THE PIGEON AS A TYPE OF BIRDS. 599 



beside the ureters, and open into the middle region of the 

 cloaca. The right ovary atrophies, the right oviduct is rudi- 

 mentary. 



The eggs have much yolk and hard calcareous shells. The 

 segmentation is meroblastic and discoidal. The allantois is 

 chiefly respiratory, though it may also help in absorbing the 

 nutritive substance of the egg. 



The Pigeon (Columba) considered as a type of Birds. 



The varieties of domesticated pigeon with which we are 

 familiar, 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. The 

 power of rapid flight, the diet of seeds, the wooing of mates, 

 the feeding of the young by both parents, are well known. 



Form and External Characters. 



The body, well suited for rapid flight, ceases to be grace- 

 ful when stripped of its feathers. The cere above the 

 nostrils, the third eyelid hidden in the anterior upper corner 

 of the eyeball, the external opening of the ear concealed by 

 the feathers, the preen gland on the dorsal surface at the 

 root of the tail, the cloacal aperture, are external features 

 easily recognised. 



Feathers. 



The feathers most important in flight are the remiges of 

 the wing, divided into primaries borne by the metacarpals 

 and phalanges of the two fingers, and secondaries by the 

 ulna. The feathers of the tail help to guide the flight, and 

 are called rectrices. A distinct tuft of feathers borne by the 

 thumb is called the bastard wing. Covering the bases of 

 the large feathers are the coverts, wing-coverts and tail- 

 coverts, while the contour feathers give shape to the whole 

 body. In the pigeon there are no true down feathers or 

 plumules, but among the ordinary contour feathers or pennse, 



