72 



ANIMAL BIOLOGY 



the hen broods over the eggs. Gradually the cells of the 

 different external and internal organs are formed (Fig. 55) 

 from the food material furnished by the yolk and the white 

 of the egg, and at the end of three weeks the young chick 

 breaks through the shell, and soon, under the protection of 

 the mother hen, begins to search for food. When first 

 hatched, the feathers are relatively small and downy. The 

 further development of the chick is largely a matter of 

 growth in size and of change in the character of the feathers. 



55. Nests and care of young. The method of repro- 

 duction in all birds is much the same as that already described 



FIG. 56. Comparative size of the eggs of ostrich, hen, and humming 

 bird. (Photographed by E. R. Sanborn, N. Y. Zoological Park.) 



for the chick. Many birds, however, are much more help- 

 less when they emerge from the egg than are chickens, and 

 so they are sheltered in nests, and the food of the young 

 birds is brought to them by their parents until they are 

 able to fly and for several days afterwards. 



