The hen's egg: Shell shape and size parameters and their interrelations

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Abstract
The occurrence of a crack in an egg shell depends in part on the local strength of the shell, which in turn depends on its curvatures as well as its thickness, and therefore on its shape. Selection for shell shape is therefore needed; but before the breeder can do this he must have a rapid and accurate method of quantifying shell shape, and knowledge of the factors that affect its variation. A procedure is described whereby nine measurements of an egg shell—length, maximum breadth, distance from the plane of maximum breadth to the broad pole and the distances by which each pole projects into annuli of diameters 1.5, 2.5 and 3.5 cm—are used to obtain an equation describing the profile of the egg in polar coordinates. It has five parameters that measure, independently, five shell characteristics: scale, aspect (breadth‐to‐length ratio), skewness, marilynia (concordant bulging between the poles and the plane of maximum breadth) and platycephaly (discordant bulging). Egg volume and superficial area can be obtained by integration, shell curvatures by differentiation. An experiment in which the computed volumes of 61 eggs from 19 hens of four strains were compared with the volumes measured by water displacement failed to detect, in single‐yolked eggs, any discrepancy not attributable to random measurement error, which was 0–2 per cent of mean egg volume. Aspect, marilynia and platycephaly are shown to be correlated inter se and with egg size; differences between hens and strains in respect of them are shown to exist. Making the shell measurements could be automated and use of the method could lead to improved efficiency of selection for egg size as well as shape.