MOVEMENTS AND FOLDINGS 4 1 



animal is in the evolutionary scale, the simpler its 

 gastrulation. The animals which are described here 

 are selected partly because they have been particu- 

 larly well investigated, and partly as typical repre- 

 sentatives of this series of gradually increasing 

 complexity. Thus we shall begin with the simple 

 gastrulation of the sea-urchins and go on to primitive 

 vertebrates like newts and lampreys, working up to 

 more highly evolved groups of vertebrates like birds, 

 and finally to mammals, which have developed a 

 special container, the womb, in which development 

 takes place. 



The Formation of the Three Layers in Sea- Urchins 



The simplest kind of gastrulation looks just as though 

 one side of the blastula was being pushed and folded 

 inwards by an invisible finger, in the same way in 

 which one can push in one side of a rubber ball till 

 it touches the other side. This happens, for example, 

 in the embryos of sea-urchins and starfish. The first 

 sign of gastrulation is a flattening of the bottom of 

 the blastula, where the cells are usually slightly 

 bigger than at the top, although this difference is 

 not very striking, since there is not much yolk in 

 the egg. Soon the flattened part sinks deeper in 

 towards the centre of the blastula and makes a 

 groove or small hole. This holq is a very important 

 structural feature and turns up under all sorts of 

 peculiar guises in other groups; it is called the 

 blastopore, and is the entrance to the cavity lined by 

 the endoderm, or the primitive gut. It grows deeper 



