Radiographic Studies on the Formation of the Hen'S Egg Shell

Abstract

1. Radiographic methods have been used to study the rate of deposition of the hen's egg shell and the changes in volume and orientation undergone by the egg in the shell gland. 2. Shell deposition commences about 5 hr. after the yolk is ovulated and several series of radiographs were obtained tracing the process from these earliest stages through to the fully calcined shell. 3. From radiographs of calcium carbonate-gelatin mixtures it was found that, for a series of comparable objects differing only in calcium carbonate content, the densitometer readings on their radiographs were directly proportional to the density of calcium carbonate traversed by the X-rays in each object. 4. Hence, densitometer measurements on the periphery of the shell in each of a series of radiographs taken during the development of a single egg shell give values which are proportional to the density (or thickness) of calcium carbonate traversed by the tangential rays. It is shown that the radial thickness is closely proportional to the square of these values. 5. Plotting these squared densitometer readings against time indicates that the rate of deposition of mineral matter in the shell follows an S-shaped curve, with a marked acceleration in shell deposition 5-6 hr. after its onset. 6. During its first few hours in the shell gland, the egg undergoes a 25% osmotic increase in volume. This swelling is brought to a fairly abrupt halt by the increase in the rate of shell deposition and the consequent increase in the impermeability and rigidity of the shell. 7. Throughout all but the last hour or two of its 20 hr. stay in the shell gland, the egg lies with its pointed end caudal. Shortly before it is laid, however, it usually undergoes a 180° rotation in a horizontal plane. Thus the blunt end finally becomes caudal and emerges first when the egg is laid. During the rotation, the egg sinks to a more ventral position. This is necessary because, in most hens, the length of the egg plus the thickness of the walls of the shell gland is greater than the width of the pelvis. 8. The possible significance of the S-shaped curve of shell deposition is discussed. The volume, shape and orientation of the egg are considered in relation to the needs of the chick embryo and to the characteristic structure of the bird's oviduct and pelvis.