Final answer:
The three embryonic cell layers are the ectoderm, mesoderm, and endoderm, with the ectoderm being crucial for the initiation of the nervous system's development.
Step-by-step explanation:
Embryonic Cell Layers
The three layers of embryonic cells that develop after fertilization are known as the ectoderm, mesoderm, and endoderm. These layers are established during a process called gastrulation. As the embryo continues to develop, each layer gives rise to different types of cells and tissues that will form the organs and systems of the body.
Ectoderm
The ectoderm is the outermost layer, which develops into the nervous system including the brain and spinal cord, as well as the epidermis of the skin. It plays a critical role in starting the processes of the nervous system during a subsequent stage in embryonic development known as neurulation.
Mesoderm
The mesoderm is the middle layer that differentiates into muscle cells and connective tissues, including the heart, blood vessels, and bones.
Endoderm
The innermost layer, the endoderm, gives rise to the epithelial lining of the digestive tract and the respiratory systems, as well as many internal organs such as the liver and pancreas.