Author’s note: There were errors in the original version of this article. Please see the Post Scripts for more details.
In 1901, Hans Spemann revolutionized biology by doing something very strange. He had been watching various embryos grow, and he got bored. That’s the short version of his motivation.
Basically, the embryologists of the day had already spent oodles of time carefully documenting the normal development of various animals from egg to embryo to hatchling. They had established that most animals – vertebrates included – go through a succession of embryological stages called the morula, blastula and gastrula. The morula is just a dense ball of cells (In Latin, morula means “mullberry”), the blastula is a hollow ball of cells, and the gastrula is like a blastula with an indentation somewhere. The indentation keeps growing inward until it meets the other side and becomes a tube that runs through the embryo’s whole body. This tube becomes the digestive tract, which, if you think about it, is just a tube running through an animal’s whole body. In some species, called the protostomes, the original indentation becomes the mouth, but in other animals (deuterostomes) it becomes the anus. We are deuterostomes. In fact, it’s all very interesting, because the same program seems to occur in wildly different organisms, from worms to molluscs to starfish and humans.