The process of getting a full organism from its body part is called regeneration. This means that, in these organism, new organism can be reproduce from their cut body part.
The simple animals like hydra & planaria show regeneration. If hydra or planaria come how get cut into number of pieces, then each body piece can grow into a complete organism.
The regeneration of an organism from its cut body part occurs by the process of growth & development.
The cells of cut body part of the organism divide rapidly to make a ball of cells.
The cells present in the ball of cells move to their proper places within the ball where they have to form various organs.
The cells then change their shapes (or become specialized) to form different types of tissues.
These different tissues form various organs & body part of the organism. In this way a complete organism is regenerated.