In 1668, Dutch biologist and microscopist Jan Swammerdam was first to observe and describe red blood cells. Swammerdam was a pioneer in using the microscope in dissections. His techniques were employed for hundreds of years.
17th-century knowledge of insects was largely inherited from Aristotle, who considered the critters so insignificant that they were largely ignored as unworthy of scientific study, thus ensuring that entomology would be neglected for over a millennium. Swammerdam studied insects under his microscope. Swammerdam’s principal interest was in disproving metamorphosis: that insect life stages could radically differ in form. In this he was dead wrong. Nevertheless, Swammerdam observed that late-stage larval flies had undeveloped adult features, such as wings. This led him to cast his lot with preformationism.
There is never generation in Nature, only an increase in parts. ~ Jan Swammerdam