CHAPTER III 

 MOVEMENTS AND FOLDINGS 



In the blastula stage, the three fundamental layers 

 — the ectoderm, endoderm, and mesoderm — are all 

 simply different parts of the surface wall of a hollow 

 ball. If they are to arrive at their right places, with 

 the endoderm inside the ectoderm and the mesoderm 

 in between, a series of foldings has to be carried out. 

 There are several different methods of folding, as 

 we shall see, but they all lead to this same result. 

 We might compare the development of embryos to 

 making toys by folding up pieces of paper ; but with 

 embryos, whatever the final shape, the folding always 

 starts by producing a three-layered gastrula, just as 

 if we are making toys we often start by making a 

 hat shape, and then go on to further foldings which 

 turn it into a boat or a frog or whatever it may be. 

 We shall have to describe some of the different 

 ways in which the gastrula is produced, both because 

 this is the most important process in the develop- 

 ment of the embryo, and also as an example of how 

 the same process appears in a slightly different form 

 in different animals. In some embryos it is actually 

 a folding which occurs, but in others the layers 

 move into their right places by a streaming move- 

 ment, sweeping across the surface and around the 

 inside of the gastrula like glaciers moving down a 

 mountain side. As a general rule, the lower thiC 



