Peta O'Connor of Waitahuna asks :-

How do baby birds inside a fluid filled egg, and without an umbilical cord to their mother, survive so long without oxygen?

Alex Davies, a retired anatomist with Massey University's Institute of Veterinary, Animal and Biomedical Sciences, responded.

The newly laid egg of reptiles and birds contains a tiny embryo and a large nutritional store of yolk and albumen. The embryo, as an early priority, must develop a means of transporting food to itself from this larder, a system of vessels outside its body filled with blood pumped by its heart. The shell enclosing the egg must be permeable to oxygen needed for the "burning" of this food, and to the carbon dioxide produced from the process.

The flow of blood just under the shell ensures a concentration gradient of the gases that pass in and out. Therefore a functional heart and blood vessels are needed several days before the embryo looks anything like the animal it will grow into. The rate of diffusion is enough to sustain the growth processes, but not such physical activities as will occur after hatching, when it will be able to breathe and have a better supply of oxygen. The shell must be firm enough to prevent the developing embryo from being crushed and deformed at this delicate stage, but it must not be so firm that it takes more mineral from the mother than she can afford to provide, or too strong for the hatching reptile or bird to break through.

The embryo develops, outside its body, three special membranes within the egg, all a part of itself. These are the amnion that surrounds the embryo itself, the yolk sac, and the allantois that underlies the shell and carries the blood vessels for gas exchange. Animals with these membranes are amniotes. They include the mammals, in which the embryo remains internal and lives off an exchange of nutrients and gases with those in the mother's blood. There is therefore not a great difference in the way such animals first nurture their young, just either the presence of a shell or the ability to contact and invade the wall of the reproductive tract of the mother.

Perhaps the most remarkable and critical thing about life in an egg is the preparation necessary for this adventure. Provisions come from the mother's ovary (the yolk) and her oviduct (the albumen and shell). Regularly, about once a day, she provides another developing embryo for the first part of life's journey with a perfect larder inside a precisely formed permeable container. Provisions must be exactly meted out so that the embryo has enough nutrition and protection, and so that the mother does not deplete her own reserves.

At journey's end, an offspring hatches ready to breathe, eat and walk. Could the production specifications in any factory be more impressive?