The pre-embryonic 16-cell solid ball of cells that reaches the uterus approximately 3 days after fertilization is called the:

1. After fertilization, the zygote undergoes several rapid mitotic divisions called cleavage. 2. By about 3 days, a compacted, solid 16-cell mass reaches the uterus. 3. This compacted structure is called the morula. 4. Only later, once it organizes around a fluid-filled cavity, does it become a blastocyst, so that term does not apply at this stage. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 28 "Development and Inheritance", p.1325 §Pre-implantation Embryonic Development_