Which one is not present in Blastula?

Question

Accepted Answer

The correct option is (2): Blastopore

The **Blastula** represents a pivotal stage in early embryonic development, following the initial rapid cell divisions of cleavage (forming the solid morula) and preceding the formation of the distinct germ layers during gastrulation. By definition, a blastula is a hollow sphere of cells characterized by a cellular wall and a central, fluid-filled cavity. The formation of the **Blastopore** is the hallmark event that defines the transition from the blastula stage to the subsequent **Gastrula** stage. Gastrulation involves the massive movement of cells (morphogenesis) during which a pocket or invagination is created in the embryonic wall. The opening of this invagination, the entrance point for the inward-moving cells, is the blastopore. Therefore, because the blastopore only forms as the blastula begins to transform into the gastrula, it is fundamentally an absent structure during the true, established blastula stage.

The incorrect option is (1): Blastomere

The individual cells of the developing embryo, produced by the mitotic divisions of the fertilized zygote, are called **Blastomeres**. Throughout the cleavage stage, from the two-cell stage all the way to the morula and the final blastula, the blastomeres are the building blocks of the embryo. The entire cellular structure of the blastula—its outer wall—is a direct assembly of these blastomeres. An embryo cannot be classified as a blastula without these constituent cells, so blastomeres are universally present.

The incorrect option is (3): Blastoderm

The **Blastoderm** is the layer or wall of blastomeres that encloses the central cavity of the blastula. In many vertebrate and invertebrate species, the term is used to refer to this cellular sheet itself, particularly in cases where the cleavage is incomplete and restricted to a disc of cells atop a large yolk mass (e.g., in birds and fish). The blastoderm is the essential cellular boundary of the blastula, regulating the internal environment and providing the cells that will later undergo invagination to form the germ layers. The presence of this cellular layer is thus integral to the blastula's structure.

The incorrect option is (4): Blastocoel

The **Blastocoel** is the defining characteristic of the blastula stage. It is the fluid-filled, central cavity that forms within the morula as the cells begin to reorganize, marking the end of the cleavage stage and the beginning of the blastula stage. The blastocoel is created either by the movement of blastomeres or by fluid being pumped into the center of the embryo. This cavity provides space for cell migrations that occur during gastrulation. An embryo is identified as a blastula precisely because it possesses this hollow blastocoel.