AP Psychology Practice Test

At what stage does an embryo develop major body systems?

1. After the second month

2. From fertilization through the second month

3. Up to two weeks after fertilization

4. During the third trimester

The correct answer is: From fertilization through the second month

The development of major body systems in an embryo primarily occurs during the embryonic stage, which spans from fertilization through the end of the second month. This is a crucial period when foundational structures and organ systems begin to form, such as the heart, brain, and limbs. From fertilization to the end of the second month, the embryo undergoes significant changes, including differentiation of the three germ layers (ectoderm, mesoderm, endoderm) which give rise to different systems and organs. By the end of the second month, most of the essential body systems have started to develop, making this period critical for early development. The other options refer to timelines that either start after major systems are already established or focus on stages that do not align with the critical development phase of body systems. For example, the third trimester is too late for the initial development of these systems, and the two-week period after fertilization precedes the embryo stage where these critical developments occur. Thus, the time frame outlined in the correct choice encompasses the vital period necessary for the formation of major body systems.