STRUCTURE OF THE VERTEBRATES 75 



ing flesh with both claws and bill, for example the owls, hawks, 

 eagles and vultures; the sand-piper, secretary bird, and the 

 common fowl, while well equipped for flying, can run swiftly 

 along the ground; and the opposite of the last adaptation is 

 found in the humming birds, which have almost no ability for 

 ground locomotion, but are able to remain on the wing, poised 

 in one place before a flower, for a sustained period. 



It has been often stated that anatomically a "bird is a bird", 

 but this applies only to the soft anatomy. For the many modi- 

 fications, and the numerous orders, the student is referred to 

 books of natural history. As the birds are far removed from the 

 line of human evolution a full discussion cannot be attempted. 



Embryology of the Bird. Avian development is more spe- 

 cialized than that of the reptile, but is the same in all essential 

 details. This similarity was discussed under reptilian embry- 

 ology. Most birds sit upon the eggs during the entire time of 

 incubation, only a few genera which live in dry s^ndy regions 

 depending at all upon the heat of the sun. The body tempera- 

 ture is higher than that of the typical mammal, being about 103 

 degrees Fahrenheit, and this temperature must be maintained 

 for the proper development of the embryos. 



When the egg is laid development has already begun, but 

 cleavage is suppressed when the egg cools. Embryos may re- 

 main in this arrested condition for several weeks, and when heat 

 is again applied begin growth. In this way all the eggs, which 

 were laid at different times, begin development together and 

 hatch at about the same time. 



