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CONCEPTUAL LIFE SCIENCE
DevelopmeDt
Fertilization
Fertilization is the beginning ofllfe. 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 ofcell divisions known IS cleavage divisions.
Stages ofdevelopment
•
Fertnlzed Elg
2<ell sta.e
(1 st cle.age)
(ZJGote)
•
8-ceUstale
(ard cleavage)
a
•
Morula
•
4-4:ell sta...
(2nd cleaya.e)
Blastula
Q
Gastrula
Figure 18·1. Early stages ofdevelopment.
Cleavage
After fertilization, the egg becomes a zygote. The zygote begins a series of rapid
. cleavage divisioDS. During the cleavage divisions, the embryo Rmains about the same
size IS the original egg. The result is called a morula, which is a solid ball ofcells. As
the eell divisions continue, the ceD sizes continually become smaUer· in these stages.
Blastula
Cell division continues in the embryo. The blastida is a hollow ball ofcells that
results from these divisions. Early differentiation is seen because there are larger cells at
oneend of the embryo. This end with the larger cells is called the -vegetalpole. The
opposite end with the smaller ceDs is called the animal pole. The hollow iDterior part of
the blastula is called the blastocoel.
18-1
18-2
Gastrula
Gastrulation is the infolding ofthe 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 ofa third type tailed mesoderm. The mesoderm forms between the othq 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 XVm-1. Em
Skin
Lining ofthe mouth
Nervous system
d, 8D8l C8DBl r
Lcnsof
muscles
e CODDeCtive tissue
Others
Endoderm
Alimentary C8DBl
Pharyngeal derivatives
.