Final answer:
The ectoderm germ layer is responsible for forming organs and structures that maintain contact with the external world, such as the nervous system and the epidermis. This process occurs during gastrulation, which is followed by organogenesis, where each germ layer differentiates into specific tissues and organs.
Step-by-step explanation:
The germ layer that typically gives rise to organs and structures in contact with the external world is the ectoderm. During the developmental process called gastrulation, the three germ layers are formed, which include the endoderm and mesoderm as well. The organogenesis phase is where these layers give rise to specific tissues and organs.
The ectoderm is responsible for forming the nervous system, the skin's epidermis, and other tissues. For instance, this layer leads to the development of sensory organs like eyes and ears, as well as the epidermis which includes hair and nails. Similarly, the endoderm forms essential internal structures such as the epithelial lining of the gastrointestinal tract and the lungs, while the mesoderm gives rise to various systems, including muscles, the circulatory system, and bones.
The organized differentiation process allows embryonic cells to express specific genes, leading to their development into the destined cellular structures. This showcases the remarkable complexity and precision of biological development from the simplest cell layers into a fully formed organism.