Individual from birth to the beginning of adolescence (usually around the age of

twelve or thirteen). Adolescent psychology is also a subset of developmental

psychology. It concerns itself primarily with the study of the individual from the

beginning of adolescence to its end (usually around the age of eighteen). Sometimes

child psychology refers loosely to both child and adolescent psychology.

Biological Aspects of Development: From Fertilized Egg

to Infant

Freud said, “Biology is destiny.” Although Freud is usually thought of as a psychologist, not a biologist, his early academic love was the study of biology. He

was trained as a biologist before he became a medical doctor. Freud’s statement

recognizes that, although learning and experience shape behavior, much of our

behavior is based on a foundation of genetic givens. For example, if a fertilized

egg contains an XX chromosome pattern, the individual will become a female.

If the fertilized egg contains an XY pattern, the individual will become a male.

The fact that one is a female or a male will be an important determining factor

in countless behaviors from birth to death. For a second example, let’s say that

a fertilized egg contains three chromosomes where normally there is a twentyfirst

pair of chromosomes. This is a chromosomal anomaly known as trisomy

21. The individual will suffer from Down’s syndrome, a pattern characterized

by mental retardation and poor health. Freud’s view that biology is destiny has

much to recommend it. (A chromosomal anomaly is an abnormal chromosome

pattern.)

The individual begins when a given sperm and a given ovum unite. Provided

by the father, the sperm, or more completely spermatozoon, is a highly mobile

cell with a tail. Provided by the mother, the ovum is a single egg cell. Both the

sperm and the ovum contain twenty-three single chromosomes. When the egg is

fertilized, there will be twenty-three pairs of chromosomes. Meiosis is the process

that reduces pairs of chromosomes to the individual chromosomes found in either

the sperm or the ovum. Mitosis, on the other hand, is the process that allows a

cell to reproduce itself. This process starts with twenty-three pairs of chromosomes,

and all twenty-three pairs are replicated. It is mitosis that makes possible

the growth of the individual from one cell, the fertilized egg, to billions of cells.

A chromosome is a rodlike structure that contains genes. A chromosome is

so named because it is capable of picking up a dye, making the structure visible

under a microscope. Chromo refers to color, and soma refers to body. Thus a chromosome

is a “colored body.”

A gene is the basic unit of heredity. It is made up of strands of deoxyribonucleic

acid (DNA), a complex organic molecule with the unique ability to

replicate itself. It is the genes that do all of the active work associated with hereditary

influence. The relationship of a chromosome to a group of genes is similar to

the relationship of a ship to its crew. The chromosome is the ship. The genes are

the members of the crew.

There are four stages associated with conception and birth: (1) zygote, (2)

embryo, (3) fetus, and (4) neonate. When a sperm and an ovum unite to form a

fertilized egg, the new being is called a zygote. The stage of the zygote lasts for

one week. During this stage the zygote develops rapidly from a single cell to a

large group of cells. A zygote may be imagined as a ball of cells without differentiation.

From one week to seven weeks, the new being is called an embryo. As the

cells continue to divide and replicate themselves, some differentiation begins to

take place. Three basic embryonic layers emerge: (1) ectoderm, (2) mesoderm,

and (3) endoderm. The ectoderm is the outer layer of cells, and it will become

the sense organs, skin, and nervous system. The mesoderm is the middle layer of

cells, and it will become the heart, bones, and muscles. The endoderm is the

internal layer of cells, and it will become the stomach, intestines, and lungs.

From seven weeks to birth, the new being is called a fetus. Fetal development

is rich and complex. The cells continue to divide, and they become specialized in

their structures and functions. Brain cells (neurons), skin cells, hair cells, fat cells,

and many other kinds of cells form. The head, limbs, fingers and toes, and other

features of the body appear. In the typical case, the stage of the fetus lasts a little

over seven months, making the total time from conception to birth about nine

months.

At birth the new being is called a neonate. Neo means “new.” And nate means

“birth.” Thus the word neonate simply means “newborn.” If the neonate loses

weight after birth, then he or she is not referred to as an infant until birth weight

has been regained. The word infant is from Latin roots meaning “without

speech.”