Formation of notochord:
Mesodermal cells are the mid dorsal axis form a dorsal rod like structure is called notochord. It acts as a supporting organ of the embryo.
Formation of nerve cord:
The ectodermal cells at the mid-dorsal axis just above the notochord sink inward to form a neural tube is called nerve cord. The nervous system develops from nerve Cord. At this point, the embryo is called neurula.
The neurula rapidly enlongates to form a tail. Different organs and systems gradually develop from the three germinal layers. The embryo now hatches and is termed as Tadpole larva. It has a fish-like body having external gills which are soon replace by internal gills. It swims in water and feed upon aquatic vegetations.
After an active free swimming phase, the tadpole larva passes through a series of rapid changes that transform it into the tailless adult frog.
“The process involving series of changes that transform tad-pole larva into adult is known as metamorphosis.