Began series of observations of rams' testicles

Date: 
July 24, 1700

Hans Sloane, Leeuwenhoek's new editor at Philosophical Transactions, had visited Delft earlier in the year. On Christmas, Leeuwenhoek wrote to him about a series of observations that he had made the previous July.

What puzzled him was why young rams were first ready to copulate months before the young ewes were first able to conceive. His question: Were the little animals (dierkens) in the rams' sperm alive and moving?

He was trying to puzzle out the relative role of male and female, sperm and egg.

Document: 

Letter 135 of 1700-12-25 (AB 220)

I therefore ordered a butcher to deliver at my house the testicles of a young ram (that is the name we give to a male sheep) as soon as he killed a ram. On the 24th of July I received two testicles of a young ram, of which I opened the vasa deferentia, and when I placed the substance contained therein before the magnifying glass, I discovered indeed the animalcules, but I could not discern any life in the animalcules. Afterwards this did not appear strange to me when I learned that the young ram had been dead some thirty hours when I received the testicles.

Next I ordered the butcher to deliver at my house the testicles of a ram as soon as they had been cut out of the ram.