Made of mesoderm, they reach around the neural tube to meet their opposite numbers and encase the neural tube. They will become vertebrae and muscles and the deepest layers of the skin. Underneath the embryo the endoderm, which embraces an enormous, flaccid sac of yolk, retracts up into the embryo to become the gut. As the gut shrinks the two halves of the embryo that it has previously divided are drawn together. Two hitherto inconspicuous tubes, one on either side, then unite to make a single larger tube running the length of the embryo’sfuture abdomen, an abdominal tube that echoes the neural tube on its back. Within a few days this abdominal tube will begin to twist and then twist again to become a small machine of exquisite design. Though it still looks nothing like what it will become it already shows the qualities that led William Harvey to call it ‘the Foundation of Life, the Prince of All, the Sun of the Microcosm, on which all vegetation doth depend, from whence all Vigor and Strength doth flow’. On day 21 it begins to beat.
The ability of disparate organ primordia to find each other and fuse to form wholes is one of the marvels of embryogenesis. Underlying it are thousands of different molecules that are attached to the surface of cells and are, as it were, signals of their affiliation, that permit other cells to recognise them as being of like kind. These are the cell-adhesion molecules; molecular biologists speak of them as the Velcro of the body: weak individually, but collectively strong. Even so, the fusion of organ primordia is a delicate business. Neural tube fusion is particularly prone to failure. One infant in a thousand born has a neural tube that is at least partly open – a condition called spina bifida. At its most severe the neural tube in the future head fails to close. The exposed neural tissue becomes necrotic and collapses, leaving a child that has the remnant of a brain stem but in which the back of the head has been truncated, as if sliced with a cleaver.
Such anencephalic infants, as they are known, occur in about 1 in 1500 births; they have heavy-lidded eyes that seem to bulge from their heads and their tongues stick out of their mouths. They die within a few days, if not hours, of being born. As thename suggests, spina bifida is often not just a failure of the neural tube to close, but a failure in the closure of the vertebral column so that instead of being sheltered by bone the nerve cord lies exposed. It is not the only organ prone to this sort of defect. Sometimes the primordia of the heart fail to meet; the result is cardiac bifida, two hearts, each only half of what it should be.
The power of cell–cell adhesion to mould the developing body is startling. In his monograph on Ritta and Christina, Serres describes a pair of stillborn boys who are joined at the head. Oriented belly to belly, their faces are deflected ninety degrees relative to their torsos so that they gaze, Janus-like, in opposite directions. What is remarkable about these children is that each apparent face is composed of half of one child’s facefused to the opposite half of his brother’s. The developing noses, lips, jaws and brains of these two children have found each other and fused perfectly – twice.
C ONJOINED TWINS: CEPHALOTHORACOILEOPAGUS . F ROM É TIENNE S ERRES 1832
Recherches d’anatomie transcendante et pathologique.
The diversity of ways in which conjoined twins can be attached to each other seems to depend on the position of the developing embryonic discs relative to each other as they float on their common yolk sac and when they contact. The embryonic discs that gave rise to Ritta and Christina were side by side, and fused some time after closure of the vertebral column but before formation of the lower gut. In the case of the twins with fused faces the embryonic discs were head to head. The most extreme form of conjoined twinning is ‘parapagus diprosopus’, in which the fusion is so
The ability of disparate organ primordia to find each other and fuse to form wholes is one of the marvels of embryogenesis. Underlying it are thousands of different molecules that are attached to the surface of cells and are, as it were, signals of their affiliation, that permit other cells to recognise them as being of like kind. These are the cell-adhesion molecules; molecular biologists speak of them as the Velcro of the body: weak individually, but collectively strong. Even so, the fusion of organ primordia is a delicate business. Neural tube fusion is particularly prone to failure. One infant in a thousand born has a neural tube that is at least partly open – a condition called spina bifida. At its most severe the neural tube in the future head fails to close. The exposed neural tissue becomes necrotic and collapses, leaving a child that has the remnant of a brain stem but in which the back of the head has been truncated, as if sliced with a cleaver.
Such anencephalic infants, as they are known, occur in about 1 in 1500 births; they have heavy-lidded eyes that seem to bulge from their heads and their tongues stick out of their mouths. They die within a few days, if not hours, of being born. As thename suggests, spina bifida is often not just a failure of the neural tube to close, but a failure in the closure of the vertebral column so that instead of being sheltered by bone the nerve cord lies exposed. It is not the only organ prone to this sort of defect. Sometimes the primordia of the heart fail to meet; the result is cardiac bifida, two hearts, each only half of what it should be.
The power of cell–cell adhesion to mould the developing body is startling. In his monograph on Ritta and Christina, Serres describes a pair of stillborn boys who are joined at the head. Oriented belly to belly, their faces are deflected ninety degrees relative to their torsos so that they gaze, Janus-like, in opposite directions. What is remarkable about these children is that each apparent face is composed of half of one child’s facefused to the opposite half of his brother’s. The developing noses, lips, jaws and brains of these two children have found each other and fused perfectly – twice.
C ONJOINED TWINS: CEPHALOTHORACOILEOPAGUS . F ROM É TIENNE S ERRES 1832
Recherches d’anatomie transcendante et pathologique.
The diversity of ways in which conjoined twins can be attached to each other seems to depend on the position of the developing embryonic discs relative to each other as they float on their common yolk sac and when they contact. The embryonic discs that gave rise to Ritta and Christina were side by side, and fused some time after closure of the vertebral column but before formation of the lower gut. In the case of the twins with fused faces the embryonic discs were head to head. The most extreme form of conjoined twinning is ‘parapagus diprosopus’, in which the fusion is so
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