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PHYLUM CHORDATA : CLASS AVES — BIRDS 



The eggs have much yolk and hard calcareous shells. The 

 segmentation is meroblastic and discoidal. The allantois is 

 chiefly respiratory, though it helps in absorbing the nutritive 

 substance of the egg, and acts as a receptacle for the embryo's 

 waste products. 



Fig- 433- — Young bearded griffin {Gypaetus barbatus). — After Nitzsch. 



■ Showing the feather-tracts or pterylae, for instance those on the breast 

 (PT.). E., Ear ; P., web or propatagium ; TH., thumb ; PR., bases 

 of primary feathers ; S., bases of secondary feathers ; B.S., bare streak 

 without pterylae ; CL., cloaca ; R., bases of rectrices or tail feathers. 



The Pigeon {Colutnba) as a Type of Birds 



The numerous varieties of domesticated pigeon (pouter, 

 fantail, tumbler, etc.) are all descended from the rock-dove, 

 Columba livia, and afford vivid illustrations of variation, and 

 of the results of artificial selection. Certain variations, 

 e.g. in beak or tail, crop up, we know not how ; and similar 

 forms are bred together until a new breed is established. 



