232 MANUAL OF ANIMAL BIOLOGY 



tion, water is drawn into the mouth and forced out through the 

 lateral gill slits in the wall of the pharynx, where it comes into con- 

 tact with the gill filaments. This fish-like method of breathing is 

 continued until the lungs are developed, after which the animal 

 secures oxygen from the air. The gills on the right side of the 

 embryo degenerate first. 



Excretion of urea in the tadpole is carried on in the early larval 

 stages by tubular organs, known as the pronephroi. A pair of 

 these is developed very early, one on either side of the body. Each 

 pronephros consists of three tubules which open into the coelom at 

 one end. They unite at the other end to form the common pro- 

 nephric duct which runs posteriorly on each side of the body and 

 empties into the cloaca. (W. f. 129.) 



The anterior portion of each pronephric duct degenerates in 

 later larval life, and another group of tubules develops posteriorly 

 on each side of the body, which becomes connected with the vas- 

 cular system. These form the permanent kidneys, or mesoneph- 

 roi, of the Frog which open into the posterior part of the former 

 pronephric ducts. The latter remain as the permanent ducts. In 

 the higher Vertebrates, the permanent kidneys develop later and 

 are known as the metanephroi. 



B. Development of the Chick 



The male and female reproductive organs of the Birds have the 

 same fundamental type of structure that is found in the Frog and 

 other Vertebrate animals. They are paired structures in their 

 early development, but in the later stages the ovary and associated 

 structures on the right side of the female undergo degeneration so 

 that the functional reproductive organs in the female are present 

 only on the left side. 



The ovary, as seen with the naked eye, consists of a mass of 

 various-sized eggs which appear as yellowish globules. The ovary 

 is suspended from the dorsal wall of the abdomen by a connective 

 tissue mesentery, known as the mesovarium. Lying near the 

 ovary is a large, glandular oviduct which ends anteriorly, as in the 

 Frog, in a large ciliated opening, the ostium, and connects pos- 

 teriorly with the cloaca. The oviduct consists of three portions : 

 (1) the ostium, which is followed by (2) a glandular portion, the 

 cellular walls of which secrete both the 'white' of the egg and the 

 shell ; and (3) a thin- walled portion, which opens into the cloaca. 



