History of embryology
General issues of embryology were already discussed in Greek times, notably by Hippocrates and Aristotle. But even in the 1700s it was still thought that perhaps every embryo started from a very small version of a complete organism. In the 1800s, however, detailed studies revealed the progressive development of complexity in the growth of an embryo. At the end of the 1800s experiments based on removing or modifying parts of early embryos began, and by the 1920s it had been discovered that there were definite pieces of embryos that were responsible for inducing various aspects of development to occur. That concentrations of diffusing chemicals might define where in an embryo different elements would form was first suggested in the early 1900s, but it was not until the 1970s and 1980s—after it was emphasized by Lewis Wolpert in 1969 under the name "positional information"—that there was clear experimental investigation of this idea. From the 1930s and before, it was known that different genes are involved in different aspects of embryo development. And with the advent of gene manipulation methods in the 1970s and 1980s, it became possible to investigate the genetic control of development in organisms such as fruit flies in tremendous detail. Among the important discoveries made were the homeobox genes (see note above).