A blastopore is evident during which developmental stage?

The presence of a blastopore is a key characteristic of the gastrula stage of embryonic development. During gastrulation, cells begin to migrate and reorganize, leading to the formation of the three primary germ layers: ectoderm, mesoderm, and endoderm. The blastopore is the opening that forms during this process and ultimately contributes to the development of the digestive tract in the organism.

In contrast, the zygote is the very first stage after fertilization, where the fertilized egg begins its initial divisions. The blastula represents a later stage characterized by a hollow ball of cells, known as a blastocel, but does not yet exhibit a blastopore. The eight-cell stage is even earlier, where the zygote has divided into eight cells and is still very much in the early stages of cleavage.

Thus, the correct answer highlights the crucial developmental milestone of the gastrula stage, where the establishment of the blastopore plays a fundamental role in defining the body plan of the organism.