Which of the following is a stage of embryonic development in which the embryo is a hollow structure?

The correct choice is the blastula, which is a significant stage of embryonic development where the embryo forms a hollow sphere of cells. During this phase, the embryo consists of a single layer of cells surrounding a fluid-filled cavity known as the blastocoel. This structure is crucial because it sets the stage for further development, including gastrulation, where the cells will begin to differentiate and organize into distinct layers that will eventually develop into various tissues and organs.

In contrast, the gastrula stage follows the blastula and involves complex cell movements, leading to the formation of the three germ layers (ectoderm, mesoderm, and endoderm), not a hollow structure. The fetus is a later stage in development when the main structures of the organism are already formed and is characterized by more complex development, typically no longer being in a hollow form. The neural tube, meanwhile, is a structure that develops from the ectoderm and gives rise to the central nervous system, but it is not a hollow structure like the blastula. Understanding these stages is essential for grasping the overall process of embryonic development.