35 Life Cycles-Mid

Life Cycles Written by Ned Jensen

Introduction Another word for a living thing such as a plant or animal is an organism. Most organisms go through several stages from the very beginning of their lives to the end of their lives. In each stage, the organisms change. The stages an organism goes through are called its life cycle. You are an organism, too. You have gone through stages of change. And you will continue to change as you grow older.

Organisms of the same type go through the same stages of change. Humans, dogs, cows, and bears are all mammals. They all go through the same kinds of stages. Different types of animals, such as birds, frogs, and snakes, have different kinds of life cycles. They go through different stages of change. The life cycles of plants are different from the life cycles of animals. They go through stages of change, too. As you read this book, you will learn how the life cycles of these organisms are different.

Human Life Cycle Let's begin with a life cycle that is important to you— the human life cycle. The human life cycle begins with a tiny egg. This egg grows inside a mother organism. The egg soon becomes something called an embryo. After about two months, the human embryo begins to look like a human. It is still very small, but it has a head, arms, legs, and other body parts. For another seven months, the human organism develops inside its mother's body.

After nine months inside its mother, the baby is born. This stage is called infancy, and the organism is called an infant. An infant needs its mother for everything. It cannot walk, talk, or feed itself. Word Wise Here are three prefixes that represent numbers. tri = 3 quad = 4 quint = 5 A mother may have more than one egg growing inside her at the same time. If two eggs are growing, twins will be born. Three eggs will produce triplets. Four eggs will produce quadruplets. And five eggs will produce quintuplets. Sometimes one egg splits into more than one embryo. When this happens, more than one baby is born.

At about two years old, an infant enters a stage called childhood. Childhood lasts for about ten years. Then a child enters a stage called adolescence. In this stage, a person can start to make babies. A human is an adolescent until it is about 18 to 20 years old. Then the adolescent becomes an adult. Adulthood is the longest stage in the human life cycle. A human is an adult for the rest of his or her life. Do You Know? The oldest known human was a French woman who lived 122 years.