Preformationism, a philosophical theory of heredity, claimed either the egg or the sperm (exactly which was a contentious issue) contained a complete preformed individual called a homunculus. Development was therefore a matter of enlarging this into a fully formed being.
The term homunculus was later used in the discussion of conception and birth, Nicolas Hartsoeker discovered "animalcules" in the semen of humans and other animals. This was the beginning of spermists' theory, who held the belief that the sperm was in fact a "little man" (homunculus) that was placed inside a woman for growth into a child. This seemed to them to neatly explain many of the mysteries of conception. It was later pointed out that if the sperm was a homunculus, identical in all but size to an adult, then the homunculus may have sperm of its own. This led to a reductio ad absurdum, with a chain of homunculi "all the way down". This was not necessarily considered by spermists a fatal objection however, as it neatly explained how it was that "in Adam" all had sinned: the whole of humanity was already contained in his loins. The spermists' theory also failed to explain why children tend to resemble their mothers as well as their fathers, though some spermists believed that the growing homunculus assimilated maternal characteristics from the womb environment in which they grew.
The homunculus is commonly used today in scientific disciplines, such as psychology, to describe the distorted scale model of a human drawn or sculpted to reflect the relative space human body parts occupy on the somatosensory cortex (sensory homunculus) and the motor cortex (motor homunculus). The lips, hands, feet and sex organs have more sensory neurons than other parts of the body, so the homunculus has correspondingly large lips, hands, feet, and genitals. Well known in the field of neurology, this is also commonly called "the little man inside the brain." This scientific model is known as the cortical homunculus.
In medical science, the term homunculus is sometimes applied to certain fetus-like ovarian cystic teratomae. These will sometimes contain hair, sebaceous material and in some cases cartilaginous or bony structures.