36 Life Cycles-High

Life Cycles Written by Ned Jensen

Introduction Most living things, or organisms, go through several stages during their lives. During each stage, these organisms change. When they reach the adult stage, they often produce new living things of their own type. The adult stage ends when an organism dies. The stages an organism goes through are called its life cycle. All plants and animals go through changes from the very beginning of their lives to the end of their lives. They all have life cycles. You, too, are an organism. 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, cats, dogs, cows, and bears are similar organisms. They are called mammals. Mammals go through similar stages. Different types of animals, such as birds, frogs, and snakes, have different kinds of life cycles. They go through different stages of change. Plants have life cycles that are different from all animal life cycles. This book is about the life cycles of different organisms. As you read you will learn how the life cycles of these organisms are different.

Human Life Cycle Let's begin by looking at 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 a small embryo. An embryo is a bunch of cells that are just starting to grow into a baby organism. 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 human body parts. For another seven months, the human organism develops inside the mother's body.

After nine months inside the mother, the baby is born. At this time, the stage is called infancy, and the organism is called an infant. An infant depends on 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.

At around two years of age, an infant leaves the infancy stage and enters a stage called childhood. Childhood lasts for about 10 years. Then a child enters a stage called adolescence. In this stage, a human develops the ability to make a baby. A human is an adolescent until about 18 to 20 years of age. The adolescent then becomes an adult. The adult 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.