GASTRULATION
The mechanism by which the blastula is transformed into a three-layered embryo, or gastrula, is called gastrulation. Early development passes through the following stages:

Zygote early cleavage stages morula blastula gastrula

During gastrulation, the embryo starts to take on a rough approximation of the arrangement of its body as cells distribute themselves into three distinct germ layers, or embryonic tissue layers:

• the ectoderm (outermost layer)
• the endoderm (innermost layer)
• the mesoderm (middle layer)

Throughout gastrulation, additional cell divisions occur, causing many cells to lose their old cell-to-cell contacts and establish new ones. Cell recognition and adhesion processes involving complex interactions among the integrins and other plasma membrane proteins take place. Many cells undergo cytoskeletal changes, especially alterations in the distribution of actin microfilaments.