Biological Systems MCAT Practice Exam

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What is the first stage of embryogenesis called when the zygote undergoes rapid cell division?

Blastulation
Cleavage
Gastrulation
Neurulation

The correct answer is: Cleavage

The first stage of embryogenesis characterized by rapid cell division of the zygote is known as cleavage. During this phase, the single-celled zygote undergoes a series of mitotic divisions, resulting in a multicellular structure called the blastula. These divisions are unique because they increase the number of cells without increasing the overall mass of the embryo, leading to smaller cells known as blastomeres. Cleavage is crucial as it sets the stage for subsequent embryonic development, including the processes of implantation and formation of the embryonic layers. Blastulation occurs after cleavage, when the blastula is formed, while gastrulation follows, marking the differentiation of the embryonic layers. Neurulation is a later step involving the formation of the neural tube and is also well after the cleavage stage. Each of these subsequent stages builds upon the foundation laid by cleavage, emphasizing the importance of this initial phase in the overall development of the embryo.