Optometry Admission Test (OAT) Practice

What is the outcome of the cleavage stage in early development?

1. Formation of the placenta

2. Division of the fertilized egg into smaller cells

3. Completion of mitosis

4. Initiation of genomic imprinting

The correct answer is: Division of the fertilized egg into smaller cells

The cleavage stage in early development refers to the series of rapid cell divisions that occur following fertilization. During this stage, the fertilized egg, or zygote, undergoes mitotic divisions without significant growth, leading to a rapid increase in the number of cells. As a result, the single-celled zygote is transformed into a multicellular structure comprised of smaller cells called blastomeres. This process is crucial because it sets the foundation for further stages of development, such as the formation of the blastula and eventual implantation in the uterine wall. The cells retain the same total volume as the original zygote, but their size decreases with each cleavage division. Importantly, during this phase, the cells are not yet differentiating into specialized cell types, as this occurs in later stages. The other potential outcomes listed do not accurately describe the cleavage stage. For instance, the formation of the placenta occurs much later in development, after implantation has taken place; completion of mitosis refers to a process that happens continuously, rather than being an outcome of a specific developmental stage; and genomic imprinting is a phenomenon that has its active role in later stages of embryonic development. Therefore, the correct response highlights the essential biological activity occurring during