INTRODUCTION 1 7 



been embryologists. Aristotle, in fact, founded the 

 science. He opened hens' eggs after they had been 

 incubated for xarious lengths of time, and described 

 what he saw. For centuries, embryology remained 

 a purely descriptive science. The changes which 

 embryos go through as they develop are so many 

 and complicated that it took an enormous amount 

 of careful and painstaking work simply to describe 

 them. Scientists have always asked why the changes 

 occur; but only in the last fifty years or so have 

 they been able to perform experiments to try to find 

 out ; before that they could only guess, and, naturally 

 enough, their guesses were usually wide of the mark. 

 Even now we know very little about the causes 

 which underlie embryonic development, but this is 

 the most important and interesting part of the 

 subject, and in this book I shall lay more emphasis 

 on the tentative beginnings of our knowledge about 

 the causes of development than on the description 

 of the changes which occur. 



One very important fact has been discovered and 

 will be described later on in the book. It has been 

 found that at a rather late stage the organization 

 of an embryo is comparatively loose and the various 

 parts are to a large extent independent of one another 

 and of the whole embryo as regards the way they 

 develop. 1 At an earlier stage, on the other hand, it 



1 Though not, of course, as regards the way they work : in 

 this stage a lung can develop quite independently of the heart, 

 but it cannot function to aerate the blood without the help of 

 a heart. 



