The final version of the cell theory is therefore the combination of Schleiden and Schwann’s first theory with the conclusion of Remak and Virchow: “all living creatures are made of cells and of cell products, and cells are always generated by the division of other cells”.

The problem of generation

At the very center of biology there are two complementary problems: “how does an organism produce an egg?” (the problem of generation), and “how does an egg produce an organism?” (the problem of embryonic development). These questions have been debated since antiquity – both Hippocrates and Aristotle wrote at lenght about them – but only the microscope made it possible, in the 19th century, to make the crucial observations that led to a solution.
With the cell theory, organisms became societies of cells, and the problem of generation became the problem of understanding which and how many cells are forming the germ of a new individual. Botanists believed that any seed had to be fertilized by a high number of pollen grains, and it was widely held that the greater that number the stronger would be the resulting plant. As for animals, it was again thought that an egg had to be fertilized by many spermatozoa, each carrying a fraction of the heredity material, because it was an experimental fact that an egg is always surrounded by a multitude of spermatozoa at fertilization, and it was taken for granted that a single spermatozoon could not possibly carry all the hereditary traits of an adult body.
It was Oskar Hertwig, in 1875, that solved this problem. By studying sea urchins, animals which are particularly suitable for microscopy studies because of their transparency, he noticed that eggs contain a single nucleus before fertilization and two nuclei immediately afterwards. He realized that the second nucleus had come from a spermatozoon, and therefore that a single spermatozoon can fertilize an egg. Hertwig’s discovery was completed in 1879 by Hermann Fol, who managed to inject many spermatozoa in a single egg, and found that in this case development is always abnormal, thus proving that fertilization can and must be realized by a single spermatozoon.