The Abstract of Two Letters Sent Some Time Since by Mr. Anth. Van. Leeuwenhoeek to Dr. Gale and Dr. Hooke

IV. The Abstract of two Letters sent some time since by Mr. Anth. Van Leeuwenhoek to Dr. Gale and Dr. Hooke. Having examined the Testicles of a Rat, and the Liquor press'd out of the Seminal Vessels thereof, I found a great number of Amalcules long and serpent-like, as is represented Fig. 4. which shews one of them dead. The Liquor itself in which they floated was transparent and oily, full of irregular Parts, besides the Amalcules, some whereof lay turned up round, and seemed not to be full grown or living; and I conceived the irregular Particles were the Beginnings or Rudiments of these Animalcules which were generated in the Thred-like Vessels of the Testicles. I suppose the manner of their Generation differs from that of the Eels I have formerly observed in Vinegar, which carry their Young in their Body, whereas I believe these are produced from an Egg. And possibly they may have their beginning with the Generation of the Animal, and come to perfection together with it, remaining in the Seed in the Testicles in Fieri, till the Animal itself is fit for Copulation; as in Man till about Fourteen Years, and then first have Life and Perfection. The particular Coat of these Thred-like Vessels is exceeding thin; and the Amalcules therein contained so very minute, that 10000 of them equal not one Thred; whence may be computed the vast number one Testicle is capable of containing. These Threds are more conspicuous in the Testicle of a Rat than any Animal. Endeavouring to examine the Seed of Muscles in March when they spawn, I was somewhat prevented; only in the Liquor contained in their Shells, I found many many small living Animals; and the Muscle itself had two thin Films, consisting of long slender Threads, with little knobby parts thereon, something like the Beards or Fins of Oysters, but were more easily separable from the Fish: These Strings were thick beset with Hairs continually moving; and through the Threads themselves I saw a great number of small Animals. In the Liquor in Oysters I found several sorts of Amalcules, but in their Beards or Gills none, possibly because the Oysters were dead. Examining the Sap that bleeds from Vines, I found therein several Amalcules of different sizes, such as I had formerly observed in Water. And I suppose that the Sun may have raised the very small Eggs of these Amalcules, and let them fall in Dew or Rain on the outsides of Vines which stop by the Leathers that fasten them to the Wall, are there hatch'd and carried down with the trickling Sap, which I am confirmed in, by reason after Rain I found of these Amalcules in the Water so stop by the Leathers; and at other times I have found no Amalcules in the Sap of Vines. I have sometimes found in this Sap an Aurelia, such as Fig. 5, but a Million of times smaller than a course Sand. Some of these I brought home, but those in the Air dried up to nothing, and some that I kept in Water were lost. V. Some Observations in the Dissection of a Ratt, communicated by Mr. R. W. S.R.S. The Fore-Feet of a Rat resemble those of the Castor. The Hair is also some Fine, some Course; as in that Animal. The Tail scaly, with Hairs between every