Details about amphibian embryology
Amphibian
embryology:
Most amphibians
lay eggs in watery environments and the eggs are fertilized as the female
releases them. Frog eggs have a pigmented animal pole. Because the vegetal pole
is heavily laden with yolk, the eggs rotate in their jelly coats so that the
less dense, darky pigmented animal pole is oriented up. This rather simple
series of events has interesting adaptive significance. Amphibian eggs develop
with little parental care. The pigmentation helps camouflage developing embryos
from predators. When viewed from below, the light colour of the vegetal end of
floating eggs blends with the sky above. When viewed from above the dark colour
of the animal end blends with the bottom of the pond, lake or stream. The dark
pigment of the animal pole also absorbs heat from the sun and the warming
promote development.
Cleavages/Morula:
The first
longitudinal cleavage begins at the animal pole and divides the grey crescent
in half, because of the large amount of yolk in the vegetal end of the egg
cleavages are slower there than in the animal end.
Morula consists
of many small cells at the animal end and fewer, larger cells at the vegetal
end.
Comparison:
The amphibian
blastula forms in much the same way as the echinoderm blastula except that the
yolky vegetal cells cause the blastocoel to form in the animal half of the
embryo; the blastula wall of amphibians has multiple cell layers.
Gastrulation:
The cells of
blastula that will develop into specific structures are grouped on the surface
of the blastula. During gastrulation some of these cells move into the interior
of the embryo. Embryologists use dyes or carbon particles to mark the surface
of the blastula and then follow the movements of these cells during
gastrulation.
Embryonic cells
are designated according to their future development by “presumptive
notochord”, “presumptive endoderm” and so forth.
The first sign
that gastrulation is beginning is the formation of blastopore between the grey
crescent and the vegetal region of the embryo. Cells at the bottom of the
groove move to the interior of the embryo and the groove spreads transversely.
Superficial cells begin to roll over the dorsal lip of the blastopore in a
process called involution. Cells spread from the animal pole toward the
blastopore and replace those moving into the interior of the embryo.
In the process,
the ends of the slit like blastopore continue to spread transversely and
downward toward the vegetal pole until one end of the slit meets and joins the
opposite end of the slit. A ring like blastopore now surrounds the protruding,
yolk filled cells near the vegetal end of the embryo. These protruding cells are
called the yolk plug. The blastopore is said to have closed. During the closing
of the blastopore two occur movements occur:
(1) The
spreading of cells from the animal pole toward the dorsal lip of the blastopore
and the rolling of cells into the blastopore from the archentrons. As these
mesodermal and endodermal cells roll into the interior of the embryo, the
archentrons become larger and the blastocoel becomes smaller. Gastrulation
results in spreading and thinning of ectodermal cells toward the blastope. In
addition ectoderm spreads over the entire embryo, a process called epiboly.
Mesoderm
formation:
Some of the last
cells to roll over the dorsal lip into the blastopore are presumptive notochord
and presumptive mesoderm. Initially cells makeup the dorsal lining of
archentron near the blastopore. Later they detach from the endoderm and move to
a position between endoderm and ectoderm called chorda mesoderm, will
differentiate into notochord. Lateral to the notochord mesoderm spreads and
thickens along the sides of the embryo. These thickenings called somites are
visible externally as a row of bumps on either side of the embryo.
As mesoderm
continues to spread ventrally it splits to form the coelom and the mesodermal
lining of the body wall and gut.
Neural tube
formation:
During late
gastrulation external changes along the upper surface of the embryo begin to
form the neural tube, a process called neurulation. After gastrulation is
complete on oval shaped area on the dorsal side of the embryo marks the presumptive
neural tube. This region is the neural plate. Microfilaments in neural plate
cells flatters and thicken the neural plate. The edges of the neural plate roll
up and over the mid line of neural plate as neural fold which meet dorsally to
form the neural tube. The portion of the neural tube that will become the brain
is the last to close.
Larva:
With further
development of the mesoderm, the amphibian embryo gradually takes on the form
of a tadpole larva. Yolk in cells lining the floor of the gut is gradually
depleted and the larva is herbivore.
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