Abraham van Bleyswijk wrote Letter L-543 to Herman Boerhaave to report on research that he conducted with Leeuwenhoek about rabbit sperm

November 4, 1716
Collected Letters volume: 

Latin transcription in: Biblioteca Nazionale Braidense, Ms. AD. XV. 18/14, libr. II, fol. 9r-9v.

This letter is undated, but L. mentions the date in Letter L-546 [XXXI] of 21 November 1716, in this volume.

In response to Letter L-541 of 10 October 1716 from Boerhaave to Leeuwenhoek, in which letter Boerhaave asked a number of specific research questions about the presence of sperm in different parts of a male rabbit, Van Bleyswijk reports on the research he conducted together with Leeuwenhoek.