Has the semen soul, or not? ~ Aristotle
The name of 6th-century-bce Greek philosopher Pythagoras would live on for his triangle theorem, among other mathematical feats. Pythagoras also originated spermism: the notion that fathers provided the essential characteristics of their offspring, while mothers merely contributed material substrate. This testosterone-fueled foolishness reverberated through history.
A century and a half after Pythagoras, Aristotle further ejaculated on spermism. In his treatise On the Generation of Animals, Aristotle fixated on semen, wildly speculated on inheritance, and tossed about preformationism and epigenesis, whereupon he came down solidly in favor of preformationism. In the inevitable cerebral coin tosses of ignorance, one must ultimately pick a side.
The microscope was instrumental in resolving developmental biology. But a tool is only as good as its user.
In the 1660s, Dutch lens-grinder Antonie van Leeuwenhoek considerably improved the resolving power of the microscope, and was one of the first to see spermatozoa, going on to chronicle the sperm of about 30 species. His description confirmed spermism, upon seeing what he took to be:
… all manner of great and small vessels, so various and so numerous that I do not doubt that they be nerves, arteries and veins. And when I saw them, I felt convinced that, in no full-grown body, are there any vessels which may not be found likewise in semen.