During which embryonic stage is the blastopore formed?

The formation of the blastopore occurs during the gastrulation stage of embryonic development. Gastrulation is a critical phase where the single-layered blastula reorganizes into a multi-layered structure called the gastrula. This process establishes the primary germ layers: ectoderm, mesoderm, and endoderm, which will later develop into various tissues and organs.

During gastrulation, the blastopore, which is the opening that connects the developing gastrointestinal tract to the outside, forms as cells move inward. This inward movement is essential for the proper organization of embryonic tissues and the establishment of body axes. The fate of the blastopore varies among different organisms; in some, it becomes the mouth, while in others, it becomes the anus.

Other stages mentioned in the options relate to different developmental processes. Neurulation follows gastrulation and involves the formation of the neural tube, while blastulation precedes gastrulation and is when the blastula forms. The archenteron is a structure that forms during gastrulation but is not the stage itself where the blastopore is formed. Thus, gastrulation is the accurate and relevant stage for the formation of the blastopore.