How does a human zygote become a morula?

A human zygote becomes a morula through a process called cleavage, which consists of a series of rapid mitotic cell divisions.12

As the zygote travels down the uterine tube, it undergoes these divisions to produce smaller cells known as blastomeres. Because the human egg contains very little yolk, these divisions are equal, resulting in blastomeres of similar size and composition. During this stage, there is no significant growth in the total mass of the organism; the cells simply subdivide into smaller units while remaining enclosed within the zona pellucida, a gelatinous capsule.132

By approximately the fourth day after fertilization, the zygote has developed into a solid cluster of cells—typically consisting of 12 or more blastomeres—known as a morula. The name reflects its resemblance to a mulberry. As the number of cells continues to increase, the morula eventually transitions into a blastocyst before implanting in the uterine lining.14