Continued observations of rams' testicles

July 27, 1700

The first observation on Saturday, July 24, had been inconclusive, so Leeuwenhoek ordered more rams' testicles to continue his observations.

On Tuesday, July 27, he found "clouds" of sperm, which he described with astonishment.

The following morning, he checked again. At seven, the sperm were still alive. However, by noon, they had ceased to move.


Letter L-380 of 1700-12-25 (AB 220)

I then received on the 27th of July, at about nine o'clock in the evening, the testicles of a young ram.

Since the first skin had been stripped off the testicle by the butcher, I stripped off the second skin or membrane, in order that theĀ vasa deferentia (see below) might be naked before my eyes.

I then first opened the seminal vessels on the outside of the testicle and I removed the male sperm from them (which appears white to the naked eye) and placed it before the magnifying glass, and thus before my eyes, and then I perceived the male animals (mannelijke dierkens) in such incredible numbers that no one would believe it unless he were an eye-witness. They were floating or swimming about in large groups, like clouds; many of them followed the same course, and then some thousands of them broke away from the group and joined another group, following the latter. Briefly, the wonderful and nimble swimming of the animalcules cannot be properly described. I further placed some of the seminal vessels before the magnifying glass in order to see, if possible, the little animals living and moving therein, but I could not discern this.

I followed the vasa deferentia to the top of the testicle, near the place where the vasa efferentia and the vasa deferentia are very close together, where the vas deferens also comes from the testicle. In these seminal vessels I could discern the animalcules in very large numbers, but they could not be seen to be alive.

Next I opened the testicle where the last-mentioned seminal vessel enters the testicle, but I could not discern any little animals at all in the said substance, and all that I saw in that substance seemed to me to be extremely small globules, some of them as big as the bodies of the said animalcules and many particles which were smaller.

The next morning, at about seven o'clock. I again opened some of the seminal vessels, very close to the place where I had discovered the living animalcules the evening before, and I saw them living as well as the evening before, but when I pursued my research about noon, I saw that none of the animalcules were alive.

A note on the translation

vasa deferentia and vasa efferentia date from the 1780s. Leeuwenhoek called them seed vessels (zaadvaaten) and draining seed vessels (afdragende zaadvaaten). He did not seem to distinguish a separate organ, what we now call the epididymis. He did see the two kinds of vessels, those transporting sperm to (toe voerende), which we now call vasa efferentia, and those transporting sperm away (af voerende), toward the urethra, one vasa deferens per testicle.

the phrase "little animals" is my translation of Leeuwenhoek's dierkens. Alle de Brieven / Collected Letters translates it as "animalcules".