Life Science: Session 3
The Incredible Egg
What is an egg?
When we talk about an egg, we’re usually referring to the kind that you can buy at a supermarket — a chicken egg. The word “egg,” however, means something different in biology. The scientific definition of the word “egg” is the female sex cell or ovum, also called a gamete. The fertilized egg, also known as the zygote, is formed when sperm, the male gamete, fuses with the egg, the female gamete.
“Egg” also refers to the physical structure produced by many animals that houses a developing embryo. Such eggs are produced by fish, birds, reptiles, and amphibians. There are even mammals that produce eggs, such as the duck-billed platypus.
How do eggs vary?
There is an enormous diversity of adaptations found among egg-producing animals. In aquatic environments, fish often produce hundreds of eggs. Because they do not take care of their offspring, it is better to produce more eggs to ensure some survive and escape predators. In organisms that invest parental care, such as birds, fewer eggs are typically produced. Biologists consider this a trade-off: more eggs with less energy devoted to care or fewer eggs with more energy devoted to care. In each case, the adaptation is successsful only if an appropriate number of young survive to reproduce again.
Another variable feature having to do with eggs is the yolk, which feeds the developing embryo. The investment in yolk, which is high in protein and other nutrients, is often greater in those organisms with fewer total eggs. Some animals produce eggs containing little yolk. In these cases, offspring hatch relatively quickly and are immediately able to fend for themselves and procure food. The contents of an egg thus serve as an important food source for some species but not for others.
An additional adaptation commonly seen in egg structure is their resistance to drying. We can all relate to cracking open a chicken egg. In fact, the eggs of all terrestrial organisms, such as birds and reptiles, have hard shells. The hard shell prevents the egg from drying out and protects the developing embryo. This is in contrast to soft, jelly-like eggs seen in fish and amphibians, which are aquatic organisms.
There are other differences among animals: The eggs described above hatch outside of the body. Some animals carry accessory egg “sacs” that keep the fertilized eggs with one of the parents. In most fish and amphibians, the female will possess the fertilized eggs although there are some species where the male carries them.
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