654 BIRDS. 
substance of the egg, and acts as a receptacle for the embryo’s 
waste products. 
Tue Piczon (Columba) as A TYPE OF BiRDS 
The numerous varieties of domesticated pigeon (pouter, 
fantail, tumbler, etc.) are all descended 
i from the rock-dove, Columba livia, and 
afford vivid illustrations of variation, and 
\ hy 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. 
External characters.— The form of 
the body, well suited for rapid flight, 
i Be Cc ceases to be graceful when stripped of 
_ its feathers. The cere above the nos- 
trils, the third eyelid in the anterior 
upper corner of the orbit, the external 
opening of the ear concealed by the 
feathers, the preen gland on the dorsal 
surface at the root of the tail, and the 
cloacal aperture, are external features 
easily recognised. 
The feathers most important in flight 
are the twenty-three remiges of the 
wing, divided into eleven primaries 
borne by the metacarpals and phalanges 
Fie, -459,-Atter of the two fingers, and twelve secondaries 
“Nitzach, borne by the ulna. Twelve tail feathers 
A. Filoplume. B., very OF rectrices serve as a brake, and help 
young feather within g little in steering. A distinct tuft of 
its sheath (s%.) ; c., the 5 
core of dermis; 2., the feathers borne by the thumb is called 
barbs. C., the same, the bastard wing. Covering the bases 
of the large feathers are the coverts, 
—wing-coverts and tail-coverts,—which belong to the 
series of contour feathers which give shape to the whole 
body. In the pigeon there are no true down-feathers or 
plumules, but among the ordinary contour feathers or penne 
there are little hair-like feathers (filoplumes) with only a few 
