618 IMPROVEMENT OF LIVING THINGS BY MAN 



In an older stage three layers of cells are noticeable. Those 

 of the outside, developed from the outer layer of the gastrula, 

 are called the ectoderm; this later gives rise to the skin, nervous 

 system, etc. An inner layer, developed from the inner layer of 

 the gastrula, is called the endoderm; this forms the digestive and 

 circulatory systems. A middle layer, called the mesoderm, lying 

 between the ectoderm and the endoderm, gives rise in higher 

 animals, to the muscles, the skeleton, and other internal organs. 

 At a very early stage in development, in some animals the two- or 

 four-cell stage, the so-called sex cells may be found, so that we may 

 say that almost from the beginning the sex cells are set apart from 



~*-a-_ the other cells of the de- 

 shell velo P m g orgamsm. 



jL We have already discussed 



albumen. J 



yollc the development of insects, 



of fishes, and of frogs, 

 .airspace , . , . , 



and have learned that in 



every case the fertilization of 

 the egg has resulted in the 

 development of the animal, 

 sometimes through a compli- 

 cated life history called a 

 metamorphosis. We have 

 found in all of these that the 

 egg developed without any 

 care on the part of the 

 parent. In higher forms we 

 find fewer eggs fertilized and 

 much more care taken of the 

 egg and young. In birds the 

 egg is incubated or hatched 

 outside the body of the 

 mother. But in the mam- 

 mals the young develop within the mother's body and consequently 

 are protected for a much longer time than those of the lower forms. 

 Development of birds. The white of the hen's egg is accumu- 

 lated during the passage of the real egg (which is on the yolk or 



.... -shell 

 albumen 

 ....fWict 



Above, a section through a bird's egg showing 

 tiny embryo connected to the yolk, which is sur- 

 rounded by albumen. 



Below, the egg has been incubated for some 

 days. Various organs have already developed, and 

 can be recognized. 



