Stage 4 (gastrulation)

Gastrulation is an early stage of embryo development where the single-layered blastula transforms into a multilayered structure called the gastrula. During this process, cells move and reorganize dramatically, forming three primary layers: ectoderm (outer layer, developing into skin and nervous system), mesoderm (middle layer, forming muscles, bones, and blood), and endoderm (inner layer, creating internal organs). These layers set the foundation for all the organs and tissues in the body. Essentially, gastrulation is the shaping process that establishes the basic body plan of the developing organism.