Certificate In Psychology (CPSY) Practice Exam

Which embryonic structure is the last to appear during development?

1. Ectoderm

2. Endoderm

3. Trophoblast

4. Mesoderm

The correct answer is: Mesoderm

The correct answer is based on the sequence of embryonic development. The mesoderm is indeed the last of the three primary germ layers to be established during early embryogenesis. In the process of gastrulation, which occurs after the formation of the blastocyst, the ectoderm and endoderm are formed first. The ectoderm gives rise to structures such as the nervous system and skin, while the endoderm forms the lining of the gut and organs such as the liver and lungs. Following the establishment of these layers, the mesoderm begins to form and is responsible for developing into muscles, the circulatory system, and skeletal structures. The trophoblast, which is an outer layer of cells that develops from the blastocyst and contributes to the formation of the placenta, occurs even prior to the differentiation into ectoderm, endoderm, and mesoderm. Thus, the mesoderm is the last of these structures to appear in the developmental timeline. Understanding this sequence is very important for comprehending early human development and the specialization of for various body systems.