History of Medicine Home > From 'Monsters' to Modern Medical Miracles Home > Embryology and Classification of Conjoined Twins Embryology of Conjoined Twins Identical twins develop when a single fertilized egg, also known as a monozygote, splits during the first two weeks of conception. Conjoined twins form when this split occurs after the first two weeks of conception. The monozygote does not fully separate and eventually develops into a conjoined fetus that shares one placenta, one amniotic sac, and one chorionic sac. Because the twins develop from a single egg, they will also be the same sex. The extent of separation and the stage at which it occurs determine the type of conjoined twin, i.e., where and how the twins will be joined.
History of Medicine Bacterial Genetics: 1946-58 His inquisitiveness, facility for establishing connections between scientific disciplines, and grasp of institutional strategy led Joshua Lederberg to the forefront of successive advances in science: molecular genetics in the 1940s and 1950s; the search for extraterrestrial life in the 1950s and 1960s; computers and artificial intelligence in the 1960s and 1970s. His discoveries in genetics produced a deeper understanding not only of the biochemical mechanism of inheritance and mutation in microorganisms, but of the evolution of diseases, the causes of drug resistance, and the possibilities of genetic engineering and gene therapy.