THE PARTS OF THE COMPLETED EGG 



303 



of lime salts in the feed, hence the necessity of supplying lime. It 

 is estimated that one hen laying 100 eggs of average size will pro- 

 duce in a year about 22 ounces of carbonate of lime. The bird gets 

 this large amount of lime from such materials as shells, dust, stones, 

 and pieces of bone, which it constantly picks up when at liberty; 

 if confined, oyster shell is given to supply the deficiency. The egg- 

 shell itself is composed chiefly of carbonate of lime, phosphate of 

 lime, and a little animal matter. It is extremely porous, hence 

 permeable by air, which is needed to supply oxygen to the embryo 

 during incubation. Microscopic examination reveals thousands of 



Blastoderm. 



outer shell- 



membrane. 



Vlteline membrane 



Inner shell membrane. 

 FIG. 146. Parts of the completed egg. 



minute pores in the shell, by means of which evaporation is going 

 on constantly from the time of laying. The degree of this evapora- 

 tion is a sure indication of the age of the egg or the conditions 

 under which it has been kept. Evaporation proceeds much more 

 rapidly in warm weather than in cold, and in dry air than in moist; 

 hence the difficulty of keeping eggs fresh for any length of time 

 in summer. This time could be greatly increased if it were pos- 

 sible to close these pores or openings and thus stop evaporation, 

 which, however, would make the egg unfit for hatching. 



Shell Membranes. Immediately under the shell are two mem- 

 branes, called the inner and outer shell membranes. Both of 

 these adhere closely to the shell, the outer one being in direct con- 



