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.
.jpg)