author 

 should devote most attention to those aspects of the subject on which he 

 has himself worked. I am conscious that I have given more space to the 



