268 BRANCH CHORDATA 



lungs, which are attached to the ribs, are alternately enlarged 

 and contracted. 



The Excretory System. — With each expiration the lungs 

 excrete carbon dioxid and other waste products. The kidneys 

 are dark-colored, paired organs lying in the pelvic region close 

 against the back. They open into the cloaca by the ureters. 



The Reproductive System. — Anterior to the kidneys are the 

 reproductive organs, which open into the cloaca. Multiplica- 

 tion is by eggs, which are noted for their very large size. The 

 egg begins in the left ovary, the right ovary not being developed. 



Fig. 218. — Diagrammatic longitudinal section through an undeveloped 

 hen's egg: Bl, Germinal disk; GD, yellow yolk; WJ), white yolk; DM, 

 vitelline membrane; EW, albumen; Ch, chalaza^; S, shell membrane; KS, 

 calcareous shell; LR, air-chamber. (After Allen Thompson.) 



It passes into the oviduct, where it is fertilized. As it passes on 

 down this duct it acquires the yolk, the white, the linings, and 

 the shell from glands that secrete these essential parts of the egg 

 (Fig. 218). The birds are developed by the large amount of 

 food within the shell. 



Incubation, or sitting upon the eggs for a definite period of 

 time, is peculiar to birds, though the python is said to coil upon 

 its eggs. The number of eggs in a clutch varies from one to a 

 score or more. Some birds, like the pigeon, are monogamous, 

 choosing one mate for life, but many are polygamous, like the 

 barnyard fowls. It is said that most pretty birds are flirts. 



