Blastuli

Blastuli is the plural of blastulus, referring to blastulae, the hollow, sphere-like stage of an animal embryo formed after cleavage of the fertilized egg. During early development, rapid mitotic divisions—cleavage—increase cell number without overall growth, producing a solid ball called the morula, which reorganizes into a hollow blastula that contains an interior cavity known as the blastocoel.

In the blastula, cells surrounding the blastocoel are called blastomeres. In mammals, the blastula stage is typically termed the blastocyst and features an outer layer of trophoblast cells that will contribute to the placenta and an inner cell mass that will form the embryo proper. In many other vertebrates, the blastula remains a relatively uniform hollow ball, though a blastocoel is still present. The specific organization and timing of these events vary among species and cleavage patterns.

Blastuli precede gastrulation, the process by which the embryonic axes are established and germ layers are formed. The arrangement of cells and signaling interactions within the blastula set up later developmental events that shape the body plan.

Terminology and usage vary by field and species. Some texts use blastula or blastulae more commonly, while blastulus and blastuli appear in certain zoological or historical contexts. Overall, the blastula represents a conserved early phase of embryogenesis across many animal phyla, marking the transition from a unicellular zygote to a multicellular, patterned embryo.