The hen's egg: Relationship between shell thickness and the amount of organic matter in the shell

Percentage weight loss on ashing was measured in 110 shell segments: 4 segments per shell—broad polar cap, narrow polar cap and two halves of the “barrel” —from 28 shells from 4 strains of chicken. The membranes had been removed, by hand, from each shell segment and shell thickness measured with a biconvex anvil micrometer. For each segment the function γ = T (1 + Wodm/Wmdo) was calculated, where T is the mean thickness, Wo the weight loss upon ashing, Wm the ash weight, d0 the density of the organic matter in the shell (assumed to be 1.0075 g/cm3) and dm the density of the mineral matter (2.386 g/cm3). Govariance analysis of the regression of T on T for each shell segment within each strain showed that the regression was linear, that the lines did not differ in slope, but that there were significant differences between the adjusted means for strains and for segments. This is interpreted as demonstrating that (a) the amount of organic matter in incremental shell is constant, estimated at 0.68 per cent by weight, and (b) the amount of organic matter outside incremental shell is variable. The equivalent thickness of the latter was estimated for each segment of each shell with a standard deviation of 0.4 μm. Other evidence suggests that the variation in organic matter outside incremental shell is due mainly to variation in cuticle thickness. Implications for poultry breeding and for future research are discussed.