CONCEPTUAL LIFE SCIENCE
Development
Fertilization
Fertilization is the beginning of life. The sperm and egg unite to form a fertilized
egg, which is also known as a zygote. The zygote begins to develop by undergoing a
rapid series of cell divisions known as cleavage divisions.
Stages of development
Figure 18-1. Early 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 cells at
one end of the embryo. This 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.
18-1
18-2
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 cell layer is
the endoderm. 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.
Table XVIII-1. Embryologic origin of body structures.
Ectoderm
Skin
Epidermis, skin glands, hair, nails, and corresponding
parts of other animals (fur, hooves, claws, scales, etc.)
Lining of the mouth
Enamel of teeth, glands, covering of tongue and lips,
anterior pituitary gland
Nervous system
Brain, spinal cord, cranial and spinal nerves, pineal gland,
sensory parts of the sense organs
Others
Lens of eye, eye muscles, pineal gland, anal canal lining
Mesoderm
Muscles
All types except eye muscles
Skeleton
Bone, cartilage, connective tissue
Excretory organs
Kidneys and their ducts
Reproductive organs
Gonads and related structures
Circulatory system
Heart, blood vessels, blood, lymphatic system, bloodforming tissues
Others
Dentine of teeth, dermis of skin, adrenal cortex, lining of
body cavities, other parts of the eye
Endoderm
Ectoderm forms linings of other organs. Each organ
below is composed of tissues derived from mesoderm,
except for the lining, which comes 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, urethra