Herman Boerhaave wrote Letter L-541 to Leeuwenhoek in response to Letter L-540 about coconuts

October 10, 1716
Collected Letters volume: 

In Letter L-544 of 5 November 1716 to Herman Boerhaave, Leeuwenhoek noted that he had

received, highly learned Sir, your letter of the 10th of last month. In it, highly learned Sir, Your Honour extols my work far more than it deserves in my opinion.

Boerhaave wrote in response to Leeuwenhoek's letter of 28 September 1716 about coconuts, a follow-up to the visit that Boerhaave, Fredrick Ruysch, and Abraham van Bleiswijk made to Leeuwenhoek in early September.

In his response, Boerhaave asked about reproduction, because Leeuwenhoek continued:

I have shared the letter with Mr Abraham van Bleyswyk, Medicinae Doctor. Thereupon, Mr van Bleyswyk has partially dissected a male rabbit at my house. He will inform Your Honour about that which was observed with regard to this.

This dissection must have occurred in late October or very early November 1716, around the time when Leeuwenhoek turned 84.

In the same letter Your Honour asks me whether I have seen a difference in the size of the little animals in the male semen of animals.

Leeuwenhoek had noted the size similarities in previous letters. [list them?]