THE FEMALE ORGANS OF GENERATION. 589 



In the third division of the oviduct (f), which is about nine centi- 

 metres in length, the longitudinal folds of the mucous membrane are 

 narrower and more closely packed than in the preceding portion. Its 

 secretion condenses into a fibrous covering, composed of three layers, 

 closely embracing the surface of the albuminous mass, and forming a 

 tough, flexible, semi-opaque envelope for the whole. These layers are 

 known as the external, middle, and internal fibrous membranes. 



Finally the egg passes into the fourth division of the oviduct (g), 

 which is wider than the rest of the canal, but only a little over five 

 centimetres in length. Its mucous membrane, which is covered with 

 abundant leaf-like villosities, exudes a fluid rich in calcareous salts. 

 The external fibrous layer is permeated by this secretion ; and after- 

 ward, owing to the reabsorption of fluid, the calcareous matter is 

 deposited in its network. The deposit goes on, growing thicker and 

 more condensed, until the membrane is converted into a white, opaque, 

 brittle, calcareous shell. The egg is then forced through a narrow por- 



FIG. 163. 



DIAGRAM OF FOWL'S EGG. a. Yolk. ." . Yitelline membrane, c. Chalazlferous membrane, d. Albu- 

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



tion of the oviduct (h), and, gradually dilating the passages by its conical 

 extremity, is discharged from the external orifice. 



The egg of the fowl, after expulsion, consists, accordingly, of the 

 yolk and vitelline membrane, with various additional parts acquired 

 during its passage through the oviduct. 



After the discharge of the egg there is a partial evaporation of its 

 watery ingredients, which are replaced by air penetrating through the 

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

 accumulates between the middle and internal fibrous membranes, form- 

 ing a cavity or air-chamber (Fig. 163, g), at the rounded end of the egg. 

 Very soon, the external layers of the albumen liquefy ; and the vitellus, 

 being specifically lighter, rises toward the surface of the egg, with the 

 cicatricula uppermost. This part presents itself almost immediately on 

 breaking open the egg at any point corresponding to the equator of the 



