What is the embryonic stage called in animals characterized by the development of a body plan?

The embryonic stage characterized by the development of the body plan in animals is known as the gastrula stage. During this phase, significant morphological changes occur as the blastula undergoes gastrulation, resulting in the formation of the three germ layers: ectoderm, mesoderm, and endoderm. These germ layers give rise to various tissues and organs in the developing organism, laying the groundwork for the body structure.

In contrast, earlier stages like the blastula simply represent a hollow sphere of cells that have not yet differentiated into layers, while the terms fetus and embryo refer to later stages of development after the body plan has already been established. Ultimately, the gastrula stage is crucial for initiating the specific developmental pathways that lead to the formation of an organized body plan, making it a key milestone in embryonic development.