552 EGG AND FEMALE ORGANS OP GENERATION. 



of the shell at its rounded extremity. The air thus introduced 

 accumulates between the middle and internal fibrous membranes 

 at this spot, separating them from each other, and forming a cavity 

 or air-chamber (g), which is always found between the two fibrous 

 membranes at the rounded end of the egg. Next we come to the 

 albumen or " white" of the egg (d) ; next to the chalaziferous mem- 

 brane and chalazs9 (c) ; and finally to the vitelline membrane (b) 



Fig. 181. 



Diagram of Fowi.'s EGG. a. Yolk. ft. Vitelline membrane, c. Chalaziferotis membrane, d. 

 Albumen, e, f. Middle and internal shell membranes, g. Air-chamber, h. Calcareous shell. 



inclosing the yolk (a). After the expulsion of the egg, the external 

 layers of the albumen liquefy ; and the vitellus, being specifically 

 lighter than the albumen, owing to the large proportion of oleagin- 

 ous matter which it contains, rises toward the surface of the egg, 

 with the cicatricula uppermost. This part, therefore, presents 

 itself almost immediately on breaking open the egg upon its lateral 

 surface, and is placed in the most favorable position for the action 

 of warmth and atmospheric air in the development of the chick. 



The vitellus, therefore, is still the essential and constituent portion 

 of the egg ; while all the other parts consist either of nutritious mate- 

 rial, like the albumen, provided for the support of the embryo, or 

 of protective envelopes, like the shell and the fibrous membranes. 



In the quadrupeds, another and still more important modification 

 of the oviducts takes place. In these animals, the egg, which is 

 originally very minute in size, is destined to be retained within the 

 generative passages of the female during the development of the 

 embryo. While the upper part of the oviduct, therefore, is quite 

 narrow, and intended merely to transmit the egg from the ovary, 



