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17 April 2019

Oscar Hertwig, the First Man to Observe Sexual Reproduction

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When he was a young and very promising scientist holding a fresh degree in medicine and having finished his doctoral thesis, German zoologist Oscar Hertwig decided to do an about face on the direction of his research. This is how he became the first man to actually observe sexual reproduction, completely and in all its glorious detail. He did so by looking at the cells of sea urchins under the microscope, a discovery that paved the way for techniques of fertilisation and assisted reproduction. He was also able to appreciate the role of the cell nucleus in the transfer of biological inheritance, which made him a pioneer in genetics as well.

Wilhelm August Oscar Hertwig (April 21, 1849 – October 25, 1922) made his great discovery right at the beginning of a long scientific career, which made him an eminent zoologist and scholar of the theory of evolution. In those early days, he formed half of a brilliant scientific duo with his brother Richard, born a year later. At the prestigious University of Jena (Germany) the Hertwig brothers soon became the most brilliant pupils of the great naturalist Ernst Haeckel, who convinced them to put aside chemistry and devote themselves to medicine.

Portrait of Oscar Hertwig made in 1906. Credit: Nicola Perscheid

After finishing their studies, both brothers abandoned medicine and followed in the footsteps of Haeckel, determined to consolidate and expand the vision of zoology held by their teacher. They decided to try to understand the functioning of animals, their entire bodies, from the study of their individual cells and the development of their embryos. Thus, they set out to solve the mysteries of sexual reproduction by studying the cells involved in a relatively simple animal: the sea urchin.

At that time, it was already known that the spermatozoon (sperm) and the ovum (egg) were the protagonists of fertilisation, but there were many unknowns regarding the exact details of how their union ignited the spark of a new life. How did both types of cells come together to give rise to an embryo? There were two very different hypotheses. The particular cellular characteristics of sea urchins allowed Oscar Hertwig to observe the process clearly under the microscope and achieve his greatest success, which he published in 1876: fertilisation occurs when the sperm enters the egg and its nuclei fuse, between 5 and 10 minutes later.

The keys to biological inheritance
Hertwig could see the climactic moment of sexual reproduction to such a high level of detail that he also discovered that it is a single sperm that fertilises the egg, although there are many more who try. Thus, when one sperm manages to penetrate an egg, it generates a membrane that prevents the entry of new competing sperm.

Oscar Hertwig continued studying fertilisation inside the ovum and observed that the key was what happened to the chromosomes in this process. In 1888, now a professor of anatomy in Berlin, he was one of the first to teach that the basis of biological inheritance resided in chromosomes. He wrote a textbook that became the reference source on the biological development of animals. And his great intuition took him one step further, suggesting that within the nucleus there is a chemical substance that is not only responsible for fertilisation, but also for the transmission of hereditary characteristics.

More than half a century later, after Hertwig’s death, it was finally proven that he was also right about this, when in 1944 the experiment by Avery-MacLeod-McCarty demonstrated that the substance responsible for biological inheritance is a nucleic acid, DNA. Recognised as an eminent scientist, in the final years of his career Oscar Hertwig immersed himself deeply in the theory of evolution and came to the conclusion that Darwin was wrong in his interpretation of evolutionary mechanisms. His most important theoretical book is: Das Werden der Organismen, eine Widerlegung der Darwinschen Zufallslehre (“The Origin of Organisms: a refutation of Darwin’s theory of chance”) published in 1916, which would have antagonised his teacher Haeckel, a convinced Darwinist. But Hertwig’s criticisms served to stimulate other scientists, who managed to refine and complete the theory of evolution.

Francisco Doménech

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