192 VERTEBRATES 



and nerves, parts of the eye and the bones of the internal 

 ear. From the mesoderm arise the muscular system, the 

 organs of the circulatory system, the bones of the skeleton 

 and the reproductive system. From the endoderm arise 

 the lining of the digestive tract, the liver and other glands, 

 and the lungs. Each part appears as an aggregation of 

 cells in its own particular location, and grows by further 

 development until it is complete and united with other de- 

 veloped organs around it. Sometime before the chick leaves 

 the egg the organs are all completed and in perfect working 

 order. When it leaves the egg it is essentially like the adult 

 form, which it becomes in a greater or less time by nutri- 

 tion and growth. 



(5) The nesting of birds is a most interesting charac- 

 teristic and can be studied in the field with great profit. Do 

 all pigeons build nests alike? Do all sparrows? If condi- 

 tions should change, do you think the birds would change 

 their way of nest-buiding? 



Habits. — Why are pigeons usually found around barns 

 or cotes made for them? Are they ever foimd wild? Do 

 they migrate in the winter? Are they gregarious or soli- 

 tary ? Are doves gregarious ? Do doves migrate ? Why do 

 some birds migrate? How do doves defend themselves 

 from their enemies? How do other birds defend them- 

 selves? What kind of food does the pigeon use? Do the 

 shapes of the bill and toes indicate any specialization for 

 food-getting ? 



Classification. — The pigeon belongs to the great class, Azies, 

 which includes all living genera of birds and some of those found 

 fossil. The different orders of birds are classified mainly on the 



