Cleavage Stage, Ovum

**Definition:** The earliest developmental stage of a fertilized ovum (ZYGOTE) during which there are several mitotic divisions within the ZONA PELLUCIDA. Each cleavage or segmentation yields two BLASTOMERES of about half size of the parent cell. This cleavage stage generally covers the period up to 16-cell MORULA.

**Tree numbers:** - A16.166

**Synonyms:** - Segmentation Stage, Ovum