(c) Both the sperm and the egg contain twenty-three chromosomes.(d) What is the process that reduces pairs of chromosomes to individual chromosomes?(e) What is the process that allows a cell to reproduce itself?Answers: (a) spermatozoon; (b) ovum; (c) single; (d) Meiosis; (e) Mitosis.A chromosomeis 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. Chromorefers to color, and somarefers to body. Thus a chro-
mosome is a “colored body.”
A geneis the basic unit of heredity. It is made up of strands of deoxyri-
bonucleic 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 hered-
itary 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.(a) A rod-like structure containing genes is called what?(b) A gene is the basic unit of.
(c) DNA stands for.Answers: (a) A chromosome; (b) heredity; (c) deoxyribonucleic acid.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 differen-
tiation.
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 ectodermis the outer layer of cells, and it will become
the sense organs, skin, and nervous system. The mesodermis the middle layer of
cells, and it will become the heart, bones, and muscles. The endodermis the
internal layer of cells, and it will become the stomach, intestines, and lungs.(a) When a sperm and an ovum unite to form a fertilized egg the new being is
called a.(b) From one week to seven weeks, the new being is called an.156 PSYCHOLOGY