PRE-CAMBRIAN AND PALEOZOIC ERAS 

 AIR SPACE EMBRYO 



169 



AMNION 



ALLANTOIS 



SHELL 



ALBUMEN 

 SHELL MEMBRANES 



FIG. 8.24. Embryo of a bird, together with shell, shell membranes, and em- 

 bryonic membranes. (Drawn by Lyman S. Rowell.) 



tioned above. Water for the needs of the embryo is present in the yolk 

 and also in the albumen ("white") of the egg. Early in development an 

 outgrowth from the body of the embryo surrounds the yolk with a yolk 

 sac. This, like the digestive system of which it is an outgrowth, is lined 

 with endoderm. The endoderm cells digest the yolk. The products of this 

 digestion are picked up by the blood, flowing in the network of blood ves- 

 sels which permeate the walls of the yolk sac, and transported to the em- 

 bryo as nourishment is needed. 



Oxygen is another necessity for the embryo. Oxygen is abundant in the 

 air surrounding the egg, but a means must be provided for securing it and 

 transporting it to the embryo. The shell and shell membranes are suffi- 

 ciently porous to permit air to enter. The blood serves as agent of trans- 

 portation, in this instance through a network of blood vessels in the walls 

 of an embryonic membrane called the allantois (Fig. 8.24). Like the yolk 

 sac, the allantois grows out from the embryo; it spreads around underneath 

 the shell membranes, where blood in its blood vessels can pick up oxygen 

 from air diffusing through. Waste products of embryonic metabolism, prin- 

 cipally uric acid (p. 93), are stored in a portion of the allantois, to be 

 discarded when hatching time arrives. 



Thus we see that by means of elaborate arrangement of shell, shell mem- 

 branes, yolk sac, amnion, and allantois, reptiles and their descendants the 



