EXCRETION IN EMBRYONIC DEVELOPMENT 



255 



which bring the circulating blood into contact with the oxygen dissolved in 

 water. In addition, the frog embryo takes in dissolved oxygen through the 

 skin, and the thin tail fins serve as additional surfaces for gaseous exchanges. 

 In the chick embryo the increased surface necessary for respiratory exchange is 

 provided by the development of the embryonic membranes. These were de- 

 scribed in Chapter 12. The allantois and the yolk sac in particular provide an 

 increased amount of surface which is important in the oxygen intake and in 

 carbon dioxide elimination of the embryo. During later development the 

 allantois grows in size and fills in the space between the yolk sac and the 

 shell. All the respiration of the embryo takes place by means of the allantois 

 (Fig. 167). 



Excretion in embryonic development 



In the metabolism of proteins, urea and uric acid are formed and must be 

 eliminated. The chick embryo has no way of getting rid of these excretions 

 until the time of hatching. They must be stored within the shell. For a while, 

 in the early period of development, the chick embryo excretes urea. Urea is a 

 rather soluble substance and must be diluted by a large volume of water or it 



SHELL ALLANTOIS 



Fig. 167. The allantois and its relation to the shell. 



