Download Fertilization

The Egg
By the next day, continued cleavage has produced a hollow ball of thousands
The frog egg is a huge cell; its volume is over 1.6 million times larger than a
it.
into a tadpole containing millions of cells but containing the same volume of
During this entire process
of cells called the blastula. A fluid-filled cavity, the blastocoel, forms within
normal frog cell. During embryonic development, the egg will be converted
material.
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The upper hemisphere of the egg — the animal pole — is dark.
of the blastula are so small, the blastula looks just like the original
egg to the unaided eye.
The lower hemisphere — the vegetal pole — is light.
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When deposited in the water and ready for fertilization, the
been run by gene products (mRNA and proteins) deposited by the
mother when she formed the egg.
Fertilization
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Meiosis II is completed.
The cytoplasm of the egg rotates about 30 degrees relative to the
poles.
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In some amphibians (including Xenopus), this is revealed by the
Gastrulation
The start of gastrulation is marked by the pushing inward ("invagination") of
cells in the region of the embryo once occupied by the middle of the gray
crescent. This produces:
appearance of a light-colored band, the gray crescent.
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The gray crescent forms opposite the point where the sperm
entered.
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The haploid sperm and egg nuclei fuse to form the diploid zygote
As gastrulation continues, three distinct "germ layers" are formed:
nucleus.
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Fertilization
1.
a cluster of cells that develops into the Spemann organizer (named
remarkable inductive properties). This becomes the notochord
ventral (V) surfaces; its anterior (A) and posterior (P); its left and
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an opening (the blastopore) that will be the future anus
after one of the German embryologists who discovered its
It foretells the future pattern of the animal: its dorsal (D) and
right sides.
Not until the blastula contains some 4,000 cells is there any
transcription of zygote genes. All of the activities up to now have
haploid egg is at metaphase of meiosis II
Entrance of the sperm initiates a sequence of events:
there has been no growth of the embryo. In fact, because the cells
ectoderm
mesoderm
endoderm
Frogs practice external fertilization, meaning the male fertilizes
eggs outside the female’s body. Some frog eggs after fertilization develop
light-gray bands around them. The band expands on the opposite of the
egg where the sperm cell entered. The way the band fans out determines
in which direction the frog will develop inside the egg. During
fertilization, the sperm and egg nucleus fuse into a diploid zygote
nucleus.
Cleavage
The zygote nucleus undergoes a series of mitoses, with the resulting
daughter nuclei becoming partitioned off, by cytokinesis, in separate, and
ever-smaller, cells. The first cleavage occurs shortly after the zygote nucleus
forms.
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Some are listed in the table.
Germ-layer origin of various body tissues
Ectoderm
Mesoderm
Endoderm
skin
notochord inner lining of gut, liver, pancreas
brain
muscles
inner lining of lungs
spinal cord
blood
inner lining of bladder
all other neurons bone
sense receptors
thyroid and parathyroid glands
sex organs thymus
The Spemann organizer (mostly mesoderm) will:
A furrow appears that runs longitudinally through the poles of
the egg, passing through the point at which the sperm entered
and bisecting the gray crescent.
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Each of these will have special roles to play in building the complete animal.
This divides the egg into two halves forming the 2-cell stage 4 8
16 32 cell stage
However, as cleavage continues, the cells in the animal pole begin dividing
more rapidly than those in the vegetal pole and thus become smaller and
more numerous.
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develop into the notochord, which is the precursor of the
backbone;
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induce the ectoderm lying above it to begin to form neural tissue
instead of skin.
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This ectoderm grows up into two longitudinal folds,
forming the neural folds stage.
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In time the lips of the folds fuse to form the neural
tube.
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The neural tube eventually develops into the brain and
spinal cord.