Final answer:
The ectoderm is the outermost embryonic germ layer, forming by the third week of life. It differentiates to give rise to the nervous system, skin, and glandular tissues, as well as other structures through processes like gastrulation and neurulation.
Step-by-step explanation:
The outermost germ layer of an embryo that forms by the third week of life is known as the ectoderm. During a process called gastrulation, the ectoderm differentiates to give rise to various tissues, including the nervous system, skin, and glandular tissue. Organogenesis follows gastrulation, where specialized neuroectodermal tissues form the neural plate, which then folds to develop into the neural tube, leading to the formation of the central nervous system. Additionally, other parts of the ectoderm become the epidermis as well as many sensory organs.
Each germ layer, including the mesoderm and endoderm, also differentiates into specific cells, tissues, and organs. The mesoderm gives rise to muscle cells and connective tissue, while the endoderm forms the gut and various internal organs.
The ectoderm is not only responsible for forming the integumentary system, which includes the epidermis, hair, and nails, but also plays a crucial role in the development of the nervous system, eyes, inner ears, and many connective tissues during the embryo development.