 By day 16, the embryo consists of three layers – endoderm, mesoderm, and ectoderm. Ectoderm develops into the nervous system, and the first phase of this process is known as neuralation. Around day 18, part of the ectoderm begins to thicken. This portion is known as the neural plate, in dark blue, between the cranial end of the embryo and the primitive node. Neuralation is the process of neural plate becoming neural tube. Got a schematic animation here to try and illustrate that process. We'll break this down day by day throughout the rest of the video. On day 19, the neural plate begins to buckle in the midline. We call this dip the neural groove, and the mounds either side the neural folds. These folds progress toward each other and begin to fuse around day 21. Where they have fused, they form a hollow cylinder, which is the neural tube. Fusion then continues in synchrony toward caudal and cranial ends. The as-yet-unfused portions are known respectively as the cranial neuropore and caudal neuropore. Pore literally means minute opening. The minute opening at the cranial end closes around day 24, and the caudal end closes around day 26. A particular group of cells reside at the peaks of the neural fold. These are the neural crest cells, which form a layer over the top of the closed neural tube. These develop into most of the peripheral nervous system. To sum up, neuralation is the process of neural plate becoming neural tube. The closure of the neural folds begins in the center and zips up towards either end. The central nervous system is developed from the neural tube. It's worth mentioning that by this gestational age, the embryo has started curving into the fetal position. We've kept the ectoderm flat to keep our schematic version really simple, but these pictures from Grey's Anatomy accurately show the embryo at around 21 and 31 days. The neural tube curves through the back of them, like so. That's it for now. Hit subscribe if you liked this video, thanks for watching and we will see you next time.