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How does a fertilized mammal egg develop into a morula?

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A series of medical illustrations showing the stages of childbirth: the baby's position at the onset of labor, flexion, internal rotation of the head, extension, external rotation of the head, and the uterus immediately after birth with the placenta and umbilical cord. — birth Sequential changes in the position of the child during labour.

A fertilized mammal egg, or zygote, develops into a morula through a series of rapid cell divisions known as cleavage.21

During this process, the zygote undergoes mitosis to subdivide into smaller cells called blastomeres. Because the mammalian egg contains little yolk, these divisions are typically equal, resulting in blastomeres of similar size and composition. Importantly, there is no significant growth in the total mass of the embryo during this stage; the cells simply divide to increase in number while remaining enclosed within the zona pellucida, a gelatinous capsule.21

As these blastomeres continue to divide and adhere to one another, they form a solid, mulberry-like mass of cells. By the time this cluster reaches approximately 12 to 60 or more cells—depending on the species—it is officially termed a morula. This structure eventually transitions into a blastocyst as fluid-filled spaces develop within it, preparing the embryo for implantation in the uterus.23