What cellular changes occur in a zygote during the first 24 hours?

After fertilization, the zygote undergoes a series of changes. Within ~24 hours it initiates its first mitotic division (cleavage) to form two daughter blastomeres. The chromosomes replicate, a cleavage furrow begins, and the zygote moves down the fallopian tube toward the uterus. 

The cytoplasm reorganises, metabolic activity increases, and pronuclei merge before the first cell division. The first cleavage typically happens around 24-30 hours after fertilization. These early divisions are rapid and produce smaller cells without increasing total embryo volume (holoblastic cleavage). These early steps set the stage for multicellular embryo formation.