Geoscience Reference
In-Depth Information
ates a formidable barrier against bacteria, fungi, and other organisms
that can cause disease. However, the shell cannot be too hard, or the
embryo cannot break out of it when it is ready to hatch. In actuality,
even hard eggshell is quite porous, with microscopic holes that allow
gases and water vapor to pass in and out. Oxygen penetrates through
the pores into the egg so the embryo can breathe, and carbon dioxide
passes out through the pores into the atmosphere.
Inside the egg, a flexible container composed primarily of pro-
teins and fat called the yolk sac (the yellow part of a chicken egg)
contains food for the growing embryo and antibodies to help protect
the embryo from disease. Another membranous sac, the allantois,
serves as a receptacle for waste products. The yolk and allantois are
surrounded by albumen, a water-saturated gel that absorbs whatever
shocks might jostle the embryo developing at the center of the yolk.
The albumen also contains more chemical components to help fend
off dangerous microbes. All these structures work together to keep
the embryo at a constant temperature inside a fluid environment
that cushions it from the extremes and threats of the world outside.
Eggs are typically classified based on visible characteristics of the
egg and the structure of its shell, including size, shape, distribution
Main sites where dinosaur eggs have been found.
