11 This is about eight orders of magnitude larger than the human egg cell. In other words, if the human egg cell were the size of a tennis ball, the yolk would be roughly the size of a large lecture hall. As the yolk passes through the oviduct, it slowly rotates, allowing egg white (albumen) to be deposited evenly around it [1]. The section toward the end of the hen's oviduct is called the isthmus. In this section, two layers of shell membranes form outside of the yolk, which serve as the foundation of the eggshell. The membraned egg then enters the shell gland (uterus). Calcite (calcium carbonate) crystals deposit onto the membrane, forming the eggshell [1]. What About the Shape of the Chicken Egg? Like hen eggs, it is well known that eggs are generally pointed at one end and blunt at the other. Scientists have long speculated on the evolutionary reason behind this shape. One well-known hypothesis is that asymmetric eggs are selected among cliff-nesting birds as they roll in tight circles, which prevents egg loss [2]. In 2017, a large-scale study collected egg data of 1,400 avian species to study the evolutionary reasons behind the shape of the egg. They found that the shape of eggs varies on a spectrum of asymmetry and ellipticity, ranging from spherical, symmetrical eggs on the one end to asymmetrical and elliptical eggs on the other end. Interestingly, when scientists plotted a density map of the 1,400 species, most bird species lay more asymmetric and elliptical eggs than chickens do [2]. With a wide spectrum of egg shapes, what is the biological mechanism behind the shape of an egg? The Biological Mechanism Behind Egg Shape Scientists hypothesized that the shape of the egg is largely determined by the shell membrane [2]. Isn't it surprising? At first glance, the hard, thick eggshell seems to be the primary structure that upholds the egg. Yet, when the eggshell is dissolved by vinegar, the shape of the egg remains. If the eggshell were the sole determinant of the egg shape, the egg would collapse upon the removal of the eggshell. So, if the eggshell does not determine the shape, what does? An x-ray radiographic study has shown that a hen egg that has just passed into the shell gland has already taken up a well-defined shape [3]. Based upon these observations, scientists developed a biophysical model able to predict the shape of a membranous egg with two factors: pressure difference across the shell membranes and variations of membrane properties. The pressure difference across membrane may arise from the fluid pressure within the egg sac, the elasticity and muscular contraction of the isthmus wall. Differences in how easily the membrane Shaped the Way It Is? 偏偏長這樣？
RkJQdWJsaXNoZXIy NDk5Njg=