stretches along the oviduct axis produce ellipticity, while variations in membrane material properties between the two halves of the egg give rise to asymmetry [2]. After the egg obtains its shell membrane, the formation of the eggshell begins in the isthmus just before the egg enters the shell gland. As the process starts, the shell gland secretes calcium carbonate around the egg. Anchored on the shell membrane are small calcite buds which act as a foundation to allow calcium carbonate to attach to [4]. As time passes, the eggshell thickens. After 18 to 20 hours, the eggshell becomes fully hardened, creating a rigid, fixed egg shape thereafter [1]. Evolution of Egg Shape Now we understand the biological mechanism behind egg shapes, and the fact that most eggs are more asymmetrical and elliptical than chicken eggs. What could be the evolutionary needs that drive this trend? From the data of the 1,400 avian species, researchers found a strong correlation between better flight ability and asymmetric eggs, elliptical eggs and larger egg sizes relative to body mass. They theorized that strong flight selects for avian species with a slimmer, more streamlined body, which limits the maximum stretchability of the isthmus or the uterus. In turn, the egg takes up a more asymmetrical and/or elliptical shape to maximize the egg volume without increasing its girth [2]. Perhaps this explains why chicken eggs are less asymmetric and less elliptical than those of most other birds. Interestingly, the researchers did not discover a correlation between cliff-nesting birds and asymmetrical eggs [2]. Conclusion Every day, countless eggs are cracked open in kitchens around the world. The translucent egg white and the yellow yolk fall out of the shell into hot pans, following with sizzling sound, white smoke and the smell of oil. The humble shell gets tossed into the bin in the corner of the kitchen. To the average person, the shell is just a piece of packaging for the egg, something to be discarded without a second thought. Yet, as we have seen, its shape is the result of a delicate interplay between physical forces, biological structures and evolutionary pressures, refined over countless generations. Perhaps this raises a broader question: How many other everyday objects around us carry similarly hidden magnificent science, waiting to be “cracked open”?
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