ADDITIONAL ANIMAL STUDIES 191 



tiles, except that the eggs are usually laid in the sand and left to 

 develop by the warmth of the sun. There are, however, certain 

 exceptions to the general statements made above. Some of the 

 sharks, for example, and certain of the snakes, instead of depositing 

 eggs that develop into embryos in the water or on land, retain the 

 eggs, and the young are born in a form much like that of the adult. 



Very few of the animals belonging to the classes that we have 

 been discussing (namely, the fishes, amphibia, and reptiles) ever 

 take any care of their young. The great majority of birds, however, 

 not only build nests in which to lay their eggs, but they also brood 

 over their eggs until they are hatched, and then the parents feed 

 the young until they are ready to fly. 



A few of the lowest mammals, like most of the vertebrates named 

 above, lay eggs. By all the common mammals, however, the eggs 

 are not laid, but as was the case with certain sharks and snakes, the 

 eggs develop into a form resembling the parent, before being born. 

 All mammals at birth, unlike birds, are unable to eat the food that 

 is used by their parents. Hence, a form of food that is easily digest- 

 ible must be furnished. This is secreted by certain cells of the 

 adults in the form of milk. The masses of cells that secrete milk 

 are known as mammary glands, and because of the presence of these 

 glands in all animals of this the highest group of vertebrates, this 

 class is known as the mammals. 



