Sex cells and embryos

During the 17^th and 18^th centuries, descriptive anatomical studies continued, with most of the work being done on fetuses in the last trimester of pregnancy (last three months). The fetus of this age is of unmistakable human appearance so these studies failed to appreciate the physical changes that take place earlier in prenatal life. Some biologists continued to believe in preformation —that organisms develop from miniature versions of themselves already contained within the egg cell or the spermatozoa, as in image ‘A’ above. William Harvey in 1651 established that the embryo is not a preformed adult. Harvey showed that during prenatal development there are a series of embryological stages that are distinct in appearance from the form visible just before and after birth. Harvey declared that «everything comes from the egg», Amazingly, it was not until 1875 that Oscar Hertwig, a German biologist who is mostly unknown today, first proved that fertilization results from a single sperm penetrating an egg cell, fusing with the nucleus, after which the cell begins to divide. He observed this using the sea urchin. Images ‘B’ and ‘C’ above are 20^th century medical diagrams of human female and male reproductive cells (not to scale) and a human embryo 4.5 weeks after fertilization. [Barry Bogin]