EGG AND FEMALE ORGANS OF GENERATION. 691 



calcareous salts. The most external of the three membranes above 

 described is permeated by this secretion ; and soon afterward, owing to 

 the reabsorption of its fluid parts, the calcareous matter begins to crys- 

 tallize in the fibrous network of the membrane. This deposit of calcare- 

 ous matter goes on, growing thicker and more condensed, until the 

 external envelope is converted into a white, opaque, brittle, calcareous 

 shell, which incloses the remaining portions and protects them from 

 injury. The egg is then forced through a narrow portion of the oviduct 

 (/?,), and, gradually dilating the passages by its conical extremity, is 

 finally discharged from the external orifice. 



The egg of the fowl, after its expulsion, consists, accordingly, of vari- 

 ous parts ; some of which, as the yolk and the vitelline membrane, 

 entered into its original formation,' while the remainder have been 

 deposited round it 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 (</), at the rounded end of the egg. Very 



Fig. 227. 



Diagram of FOWL' 8 EGO. a. Yolk. b. Vitelline membrane, c. Chalaziferous 

 membrane, d. Albumen. e,f. Middle and internal shell membranes, g. Air-chamber. 

 h. Calcareous shell. 



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

 specifically lighter than the albumen, 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 yolk, 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 constituent part, even in 

 the large and highly complicated fowl's egg ; while the remainder con- 



