Mutants by Armand Marie Leroi Read Free Book Online

a ball of their own, so that by day 9 the embryo is rather like one of those ingenious Chinese toys composed of carved ivory spheres within spheres within spheres. By day 13 it has disappeared within the uterine lining, and the wound it has caused has usually healed. The embryo is beginning to build itself.
    Its first task is to make the raw materials of its organs. We are three-dimensional creatures: bags of skin that surround layers of bone and muscle that, in turn, support a maze of internal plumbing; and each of these layers is constructed from specialised tissues. But the embryo faces a problem. Of the elaborate structure that it has already built, only a minute fraction – a small clump of cells in the innermost sphere – is actually destined to produce the foetus; all the rest will just become its ancillary equipment: placenta, umbilical cord and the like. And to make foetus out of this clump of cells, the embryo has to reorganise itself.
    The process by which it does this is called ‘gastrulation’. At about day 13 after conception, the clump of cells has become a disc with a cavity above it (the future amniotic cavity) and a cavity below it (the future yolk sac). Halfway down the length of this disc, a groove appears, the so-called ‘primitive streak’. Cells migrate towards the streak and pour themselves into it. The first cells that go through layer themselves around the yolk cavity. More cells enter the streak and form another layer above the first. The result is an embryo organised into three layers where once there was one: a gastrula.
    The three layers of the gastrula anticipate our organs. The top layer is the ectoderm – it will become the outer layers of the skin and most of the nervous system; beneath it is the mesoderm – future muscle and bone; and surrounding the yolk is the endoderm – ultimate source of the gut, pancreas, spleen and liver. (
Ecto-, meso-
and
endo-
come from the Greek for outer, middle and inner
derm
– skin – respectively.)
    The division sounds clear-cut, but in fact many parts of our bodies – teeth, breasts, arms, legs, genitalia – are intricate combinations of ectoderm and mesoderm. More important than the material from which it builds its organs, the embryo has also now acquired the geometry that it will have for the rest of its life. Two weeks after
egg
met sperm, the embryo has a head and a tail, a front and a back, and a left and a right. The question is, how did it get them?
    In the spring of 1920, Hilda Pröscholdt arrived in the German university town of Freiburg. She had come to work with Hans Spemann, one of the most important figures in the new, largely German, science of
Entwicklungsmechanik
, ‘developmental mechanics’. The glassy embryos of sea urchins were being bisected; green-tentacled
Hydra
lost their heads only to regrow them again; frogs and newts were made to yield up their eggs for intricate transplantation experiments. Spemann was a master of this science, and Pröscholdt was there to do a Ph.D. in his laboratory. At first she floundered; the experiments that Spemann asked her to do seemed technically impossible and, in retrospect, they were. But she was bright, tenacious and competent, and inthe spring of 1921 Spemann suggested another line of work. Its results would provide the first glimpse into how the embryo gets its order.
    Then as now, the implicit goal of most developmental biologists was to understand how human embryos construct themselves, or failing that, how the embryos of other species of mammal do. But mammal embryos are difficult to work with. They’re hard to find and difficult to keep alive outside the womb. Not so newt embryos. Newts lay an abundance of tiny eggs that can, with practice, be surgically manipulated. It was even possible to transplant pieces of tissue between newt embryos and have them graft and grow.
    The experiment which Spemann now suggested to Hilda Pröscholdt entailed excising a piece of tissue from the