What embryonic stage follows the blastula?

The stage that follows the blastula in embryonic development is the gastrula. The blastula is a hollow sphere of cells formed after several rounds of cell division (cleavage) and is characterized by its fluid-filled blastocoel. Following the blastula stage, the embryo undergoes a process called gastrulation, during which the cells rearrange to form distinct germ layers: the ectoderm, mesoderm, and endoderm. This process leads to the formation of the gastrula, which establishes the foundational structure for further development of body plans, organ systems, and ultimately, the various tissues of the organism.

In contrast, the fetus refers to a later stage of development after the embryonic stages have completed, where significant maturation and growth occur. The neural tube is an embryonic structure that later becomes the central nervous system, but it develops from the ectoderm during gastrulation and is not a separate stage that follows the blastula. The embryonic stage is a more general term covering both the blastula and gastrula, so it doesn't specifically represent the stage following the blastula. Thus, the gastrula is the correct answer as it directly follows the blastula in the sequence of embryonic development.