248 ANIMAL ACTIVITIES. 
cases, the difference being only in the amount of fold- 
ing. The liver and pancreas of vertebrates arise in the 
same way, by folding of the walls of the food-tube. 
Other changes take place in as simple a way as this. 
Indeed, nearly all the organs of a complex body arise 
by foldings and pushings of layers of cells. When the 
egg of a very simple animal, like the hydra, develops, 
it first divides into a number of cells forming a spherical 
: E 
Fic. 202.—Growth of Frog’s Lung from Primitive Food-tube. 
body, the morula stage. One side of this sphere is 
then pushed in until two layers of cells have been 
brought near together, the gastrula stage. In animals 
higher than the hydra a third layer grows between the 
other two, and from these three layers, by pushings 
and pullings and foldings, the parts of a complicated 
animal body arise. These morula and gastrula stages, 
more or less obscured in many cases, have been found 
in the development of all the higher animals. The 
