Tim Taylor is a senior writer at Innerbody Research focusing on human anatomy and physiology. Tim earned both his Bachelor of Science degree in Biology and his Master's degree in Teaching from the University of Pittsburgh.
The two-cell stage occurs with the first cleavage of the zygote. As it reaches the 30-hour mark (that is, 30 hours after fertilization), the zygote undergoes additional divisions as it moves toward the uterus (endometrium).
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