His Greatest Discovery

In 1853, the French botanist Gustave Thuret identified the sexual reproductive elements of the marine alga Fucus. He had shown what was involved in reproduction, but he had not watched fertilisation itself happen.

Thuret had identified the machinery. Pringsheim would see it at work.

Nathanael Pringsheim chose a different organism: the freshwater alga Vaucheria sessilis. Living material could be brought to the microscope and watched as reproduction unfolded.

Watching fertilisation

In 1855, Pringsheim watched motile spermatozoids emerge from the male organ, swim through the water and reach the female reproductive organ. He observed a spermatozoid penetrate the egg. After fertilisation, the egg formed a membrane and developed into a resting spore.

For Pringsheim, the microscope was no longer simply a tool for describing biological structures. It had become a way of watching a biological process as it happened.

Pringsheim published these observations in 1855 in Über die Befruchtung und Keimung der Algen und das Wesen des Zeugungsactes. He then extended his investigations to other freshwater algae, including Oedogonium and Bulbochaete.

What came next

These later observations revealed what Pringsheim could not yet see. Fertilisation was not simply an encounter between two reproductive cells: material from the male and female cells came together inside the fertilised egg.

Pringsheim made fertilisation visible. Hertwig and Strasburger revealed what happened inside the cell.

From fertilisation to heredity

The significance became clearer as biology advanced. Gregor Mendel had discovered regular patterns of inheritance in experiments with peas, while later cell biologists connected heredity with structures inside the nucleus. Eventually chromosomes, genes and DNA would explain how information from two parents is brought together at fertilisation.

Pringsheim could not know that future in 1855. His achievement was more immediate and fundamental: he had turned fertilisation from an inferred event into something that could be watched.

Sources

Nathanael Pringsheim, Über die Befruchtung und Keimung der Algen und das Wesen des Zeugungsactes (1855) — original publication.

Oskar Hertwig, Beiträge zur Kenntniss der Bildung, Befruchtung und Theilung des thierischen Eies (1875) — original publication.

Eduard Strasburger, Neue Untersuchungen über den Befruchtungsvorgang bei den Phanerogamen (1884) — original publication.