Final answer:
The germ layers, endoderm, mesoderm, and ectoderm, are formed during the gastrulation phase of embryonic development, which is distinct from the fetal development, zygote formation, or blastocyst stage.
Step-by-step explanation:
Gastrulation Phase and Germ Layers
The germ layers, which include the endoderm, mesoderm, and ectoderm, are formed during the gastrulation phase of embryonic development. Gastrulation is a critical step that occurs after the formation of the blastula and involves the inward folding of the blastula to create a gastrula with these three distinct layers. These layers eventually give rise to different organ systems during development: the ectoderm leads to the formation of the nervous system and skin, the mesoderm gives rise to the muscles and circulatory system including the heart, and the endoderm forms the digestive system and other internal organs.
More specifically, this developmental stage is not during fetal development, zygote formation, or the blastocyst stage. It is a separate and pivotal phase where the germ layers are established and cells begin to differentiate to form the tissue types and organs of the developing embryo.