HABITS AND FUNCTIONS OF BIRDS. 673 
ovary ; that of the right side usually atrophies early in life. 
The right oviduct is represented by a small rudiment close 
to the cloaca. 
The ovary is covered with follicles containing ova at 
various stages of ripeness. As these ova become dilated 
with yolk and otherwise mature, they burst from the ovary, 
and are caught by the dilated end of the oviduct which 
opens into the ccelom. The first part of the oviduct is a 
funnel or ostium tubze, which grips the ovum and probably 
form the thin (chalaziferous) layer of dense albumin next 
the yolk. The second part is the albumin-secreting 
portion, which forms dense albumin. The third portion, 
called the isthmus, makes the shell-membrane and more 
albumin. The fourth region, badly called the uterus, 
makes the shell and 30-40 per cent. of the albumin, which 
passes by diffusion through the shell-membtane before the 
shell is formed. The fifth portion, called the vagina,’ is 
very muscular and expels the egg. It has only unicellular 
glands which perhaps secrete the external cuticle of the 
shell, plus pigment. 
A section through the oviduct shows—a peritoneal invest- 
ment, longitudinal muscles, connective tissue with blood 
vessels, circular muscles, connective tissue, a thick layer 
of convoluted branched tubular glands except in the funnel 
and the vagina, and most internally ciliated epithelium, 
except in the anterior part of the funnel. 
In sexual union the cloaca of the male is closely apposed 
to that of the female; only in a few cases (in ducks and 
geese, Crax, Tinamus, and in the Ratite) is there a 
copulatory organ. The eggs are incubated by the parents 
for a fortnight, a high temperature of about q4o° C. being 
sustained throughout. 
HaBITs AND FUNCTIONS OF BIRDS. 
Flight.—As birds are characteristically flying animals, many ot 
their peculiarities may be interpreted in adaptation to this mode of 
motion. 
(a) Shape and general structure of the body.—The resistance offered 
by the air to the passage of a body through it depends in part on the 
shape of the body, and the boat-like shape of the bird is such that it 
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