Breathing Inside an Egg
Tucked away inside its shell, a bird embryo has to breathe. But, rather than using lungs to draw air in and push carbon dioxide and water vapor out, a bird embryo relies on“diffusion”- -the natural movement of gases- -much as do insects (which also lack lungs). In fact, both insects and eggs use tiny pores, or holes, and pore canals that connect the outside with their interior. For birds there are hundreds or thousands of tiny pores distributed all over the shell surface. The pores connect, via a narrow tube, the embryo’s blood supply to the outside world. The number of pores per egg varies markedly between species, partly but not entirely related to the size of the egg. Since the pores are fairly straight and run vertically from the inner to the outer surface, their length is usually similar to the thickness of the shell. Generally, the number and size of pores determine how much and how fast oxygen diffuses into the egg. As well as taking away unwanted carbon dioxide, the pores allow water vapor to escape from the developing embryo. As the embryo grows it generates water, referred to as metabolic water, produced as a result of the metabolism of food. Different foods generate different amounts of metabolic water. Fat, for example, yields comparatively high amounts of metabolic water.
完整版题目和答案请付费后查阅
