During neurulation, which structure induces the ectoderm to thicken?

The notochord plays a crucial role during neurulation, a process in vertebrate embryonic development. This rod-like structure forms from the mesoderm and is essential for the proper formation of the nervous system. As the notochord develops, it releases signaling molecules that stimulate the adjacent ectoderm to proliferate and thicken, forming what is known as the neural plate. This thickening is a critical first step in the process of neurulation, eventually leading to the formation of the neural tube, which will give rise to the central nervous system, including the brain and spinal cord.

Other options, such as the neural crest, amnion, and yolk sac, have different functions in embryonic development. The neural crest develops later from the edges of the neural plate and contributes to various structures, but it does not induce the thickening of the ectoderm. The amnion is a protective membrane surrounding the developing embryo, and the yolk sac is primarily involved in nutrient absorption and early blood cell formation. None of these structures have the specific role of inducing the ectoderm to thicken, which is uniquely attributed to the notochord.