During gastrulation, cell movements result in a massive reorganization of the embryo from a simple spherical ball of cells, the blastula, into a multi-layered organism. (Kenyon College)
Gastrulation is the embryonic process of cell differentiation that separates and specializes various cells to various tasks in multicellular animals. Many cells move from a surface position to an interior position creating a multilayered cellular structure.
Formation of gastrula from the monoblastic blastula is called gastrulation. Gastrulation is that phase of embryonic development during which the cells of blastula move in small masses or as a sheet of cells to attain the final location. Such movements of cells is called morphogenetic movement. It is differentiated into two types. (1) Epiboly: The term epiboly is derived from Greek language, meaning 'throwing on' or, extending upon'. (2) Emboly: The term emboly is also derived from Greek language meaning "throw in". Neurulation results in the formation of the neural tube. It gives rise to both the brain and the spinal cord. Neurulation forms neural plate. Neural plate gives rise to neural crest cells.