The mesoderm partially fills the space between the ecto and endoderm. The kidneys, heart, muscles, inner layer of the skin and most other organs develop from the mesoderm. Gastrulation during frog development begins with a small crease on the blastula where the blastopore will eventually form. Invagination occurs due to a cluster of cells burrowing inward. This process produces a dorsal lip where the gray crescent was located on the zygote.
Involution then occurs.
This is the process where cells on the surface roll up and move into the embryo's interior away from the blastopore. The neural tube and notochord are the first organs to develop in frogs and other chordates. The ectoderm above the beginning notochord thickens to form the neural plate that sinks below the the embryo's surface and rolls itself into a neural tube which will become the brain and spinal cord. The notochord stretches the embryo lengthwise and forms the core around witch the mesoderm cells will develop the muscles of the axial skeleton.