THE URO-GENITAL SYSTEM 



181 



The shell membrane consists of two layers, an inner delicate, 

 and an outer thicker layer. When the egg is just laid these two 

 membranes are in all parts closely adherent to each other, and_the_ 

 egg content completely fills the shell cavity. As soon as the content 

 cools there is a slight contraction; the two shell membranes separate 

 at the large end of the egg, forming an air cell which gradually 

 enlarges as the evaporation of liquid through the pores of the shell 

 takes place. 



The shell consists of several layers. Three are easily distinguish- 

 able: first, an inner mammillary layer, consisting of minute conical 



FIG. 58. A diagram of the parts of the egg. a, The blastoderm, b, The 

 shell, c, The outer shell membrane, d, The inner shell membrane, e, The air- 

 sac at the large end of the egg. /, The albumin, g. The chalaza. h, The 

 yellow layers of yolk, i, The white layers of yolk, k, The flask-shaped portion 

 of white yolk. /, The vitelline membrane. 



deposits of calcareous material; second, a middle spongy layer, 

 composed of a thick network of fibers; third, an outer delicate, 

 cuticle-like structure. In certain breeds of poultry a pigment 

 may be added; for example, in ducks a pea green, in turkeys a 

 spotted brownish material, and in fowls pink and various shades of 

 brown. The egg shell is porous to admit the free exchange of air 

 during the process of incubation. 



An average sized hen egg weighs about 2 ounces, of which n 

 per cent, is shell, 32 per cent, yolk, and 57 per cent, white. The 

 principal chemical constituents of the egg are as follows: ash, or 

 mineral matter, 9 per cent.; fat, or hydrocarbon, 9.3 per cent.; 

 proteids, or nitrogenous matter, 11.9 per cent.; and water, 65.5 



