Growth and development
Fertilization is followed by development and growth. As you have read, the offspring of all multicellular organisms originate from the zygote, a single cell, which gradually becomes a complete individual. The changes by which a zygote is transformed into an adult are called growth and development.
Growth and development in higher animals
The zygote divides and redivides to form a cluster of cells. Then the cells undergo differentiation, a process by which different group of cells become structurally and functionally different, to give rise to different tissues, and then different organs and organ systems. Side by side, the cells keep dividing and growing in size. This is how the zygote grows into the embryo, the embryo into the baby, and the baby into the adult.
The entire process s called growth and development, or morphogenesis. Growth, one can say, is a permanent increase in size, which is usually accompanied by an increase in weight. Development comprises the changes in shape, structure and complexity that go hand in hand with growth.
In mammals, the embryo develops within the mother’s uterus. In the case of birds and reptiles, fertilization is internal, but the embryo develops outside the mother’s body. The mother lays the fertilized eggs, which she protects until the babies inside them have developed fully. The eggs then hatch and the young ones emerge.
An individual continues to grow until a particular age and passes through various stages from birth to death. The stages that human beings go though are discussed later.
Metamorphosis
Development takes place in various ways among different insects. Insects like mosquitoes, flies, silkworms and butterflies lay eggs which have been fertilized. The new organism that hatches from the egg goes through various stages of development. The first stage is the larva. The larva, which is capable of feeding itself, develops into the pupa. It is during this stage that the growing organism develops the features of an adult, though it appears to be at rest. The pupa grows within a covering called the cocoon or the chrysalis a chrysalis is like a hard shell, while a cocoon is soft and silky.
Neither the larva nor the pupa resembles the adult insect. The transformation the larval to the adult stages is called metamorphosis.
The butterfly’s larva is called a caterpillar. It grows by feeding on leaves. When it becomes too large for its skin, the skin is cast off to allow a large caterpillar to emerge. This process, called molting, takes place several times until the caterpillar surrounds itself with a cocoon and transforms itself into the pupa. The pupa undergoes many changes and emerges as a butterfly after breaking open the cocoon.
Tadpoles, which hatch out frog’s eggs, are also a type of larva. A tadpole looks like a fish and breathes trough gills instead of lungs. As it transforms itself into a frog, it sheds its tail and the gills are replaced by lungs.
