INTRODUCTION 25 



century, and it provided a broad basis of information 

 on which all our present-day knowledge of em- 

 bryology has been built up. On the whole, it was 

 found that the three germ-layers could be fairly 

 easily recognized in most animals, but there are a 

 few difficult cases, and in some very primitive 

 animals only the ectoderm and endoderm are present 

 and there is no mesoderm. The particular impor- 

 tance of the idea of the germ-layers from our point 

 of view is that it is the beginning of an analysis of 

 the pattern in which the embryo is organized. The 

 formation of the three germ-layers is usually the first 

 structural change which the embryo achieves, and 

 almost immediately after this the main organs are 

 formed. The process by which the blastula turns 

 into the gastrula is known as gastrulation^ and a great 

 deal of the discussion later on in the book as to how 

 development is brought about will be concerned 

 with this period of gastrulation when the main 

 structure is blocked out. 



