The Incredible Egg, Symbol of Spring
From ancient times to the present, humans have seen the egg as a symbol of birth, fertility, and life. For example, our modern day Easter Rabbit brings chocolate eggs and painted eggs to children, a custom rooted in eggs’ symbolism for life and resurrection, both in pagan and Christian times. Simple in appearance yet rich in symbolism, eggs were used in different cultures either to divine the future, explain the creation of the world, or as a wedding gift that newlyweds would exchange with one another. The ability of the egg to bridge life from one generation to another has inspired myth and wonder. How can a seemingly modest package, elegant in design and yet fragile, achieve such mythic proportions?
From a scientific perspective, we can explain the formation of the egg. In this case, we start with the chicken.
The ovary contains the eggs, which are released throughout a hen’s reproductive life. A given egg traverses the oviduct in about 25 hours. The raw materials of the yolk (protein and lipids) are synthesized in the liver and travel in the blood plasma to the granulosa cells, which then pass them to the oocyte (vitellogenesis). The oocyte reorganizes them into yolk spheres and fluid. There is no biochemical synthesis of yolk in the oocyte itself.
The ovum is released by the ovary where it is captured by the opening into the oviduct, the infundibulum. The egg passes through the infundibulum in about 15 minutes, where tubular glands lay down the chalaziferous layer of the albumen. This thin layer of dense albumen surrounds the yolk. The tubular part of the infundibulum forms the chalaza at each end of the egg. The chalaza at the sharp end is a double strand, while that at the blunt end is a single strand. The chalazae appear to suspend the yolk between the two ends of the egg. They become twisted, probably from the rotation of the egg as it travels down the oviduct.
The egg takes about three hours to traverse the magnum, a large section of the reproductive tract. During this time it acquires albumen secreted by the tubular glands, with a contribution also from the unicellular glands. Sodium, magnesium and calcium are added mainly in the magnum.
Movement of the developing egg slows as it transits the isthmus, taking about 75 minutes in all. The inner and outer shell membranes that eventually line the shell are the main components that are formed here (from the tubular glands). Before these are laid down, however, a small amount of protein (about 10 per cent of the total) is added to the albumen. Calcification of the shell appears to be initiated in the isthmus.
The egg occupies the uterus for about 20 hours, during which time the shell (i.e., the testa, cuticle and pigment) is formed. Plumping occurs here. This consists of the rapid addition of watery solutions (probably from the glands of the relatively short and narrow cranial part of the uterus) into the egg, mainly during the first eight hours, doubling the weight of the albumen. During plumping, calcification of the shell is slow, but during the last 15 hours it is rapid.
Calcification is remarkable in that every 15 minutes the uterus withdraws from the blood a weight of calcium equaling the total amount circulating at any one moment. The extreme vascularity of the uterus presumably contributes to this surprising activity. Potassium is added mainly in the uterus. Usually, in the domestic fowl the egg lies in the uterus with its sharp end pointing caudally, and the egg is laid with this orientation. In some species, such as gulls and ducks, the egg in the uterus turns around just before oviposition, so that the blunt end comes out first. The biological significance of this rotation of the egg, and the way the muscles of the oviduct and its ligaments achieve it, are not known. The egg travels through the vagina in a matter of seconds.
The vagina is a conduit for the egg to pass from the oviduct through the cloaca and out the vent. The process of expelling the egg is termed oviposition. The time required for oviposition varies among species. The vaginal sphincter separates the uterus and vagina. This sphincter relaxes at the onset of oviposition. It is the smooth muscles of the uterus that propel the egg through the vaginal sphincter and into the vagina. The presence of the egg produces neuronal stimulation. This causes a bearing down reflex of the cloaca, passing the egg out through the vent. Additionally, there are folds in the region of the sphincter, the spermatic fossulae or the sperm host glands. These glands represent the primary site for sperm storage, an important adaptation to allow for rapid fertilization immediately after ovulation. When these sperm are released from these glands they swim up the oviduct to the infundibulum to be ready to fertilize the egg when it is released at ovulation. Ovulation often occurs shortly after oviposition.
Obstruction of the oviduct is relatively common in both domestic and wild birds. The most common causes include the presence in the oviduct of necrotic material, a broken-shelled egg, a normal soft-shelled egg or localized infection. Amongst wild species a common factor associated with obstruction of the oviduct is senility.
So tucked away beneath the smooth surface of an egg is a remarkable process that allows life to start anew. Fragile, yet enduring, the egg allows life to span each generation to another, in an endless chain. No wonder the egg has become the enduring symbol of Spring.