The four stages of an animal's life cycle are an egg, embryo, immature individual and adult. This life cycle is true for most fish, bird, amphibians, reptiles and mammals.
The basic life cycle of an animal starts with an egg, which becomes an embryo upon fertilization. It then develops into an immature individual that resembles an adult, and finally grows into a mature adult. For instance, a bird starts out as an egg. Once the egg is laid, the embryo inside begins to grow. When enough time passes, it hatches, letting out an immature bird. That baby bird eats and grows until it develops into an adult.
Another example is the polar bear, a mammal. Instead of laying eggs, the egg inside the female is fertilized by the male. The fertilized egg becomes an embryo, developing in the uterus of the mother. After four or five months, the female polar bear gives birth to a cub. This immature adult or cub typically weighs no more than 20 ounces. It grows into an adult over the course of five or six years.
Many insects, however, have a different life cycle. The first stage is an egg, and the last stage is a mature adult, as with other animals, but the middle stages are larva and pupa. Once the egg hatches, the larva emerges, not resembling an adult at all. For instance, a caterpillar is a larva of a butterfly. The larva grows into the pupa stage where it transforms into a mature adult.