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 coelom. The first part of the oviduct is a 

 funnel or ostium tubae, 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-membrane 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 Ratitse) is there a 

 copulatory organ. The eggs are incubated by the parents 

 for a fortnight, a high temperature of about 40 C. being 

 sustained throughout. 



HABITS AND FUNCTIONS OF BIRDS. 



Flight. As birds are characteristically flying animals, many of 

 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 



