An infinite series of plants and animals were contained within the seed or the egg. They are infinite trees in only one seed. All the bodies of humans and animals were perhaps produced as soon as the creation of the world. ~ Nicolas Malebranche
As the 18th century dawned, while spontaneous generation was slipping, another theory of life generation gained public acceptance. Preformationism embodied the idea of new life coming into being as miniatures, growing only in size.
Preformationism had much earlier roots. The concept itself was preformed and merely growing in approval. Christians long believed that all life was created in the same week, and on the 7th day the Creator rested (apparently it having been a tough work week).
In stark contrast to preformationism, epigenesis posits that organisms develop through cellular differentiation, by which distinct organs and tissues form. Epigenesis was a radical idea long resisted.