Leeuwenhoek's extraordinarily detailed illustrations caused a sensation, whether of miniature, unknown “animalcules” or complex insects such as the fly.  

Such marvels and wonders provided startling new evidence for questions about the generation and development of life, a research area which became a central focus for biologists in the following century. Leeuwenhoek’s observations of spermatozoa from many diverse species led him to advocate a form of spermist preformationism, in which the young are preformed in the sperm. 

Other preformationists, the ovists, argued that adults are preformed in the egg. Against both the spermists and the ovists were the epigenecists, who argued for a process of embryological development rather than preformation. 

In the 18th century, a preference for mechanical explanations wherever possible led most scientists to prefer preformation theories, with ovism becoming the most popular.