For my part, I hold it equally impossible for a small shellfish to be produced without generation as for a whale to have its origin from the mud.

If we now plainly perceive that the passage of the blood from the arteries into the veins of the tadpole is not performed in any other than those vessels, which are so minute as only to admit the passage of a single globule at a time, we may conclude that the same is performed in like manner in our own bodies and in those of other animals.

I have often endeavoured to view the circulation of the blood in terrestrial animals, but without success, by reason that no parts of their bodies were sufficiently transparent.

Antonie van Leeuwenhoek Quotes