Sperm meets egg: finding out what really happens, in this Moment of Science.

            In the 1870's biologists knew that, as a rule, animal egg cells would start to develop only when they had been "fertilized" -- whatever that meant.  But good microscopes and new techniques of that era made it possible to see what really happens.

            In 1874 the German zoologist Oscar Hertwig studied fertilization of sea urchin eggs.  Sea urchins are relatives of starfish and sand dollars, abundant near seashores, and still favorites among biologists.  Fertilization of sea urchin eggs is easy to observe because it happens in open water.

            Hertwig saw that before fertilization, the sea urchin egg had a single nucleus, a distinct body inside the cell that divides when the cell divides; but immediately after fertilization, there were not one but two nuclei, which soon fused into one.  Oscar Hertwig concluded in 1875 that the extra nucleus had been added to the egg by a sperm cell.

            At about the same time a French biologist, Hermann Fol, actually saw a sea urchin sperm cell penetrate an egg cell.

            By about 1900 the observations of Hertwig, Fol, and others were brought together to make the now-familiar picture: a single sperm cell penetrates the egg cell; the nucleus of the sperm merges with the nucleus of the egg; then development begins.

            These observations suggested but did not yet prove that the sperm contributes something essential to the new individual.  Not till the twentieth century was it demonstrated that one function of a sperm cell is to add genetic information to the nucleus of the egg.  The other function of the sperm is to trigger development of the egg into an embryo.

This moment of science comes from Indiana University.

I'm Yaël Ksander.