2 4 HOW ANIMALS DEVELOP 



surface it often provides the first visible indication 

 that the blastula is changing into a gastrula. The 

 three layers out of which the gastrula is made are 

 named from the Greek words for skin, and for 

 outside, inside, and middle; thus the outer layer is 

 the ectoderm^ the innermost layer the endoderm, and 

 the layer between them mesoderm. 



The ectoderm, endoderm, and mesoderm are the 

 three fundamental parts out of which an animal is 

 built. We might almost say that they correspond to 

 the three major parts of a motor-car. The ectoderm 

 develops into the skin, the sense organs, and the 

 brain and nervous system; analogous to the body- 

 work, the lamps, and the controls. The mesoderm 

 forms the skeleton, muscles, and heart, or the chassis 

 and engine. Finally the endoderm corresponds to 

 the fuel system, and develops into the stomach and 

 intestines and all the apparatus for absorbing food 

 (i.e. fuel) ; this has to be much more complicated 

 in an animal than the fuel system is in a car, because 

 animals cannot get their nourishment poured into 

 them in a form in which it can be used at once, as 

 petrol is poured into the tank ; it is as if a car had 

 to carry round with it a whole refinery for turning 

 crude oil into motor spirit. These three layers are 

 called the Germ-Layers, and, when it was first pro- 

 pounded, the idea that they could be found in some 

 form or other in all animals stimulated scientists to 

 investigate as many different kinds of embryos as 

 possible to see if the hypothesis was true. Most of 

 this work was completed by the beginning of this 



