GENERAL CHARACTERS OF BIRDS. 653 
_ The (metanephric) kidneys are three-lobed, and lie embedded 
in the pelvis » the ureters open into the cloaca; there is no 
bladder ; the urine is semi-solid, and consists chiefly of urates. 
Water must be mainly got rid of by evaporation from the 
walls of the air-sacs and atr-passages. 
The testes lie beside the kidneys ; the vasa deferentia run 
Fic. 358.—Young bearded griffin (Gypaétes barbatus).—After Nitzsch. 
Showing the feather-tracts or pteryle, for instance those on the breast 
(PT.). £., ear; P., web or propatagium; 7H., thumb; PR., bases 
- of primary feathers; S., bases of secondary feathers; B.S., bare streak 
without pteryle; CL., cloaca; F., bases of rectrices or tail feathers. 
outside the ureters, and open into the middle region of the 
cloaca. The right ovary atrophies, the right oviduct ts rudt- 
mentary. There ts rarely any copulatory organ, but it ts 
large in ostriches, ducks, geese, and some other 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 
