Immediately following fertilisation in Drosophila and many other arthropods, the embryo undergoes a series of rapid syncytial nuclear divisions. These divisions are driven by maternally supplied components and occur in the absence of zygotic transcription. Unlike typical cell divisions, these divisions alternate between S and M phases, resulting in cell cycles that last only from 10 to 25 min. After four rounds of division, the nuclei undergo axial expansion, a process that relies on microfilaments. Subsequently migration of the nuclei to the cortex relies on microtubules. Once at the cortex, the nuclear divisions occur on a single plane and rely on partial cleavage furrows to maintain an even distribution. The cortical nuclear divisions continue until the mid-blastula transition (MBT), at which time cellularisation results in the formation of a multicellular embryo.

Key Concepts:

• Fertilisation triggers a series of events that induces the first mitotic cycle, a gonomeric division between the male and female pronucleus.
  
• After fertilisation, the embryo undergoes 13 synchronous divisions within a syncytium.
  
• Divisions 10–13 occur at the cortex of the embryo and require reorganisation of actin and membrane into metaphase furrows.
  
• At cycle 14, the cell cycle pauses and cellularisation occurs forming individual somatic cells.
  
• Cellularisation, a key feature of the mid-blastula transition, marks the time at which zygotic transcription occurs and maternal products are degraded.