"([Fig. e.]) Delineatur vivum animalculum, quemadmodum in semine canino sese aliquoties mihi attentius intuenti exhibuit. E F G, caput cum trunco indigitant, G H ejusdem caudam, ([fig. f, g, h,]) alia sunt in semine canino quæ motu & vita privantur, qualium etiam vivorum numerum adeo ingentem vidi, ut judicarem portionem lymphæ spermaticæ arenulæ mediocri respondentem, eorum ut minimum decena millia continere."
By another letter written to the Royal Society, the 31st of May, 1678, Leeuwenhoek adds, "Seminis canini tantillum microscopio applicatum iterum contemplatus sum, in eoque antea descripta animalia numerosissime conspexi. Aqua pluvialis pari quantitate adjecta, iisdem confestim mortem accersit. Ejusdem seminis canini portiuncula in vitreo tubulo unciæ partem duodecimalem crasso servata, sex & triginta horarum spatio contenta animalia vita destitua pleraque, reliqua moribunda videbantur.
"Quo de vasorum in semine genitali existentia magis constaret, delineationem eorum aliqualem mitto, ut in figura ABCDE, ([fig. i.]) quibus literis circumscriptum spatium arenulam mediocrem vix superat."
I have copied these first remarks of Leeuwenhoek from the Philosophical Transactions, because, in matters of this kind, observations made without any systematical view are those which are the most faithfully described, and even this able naturalist no sooner formed a system on spermatic animals, than he began to vary in essential points.
It is evident by the above dales, that Hartsoeker is not the first who published, if he was the first who discovered spermatic animals. In the Journal de Sçavans, in the year 1774, there is a letter from Mr. Huguens, on the subject of a microscope, made by one small ball of glass, with which he asserts he perceived animals in the water, wherein pepper had been infused for two or three days, as Leeuwenhoek before had observed with the like microscopes, but whose balls were not so minute. "There are also other seeds, he continues, which engender such animals, as coriander seeds, &c. and I have seen the same thing in the pith of the birch tree, after having kept it for four or five days; and some have observed them in the water where nutmegs and cinnamon have been soaked. These animals may be said to engender from some corruption or fermentation: but there are others which must have a different origin; as those in the seed of animals, which seem in such great numbers, as to be almost composed of them; they are all transparent, have a quick motion, and their figures are like the tadpole."
Huguens does not mention the author of this discovery; but in the Journal of the 29th of August in the same year, there is an extract of a letter of M. Hartsoeker, in which he gives the method of forming these glass balls by means of the flame of a lamp; and the author of the Journal says, "By this method he has discovered that little animals are engendered in urine which has been kept for some days, and have the figure of little eels: he found some in the seed of a cock, which appeared of the same form, but quite different from those found in the seed of other animals, which resemble tadpoles, or young frogs, before their legs are formed." The author seems to attribute the invention to Hartsoeker; but if we reflect on the uncertain manner in which it is there represented, and on the particular manner in which Leeuwenhoek speaks in his letter, written and published above a year before, we must allow him to be the first who made this observation; but between them a contest took place as to the discovery which has never been decided. Be this as it will, Leeuwenhoek was undoubtedly the first inventor of the microscope, whose focuses are balls of glass formed by the flame of a lamp. But to return to his observations.
I shall first remark, that what he says of the number and motion of these pretended animals is true; but the figure of the body is not always the same as he describes it: sometimes the part which precedes the tail is round and at others long; often flat, and frequently broader than it is long, &c. and with respect to the tail, it is often much larger and shorter than he asserts. The motion of vibrations which he gives to the tail, and by means of which he pretends that the animalcules advance progressively in this fluid, has never appeared to me as he has described it. I have seen these moving substances make eight or ten oscillations from the right to the left, or vice versa, without advancing the breadth of a hair; and I have even seen many more which could not advance at all; because this tail, instead of being of any assistance to them was, on the contrary, a thread attached either in the filaments or mucilaginous parts of the liquor, and rather retained the moving substance like as a thread fastened to the point retains the ball of a pendulum; and when this tail had any motion, it only resembled a thread which forms a curve at the end of an oscillation. I have seen these threads, or tails, fastened to the filaments which Leeuwenhoek stiles vessels; I have seen them separate after many reiterated efforts of the moving bodies; I have seen them at first lengthen, then diminish, and at last totally disappear. I therefore think these tails should be considered as accidental parts, and not as essential to the bodies of these pretended animals. But what is most remarkable, Leeuwenhoek precisely says, in his letter to Lord Brouncker, that, besides these animals that had tails, there were also smaller animals in this liquor, which had no other form than that of a globule. "His animalculis (caudatis scilicet) minora adhuc animalcula, quibus non nisi globuli figuram attribuere possum, permista erant." This is the truth; but after Leeuwenhoek had advanced that these animals were the only efficient principle of generation, and that they were transformed into human figures, he has only regarded those as animals which had tails; and as it was consistent for animals that were transformed into human figures, to have a constant form, he never afterwards mentions those smaller animalcules without tails; and I was greatly surprised, on comparing the copy of this letter with that he published twenty years after, in his 3d volume, where, instead of the above words, the following are found: "Animalculis hisce permistæ jacebant aliæ minutiores particulæ, quibus non aliam quam globulorum seu sphæricam figuram assignare queo;" which is quite different. A particle of matter to which he attributes no motion, is very different from an animalcule: and it is astonishing that Leeuwenhoek, in copying his own works, has altered this essential article. What he adds immediately after likewise merits attention: he says, that by the desire of Mr. Oldenburg he had examined this liquor three or four years before, when he took these animalcules for globules; that is, there are times when these pretended animalcules are no more than globules, without any remarkable motion, and others when they move with great activity; sometimes they have tails, and at others they have none. Speaking in general of spermatic animals he says, "Ex hisce meis observationibus cogitare cœpi, quamvis antehac de animalculis in seminibus masculinis agens, scripserim, me in illis caudas non detexisse, fieri tamen posse ut illa animalcula æque caudis fuerint instructa ac nunc comperi de animalculis in gallorum gallinaceorum semine masculino;" another proof that he has often seen spermatic animals of all kinds without tails.
In the second place we must remark, that the filaments which are seen in the seminal liquor before it is liquefied were discovered by Leeuwenhoek, and that in his first observations, before he had made any hypothesis on spermatic animals, he considered these filaments as veins, nerves, and arteries; and firmly believed all the parts and vessels of the human body might clearly be seen in the seminal liquor. This opinion he persisted in, in defiance of the representations which Oldenburg made to him on this subject from the Royal Society: but as soon as he thought of transforming these pretended spermatic animals into men, he no longer mentioned these vessels; and instead of looking on them as nerves, arteries, and veins, of the human body already formed in the seed; he did not even attribute to them the functions they really possess, the producing of these moving bodies: and he says, vol. I. p. 7, "Quid fiet de omnibus illis particulis seu corpusculis præter illa animalcula semini virili hominum inhærentibus? Olim & priusquam hæc scriberem, in ea sententia fui, prædictas strias vel vasa ex testiculis principium secum ducere, &c." And in another part he says, that if he had formerly written any thing on the subject of these vessels found in the seed, we must pay no attention to it.