I went through a human neural development class in grad school and the most interesting thing I learned is that we have no/little idea what is going on. We know that rostralization/posteriorization is set up at initial fertilization in most vertebrates, but we have no what causes it (utube-spindles? actin responses to puncture?). Same with dorsal/ventralization. The way we know this small bit? Toads' (xenopus) have a dark/light side to their eggs and we can track that, but other 'clear' egged species are assumed to be like that, as we can't 'tattoo' a cell membrane reliably yet.
In general, I was really stunned at the insane complexity and fervor of development. I mean, you go from one tiny little cell and one tiny little sperm, and then you end up with a trillion little cells all different and all really specialized in under a year. And it mostly all works! Some of those cell migrate a very long way (from a baby elephant's head to it's toes with some nerves) almost perfectly. Some of the time, tons of cells are born, shepherd other cells about, and then die off in about a day (like apoptosis in breast-feeding). Why? No clue. Maybe evolutionary 'holdovers', dunno, more funding please. Really, the shear complexity and energy of the system, with things crashing into each other, traffic jams of cells in a neural cleft, all going about all at once, it is stunning. I can understand a lot of researchers that are in that field and how deeply religious they are, seeing and 'understanding' it all can be moving.