USA Biology Olympiad (USABO) Practice Exam

Which developmental process results in the formation of three germ layers in embryos?

1. Fertilization

2. Neurulation

3. Gastrulation

4. Cleavage

The correct answer is: Gastrulation

The formation of three germ layers—ectoderm, mesoderm, and endoderm—in embryos occurs during a developmental process known as gastrulation. This crucial phase follows cleavage and involves extensive cell movement and reorganization. During gastrulation, cells migrate and differentiate to establish these distinct layers, each of which will develop into specific tissues and organs in the organism. The ectoderm typically gives rise to structures such as the skin and nervous system, the mesoderm develops into muscles, bones, and the circulatory system, while the endoderm forms internal structures, including the digestive tract and some glandular organs. The other processes mentioned either happen before or after gastrulation. For instance, fertilization is the union of sperm and egg but does not involve layer formation. Cleavage refers to the early cell divisions that occur immediately after fertilization, resulting in a ball of cells known as the blastula, but it does not yet establish the germ layers. Neurulation is a subsequent process that focuses on the formation of the neural tube from the ectoderm, occurring after gastrulation has already defined the germ layers. Thus, gastrulation is the key developmental stage responsible for establishing the foundation of the body's structure through these three germ layers