Examined silkworm eggs

September 7, 1686

In Letter 56 of July 11, 1687, addressed to the Royal Society, Leeuwenhoek wrote:

On the 7th September 1686 I examined silkworm eggs that were six to eight days old.

This letter continued with Leeuwenhoek's nine-month examination of the stages of a silkworm's development. He noted the month and day and sometimes even the hour as he went along. For example:

On the morning of the 23rd May I again opened several silkworm eggs. But I found little difference, and we had no heat that day.

On the 24th May, at six o'clock in the morning, I again opened three or four silkworm eggs. And whereas I had always tried to open those eggs wherein I thought (to judge from the outside) there were the most complete silkworms; I now searched for those that contained the least developed silkworms.