Oscar Hertwig (21 April 1849 in Friedberg – 25 October 1922 in Berlin) was a German embryologist and zoologist known for his research in developmental biology and evolution.

Hertwig is credited as the first person to observe sexual reproduction at the cellular level, by looking at the cells of sea urchins under the microscope.

By studying sea urchins he proved that fertilization occurs due to the fusion of a sperm and egg cell.

[4] In 1885 Hertwig wrote that nuclein (later called nucleic acid) is the substance responsible not only for fertilization but also for the transmission of hereditary characteristics.

[6] A history of the discovery of fertilization for mammals including scientists like Hertwig and other workers is given by the book "The Mammalian Egg" by Austin.