The Human Question

16,32, 64, 128: I don't know how many cells make up a blastocysts, but Pinker points out that one can be removed to form an identical twin. So far so good. But he doesn't note that there is something orderly about these stages of life in a human: when there is 4 cells, for example, there is some difference in the material. There is, as it were, a top and a bottom. Not just a messy bunchacells: rather, a non-obvious structural unity. Yes, you can take one away and grow a new human. But you may be able to do that with iPSCs some day.