THE INCUBATION OF IIEN.S' EGGH 179 



"The shell is composed of three layers: (1) the inner or 

 mammillary layer, ('!) the intermediate spongy layer, and 

 (3) the surface cuticle. 



"The mammillary layer consists of miiuite calcareous 

 particles welded together, with conical faces impinging 

 on the shell membrane; the minute air spaces between the 

 conical inner ends of the mammilhe communicate with the 

 meshes of the spongy layer, which is several times as thick, 

 and which is bounded externally by the extremely delicate 

 shell cuticle, spoken of on commercial eggs as the "l)loom." 

 The spong3' layer consists of matted calcareous strands. 

 The shell cuticle is porous, but apparently quite structureless 

 otherwise. The cuticular pores coumumicate with the 

 mesh work of the spongy layer; thus the entire shell is per- 

 meable to gases and permits emljryonic respiration and 

 evaporation of water."' 



" Under normal conditions the shell is bacterium ])roof. 

 Moisture lessens its impervious character, however, and, when 

 combined with dirt or filth, makes it possible for micro- 

 organisms to enter and bring aliout decay. "- 



Size of Eggs. — The eggs of the different breeds of chickens 

 vary somewhat in size. The average hen's egg is 2.27 inches 

 long and 1.72 inches in diameter, where the circumference 

 is greatest, and weighs about two ounces. This brings 

 the weight of eight eggs of good size to a pound and the 

 weight of a dozen eggs to a pound and a half. In some 

 of the eastern states, where egg production has become 

 a specialty, the average weiglit of hens' eggs is somewhat 

 above two ounces. In most of the heavy-producing western 

 states the average is somewhat below two ounces. The 

 state of Iowa has a legal weight of one and a half pounds to 

 the dozen, but the law is not enforced as the law concerning 

 the moisture content of butter is enforced in many states. 



Eggs laid by pullets are nearly always smaller than the 

 eggs laid by the same individuals a >'ear later. It was found 

 by Atwood,-^ at the West Virginia Experiment Station, that 



1 Lillie's "Development of the Chick." 

 =^ Rettger, Storrs' Bulletin No. 75. 

 ' West Virginia Bulletin No. 124. 



