chapter  III
Pages 23

In the blastula stage, the three fundamental layers — the ectoderm, endoderm, and mesoderm-are all simply different parts of the surface wall of a hollow ball. If they are to arrive at their right places, with the endoderm inside the ectoderm and the mesoderm in between, a series of foldings has to be carried out. There are several different methods of folding, as we shall see, but they all lead to this same result. We might compare the development of embryos to making toys by folding up pieces of paper; but with embryos, whatever the final shape, the folding always starts by producing a three-layered gastrula, just as if we are making toys we often start by making a hat shape, and then go on to further foldings which turn it into a boat or a frog or whatever it may be.