Principles of Biology
By
Frank H. Osborne, Ph. D.
Development
Development
Fertilization
•Fertilization is the beginning of life. The sperm
and egg unite to form a fertilized egg which is
known as a zygote.
•The zygote begins to develop by undergoing a
rapid series of cell divisions known as cleavage
divisions.
Stages of Development
Cleavage
•After fertilization, the egg becomes a zygote.
•The zygote begins a series of rapid cleavage
divisions. During the cleavage divisions, the
embryo remains about the same size as the
original egg. The result is called a morula, which
is a solid ball of cells.
•As the cell divisions continue, the cell sizes
continually become smaller in these stages.
Blastula
•Cell division continues in the embryo. The
blastula is a hollow ball of cells that results from
these divisions.
•Early differentiation is seen because there are
larger cell at one end of the embryo.
Blastula
•The end with the larger cells is called the
vegetal pole. The opposite end with the smaller
cells is called the animal pole.
•The hollow interior part of the blastula is called
the blastocoel.
Gastrula
•Gastrulation is the infolding of the embryo
at the vegetal pole. The result is the gastrula.
•The outside layer of the gastrula is called the
ectoderm. The inside layer is called the
endoderm.
Gastrula
•As cell division and differentiation continue,
the interior space fills up with cells of a third
type called mesoderm. The mesoderm forms
between the other two cell layers.
Differentiation
•Differentiation is the process of a cell or cells
becoming different from the other cells in the
same embryo. The cells become different as
the embryo develops. The primary germ
layers; ectoderm, endoderm, and mesoderm;
give rise to the structures of the body.
Embryologic Origin of Body Structures
Ectoderm
Skin
Lining of the Mouth
Nervous system
Others
Epidermis, skin glands, hair, nails, and
corresponding parts of other animals
(fur, hooves, claws, scales, etc.)
Enamel of teeth, glands, covering of
tongue and lips, anterior pituitary
gland
Brain, spinal cord, cranial and spinal
nerves, pineal gland, sensory parts of
the sense organs
Lens of eye, eye muscles, pineal gland,
anal canal lining
Embryologic Origin of Body Structures
Mesoderm
Muscles
All types except eye muscles
Skeleton
Excretory organs
Bone, cartilage, connective tissue
Kidneys and their ducts
Reproductive organs
Gonads and related structures
Circulatory system
Heart, blood vessels, blood,
lymphatic system, blood-forming
tissues
Dentine of teeth, dermis of skin,
adrenal cortex, lining of body
cavities, other parts of the eye
Others
Embryologic Origin of Body Structures
Endoderm
Endoderm forms linings of other
organs. Each organ below is
composed of tissues derived from
mesoderm except for the lining
which is derived from endoderm.
Alimentary canal
Pharynx, esophagus, stomach,
intestine, liver, pancreas, most of the
colon
Pharyngeal derivatives
Larynx, trachea, lungs, middle ear,
thyroid, parathyroids, thymus
Others
Bladder, utethra
The End
Principles of Biology
Development