The Mystery of the Montaigne's Misshapen Lumps

I remember a while ago reading the following passage in Montaigne's Essays and being thoroughly perplexed:

...just as a woman left alone may sometimes be seen to produce shapeless lumps of flesh but need to be kept busy by a semen other than her own in order to produce good natural offspring...

Thankfully, I've since gotten my hands onto Screech's translation of the Essays, and a happy footnote at the bottom of the page explains most things:

The human egg not yet having been discovered, many believed with Galen that children were produced by an intermingling of a (weaker) female semen with the male's. By itself the female semen could at times produce moles, a misshapen lump.