Amphibian Embryonic Development and Organogenesis

Organogenesis in Pluteus Larvae

From the formation of the Pluteus larva, individual organs will separate. As different body organs develop, the larva grows until it reaches the normal adult size of the species.

Amphibian Embryonic Development

Amphibian fertilization is external; the female lays eggs enveloped in a gelatinous mass, and the male ejects sperm onto the eggs to fertilize them.

• The eggs are mesolecithal (they have a moderate amount of yolk).

Segmentation and Cleavage

The first meridional division occurs 2-3 hours after fertilization. The second is made by a plane perpendicular to the first. The segmentation is holoblastic (total) and unequal. Eight blastomeres are originated: 4 small micromeres and 4 large macromeres.

The Blastula Stage

The inner blastocoel is formed; at the top is an arched wall 3 to 4 cells thick, while the bottom is occupied by a mass of cells rich in yolk. These layers of cells surrounding the blastocoel are called the blastoderm.

Between the animal pole and vegetative pole, a process occurs which causes a circular ring of small micromere cells called the germ ring.

Gastrulation and Germ Layer Formation

It starts with the formation of a small groove at the germ ring, which is the dorsal lip of the blastopore. The multiplication of cells produces the invagination of cells into the segmentation cavity.

This movement originates the Archenteron or primitive gut. The groove deepens into a hole called the blastopore that communicates the Archenteron with the outside. As gastrulation proceeds, the ventral and lateral lips of the blastopore form.

The movement of cells from the animal pole to the vegetative pole occurs because of the higher rate of multiplication of cells in the animal pole; this is called epiboly. This movement causes the macromeres to obstruct the entry of the blastopore for some time, forming the yolk plug.

At the end of gastrulation, three embryonic layers are formed: ectoderm, mesoderm, and endoderm.

Organogenesis and Neural Development

The neural plate is formed; the edges of the plate fold to form the neural crest, which closes to form the nerve canal. Below the neural plate, the notochord forms at the lateral sides.