Final answer:
Gastrulation is the embryonic stage when tissue layers begin to form, resulting in three distinct layers: ectoderm, mesoderm, and endoderm, which will eventually develop into various cells, tissues, and organs of the body.
Step-by-step explanation:
The stage during embryonic development when embryonic tissue layers begin to form is called gastrulation. During this stage, the cells of the embryo migrate and differentiate to form three distinct cell layers: the ectoderm, mesoderm, and endoderm.
These layers are critical because they will develop into different types of cells, tissues, and organs as the embryo grows.
Gastrulation occurs during the second week after fertilization when the two-layered embryonic disc becomes a three-layered disc. Each of the germ layers formed during this process will give rise to specific tissues and organs.
The ectoderm develops into the nervous system and skin cells, the mesoderm gives rise to muscle cells and connective tissues, and the endoderm forms internal glands and organs.
Following gastrulation, the process of organogenesis begins. It's a stage within the first 8 weeks of gestation when the embryo establishes the fundamental structures of all organs and tissues derived from the ectoderm, mesoderm, and endoderm. Hence, gastrulation is crucial to the fetal development process.