A blasula is an early stage in embryonic development, specifically the stage following the cleavage of a fertilized egg. It is characterized by a hollow ball of cells, known as the blastoderm, surrounding a fluid-filled cavity called the blastocoel. In vertebrates, the blasula eventually undergoes further differentiation and development, leading to the formation of the gastrula and subsequent embryonic structures. This stage is crucial for the proper organization and formation of tissues in the developing embryo.