were formed of mud, growing from the "guts of wet soil" rather than through sexual reproduction. Many centuries passed before scientists were able to demonstrate that such spontaneous generation does not occur.

In 1777, the Italian Carlo Mondini located an eel's ovaries and demonstrated that eels are a kind of fish. In 1876, while he was still a young university student in Austria, Sigmund Freud was assigned the task of searching for the male sex organs