Amphibians are land vertebrates
(tetrapods) with aquatic, gill-breathing larvae and lung-breathing
adults that lay their eggs in water.
Origin of land vertebrates (tetrapods): The first tetrapods
(amphibians) evolved from a group of bony fishes
called Crossopterygians, who already had lungs and internal
nostrils. The critical change transformed the fleshy fins into walking legs.
Class Amphibia: Eggs are laid in contact with fresh water, then
fertilized externally. Larvae ("tadpoles") breathe with gills, then undergo
metamorphosis into an adult, usually with lungs and legs. Living species
always have slippery, moist skin. Examples: salamanders, newts,
frogs, toads, and extinct labyrinthodonts.
Embryology: The embryology of amphibians is more easily understood than the
more complex embryology of most other vertebrates.
- Early cleavages divide the entire fertilized egg (zygote) into
2 cells (vertically), then 4 cells (again vertically), then 8 (horizontally).
At the 8-cell stage, the top 4 cells are a bit smaller than the bottom 4.
- The 8 cells divide into 16 cells, then 32. By this time, the larger, yolk-rich
cells in the lower half ("vegetal hemisphere") divide more slowly than the smaller
cells in the upper half ("animal hemisphere"), so division becomes asynchronous,
and this asynchronous division generates an even greater size difference between
the smaller cells above and the larger cells below.
- Blastula: Inside the spherical ball of cells, a blastocoel cavity
appears, somewhat higher than the center. This cavity enlarges, and the hollow ball
of cells is called a blastula.
- Gastrulation: At a location close to the future tail, surface cells begin
tucking in to the interior to form the "dorsal lip of the blastopore,"
with a groove just below it. The groove grows to form a large circular opening
(the blastopore) that leads to an expanding internal cavity called the
archenteron (or "primitive gut").
- Formation of germ layers:
- Ectoderm: The cells remaining on the outer surface of the embryo
form an outer layer called ectoderm. From this layer are formed the
outer layer of skin (epidermis) and the entire nervous system including the
brain, eyes, and ear structures.
- Mesoderm and notochord: Along the roof of the archenteron, cells along an anterior-posterior
body axis begin to form a rod-like structure called the notochord.
The cells to either side of this notochord will form the middle layer or
mesoderm, as described below.
- Endoderm: The cells lining the inside of the archenteron form an inner
layer called endoderm. The inner lining of the digestive tube is formed
from this layer; so, too, are the lungs, liver, and pancreas.
Frog embryology
- Neurulation: As the embryo begins to elongate along an anterior-posterior axis,
"organizer" signals from the notochord cause the overlying ectoderm to roll up
into a neural tube, mostly cylindrical in shape, but swollen at the head end to
form a brain.
While the neural tube is being formed, the cells to either side of the notochord
begin to bud off from the archenteron
to form a series of pouches called somites, which will form the mesoderm
layer between the ectoderm and endoderm. From the mesodermal somites are derived
nearly all the bones and muscles of the body (including the muscular layers of the
digestive tract), the entire circulatory system, the entire urogenital system,
and the deep layer (dermis) of the skin.
- The embryo continues to elongate, and a long muscular tail develops.
Other organs form gradually, until a tadpole larva becomes strong enough to
wriggle out from its jelly-like surroundings.
- After spending some time as an aquatic larva, the tadpole will undergo metamorphosis
into an adult: its legs and lungs will develop, and certain larval structures
(tails in frogs, gills in most amphibians) will be resorbed.
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