586 COLLEGE ZOOLOGY 



birds. Extending forward from the angle of bifurcation of the 

 trachea is a flexible valve which is vibrated when air is forcibly 

 expelled from the lungs, thus producing a sound. A number of 

 muscles are able to alter the tension of this valve and conse- 

 quently the number of its vibrations and the pitch of the note 

 produced. 



The Excretory System. - -The kidneys are a pair of three-lobed 

 bodies situated as shown in Figure 471, U. Each discharges 

 its secretion, the urine, through a duct, the ureter (S), into the 

 cloaca. There is no urinary bladder, but the urine passes directly 

 out of the anus with the faeces. 



The Reproductive System. - - In the male are a pair of oval 

 testes. From each testis a duct, the vas defer ens, passes back 

 and opens into the cloaca; it dilates at its distal end to form 

 a seminal vesicle. The spermatozoa pass through the vasa def- 

 erentia; are stored in the seminal vesicles; and, when copula- 

 tion takes place, are discharged into the cloaca, and transferred 

 by contact to the cloaca of the female. There is no copulatory 

 organ. 



The right ovary of the female disappears during development 

 and only the left ovary persists in the adult. The ova break 

 out of the ovary and enter the oviducts. During their, passage 

 through the oviducts the albuminous substance, known as the 

 " white " of the egg, is secreted about them by the walls of the 

 middle portion. The double, parchment-like shell-membrane 

 is then secreted about the egg, and finally the shell is added by 

 the posterior part of the oviduct a short time before deposition. 



Fertilization takes place about forty-one hours before the 

 eggs are laid. Two eggs are laid by pigeons at a sitting, the 

 first usually between four and six P.M., and the second between 

 one and two P.M., two days later. They are kept at a temper- 

 ature of about 100 F. by the sitting bird for usually fourteen 

 days. At the end of this period of incubation, the young birds 

 have developed to such a stage that they are able to break 

 through the shell, i.e. they hatch. They are at first covered with 



