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Fertilization and Development
Fertilization is the union (fusion) of a monoploid sperm nucleus (n) with a
monoploid egg nucleus (n).
During meiosis the chromosome number (2n) was reduced.
In fertilization, the resulting diploid zygote has it’s number of chromosomes
restored.
n
+
(male monoploid)
n
(female monoploid)
=
2n
(zygote diploid)
Fertilization must occur in a moist environment.
There are two types of fertilization. They are:
External Fertilization and Internal Fertilization
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External Fertilization
Reproduction of many aquatic vertebrate animals such as
fish and amphibians is characterized by external
fertilization. This is fertilization that occurs outside the body
of the female, in the water. Usually, because of a lack of
protection from consumers and a harsher environment,
large numbers of eggs are released into the water to ensure
species survival.
Internal Fertilization
Reproduction in most terrestrial vertebrates, such as birds,
reptiles, and all mammals, occurs in the moist reproductive
tract of a female. This is internal fertilization. In this case,
only one or two eggs are released, because the
environment is protected.
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2
Embryonic Development
There are a few stages of development after the egg is
fertilized. After fertilization, the fertilized egg (zygote)
undergoes a series of rapid mitotic cell divisions. This is
called cleavage.
Stage 1: Cleavage
During cleavage there is no
increase in cell size, just an
increase in cell numbers.
First the many new cells forms a mass called the
morula. Then, as the mitotic cell division continues, the
cells form a blastula, made of hundreds of cells. The
blastula is a single layer of cells with a hollow ball-type
structure.
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Stage 2: Gastrulation
In certain animals,
one side of the
blastula becomes
indented, forming the
gastrula.
There is an inner layer
called the endoderm.
There is an outer layer called the ectoderm.
There is a third layer between the ectoderm and endoderm
called the mesoderm.
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Stage 3: Differentiation
Three embryonic layers differentiate and give rise to various
tissues, organs, and systems of the multicellular animal.
The muscles, circulatory system, skeleton, excretory
system, and gonads originate from the mesoderm layer.
The nervous system and the skin originate from the
ectoderm layer.
The lining of the digestive and respiratory tracts, and
portions of the liver and pancreas, originate from the
endoderm layer.
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Stage 4: Growth
Growth is an increase in cell number and cell size. Early development consists
chiefly of the differentiation, growth and development of the specialized cells,
tissues, and organs.
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8
External Development
External Development occurs outside of the female’s body in both
terrestrial and aquatic environments.
In Water:
The eggs of many fish and amphibians are fertilized externally and
develop externally in an aquatic environment. The survival rate is
generally low, which accounts for the large number of fertilized eggs
produced. The developing embryo’s source of food is the yolk stored in
the egg.
On Land
Eggs of birds, many reptiles, and a few mammals develop externally on
a land environment after internal fertilization. Since there is a better
survival rate, a somewhat fewer number of fertilized eggs are
produced. The developing embryo’s source of food is the yolk.
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9
Amniotic Egg
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10
Some adaptations for animals which develop externally on
land are a shells. Shells provide protection during
embryonic development. The shell membranes we need
to know are:
Amnion – contains the amniotic fluid. This fluid provides a watery
environment, protects the embryo from shock, and prevents the
tissues from tearing.
Yolk Sac – surrounds the yolk. Blood vessels which penetrate the
yolk sac transport food to the developing embryo.
Allantois – functions as a respiratory membrane and a storage site
for the nitrogenous waste, uric acid.
Chorion – the outer membrane surrounding the other embryonic
membranes that separates them from the environment.
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11
Internal Development
This involves the growth of the embryo within the body of the parent
which provides nutrition and protection. This has a higher survival rate
than external development.
Placental Mammals
The embryo develops in the uterus. The eggs of mammals have little
yolk and are therefore very small.
Within the uterus, an organ called the placenta is formed from the
embryonic and maternal tissues. The placenta is where the exchange
of respiratory gases, nutrients, and wastes occurs between the mother
and embryo.
There is no direct connection between maternal and embryonic
bloodstreams. The blood does not mix. Transport is accomplished by
both active transport and diffusion. An umbilical cord, containing blood
vessels, attaches the developing embryo to the placenta.
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13
Internal Development
Marsupials
Marsupials are non-placental animals. Their internal fertilization and
internal development occur without direct nourishment from the parent.
The source of food is the yolk
stored in the egg.
The embryo is born at a
relatively premature stage
compared to the placental
mammals and completes its
development externally in a
pouch which contains mammary
glands. The mammary glands
produce milk for the very small
baby.
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14
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