Mid-Blastula Transition (MBT) in Xenopus occurs at the:
Xenopus early cleavages lack gap phases and transcription, driven by maternal cyclins and stockpiled histones. Nuclear-cytoplasmic ratio increases exponentially as DNA doubles without growth. At approximately 4096 cells after twelve divisions, ratio reaches critical threshold titrating histones and replication factors, triggering mid-blastula transition. Cell cycles lengthen, become asynchronous, acquire G1 and G2 phases, checkpoints become functional, and zygotic genome activates en masse. Motility and responsiveness to induction begin now. MBT timing is independent of absolute time since fertilization, controlled by DNA amount, explaining haploid embryos undergo MBT one cycle later than diploid counterparts.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 5: Mid-blastula transition occurs at 12th cycle in Xenopus.