42 HOW ANIMALS DEVELOP 



and deeper as gastrulation goes on. The third layer 

 or mesoderm consists of a loose mass of cells, which 

 separate off from the endoderm (Fig. 8). 



In Frogs and Newts 



A rather more complicated gastrulation process 

 occurs in newts and frogs, which belong to the group 



fries 



Fig. 8. — The blastula {A) and gastrula {B) of a sea-urchin. The 

 mesoderm is being, formed from the endoderm in {B). (From 



MacBride, after Field.) 



of Amphibians. All amphibian eggs contain a fair 

 amount of yolk, which, as has been described, makes 

 the blastula asymmetrical and gastrulation rather 

 more difficult ; moreover, being more highly evolved 

 creatures than sea-urchins, their gastrulation has 

 diverged more from the simple ideal type. In fact, 

 it is only by the application of modern methods of 

 investigation that we have found out what actually 

 happens. All the old methods involve killing the 

 embryos at different stages in their development and 

 comparing them. The embryos are usually killed in 

 such a way as to make them very hard, and then, 



