CHAPTER VII.

COMPARISON OF MY OBSERVATIONS WITH THOSE OF LEEUWENHOEK.

Although I made the preceding experiments with all the circumspection possible; and although I repeated them a number of times, I am persuaded that many things escaped my notice; I have only related what I saw, and what all the world may see, with a little art and much practice. In order to be free from prejudices, I endeavoured to forget what other naturalists asserted to have seen, conceiving that by so doing, I should be more certain of only seeing in fact what really appeared; and it was not till after I had digested my observations, that I compared them with those of Leeuwenhoek, &c. I by no means pretend to have greater abilities in microscopical observations than that great naturalist, who passed more than sixty years in making various experiments.

Notwithstanding the authority his observations may justly claim, it is surely permitted to examine and compare others with them. Truth can only be gained by such examinations, and errors discovered, particularly as we do it without any partiality, and in the sole view of establishing something fixed and certain on the nature of those moving bodies seen in the seminal liquors.

In November 1677, Leeuwenhoek, who had already communicated to the Royal Society of London many microscopical observations on the optic nerve, the blood, the juice of the plants, the texture of trees, rain-water, &c. addressed to Lord Brouncker, President of the Society, in the following words: "Postquam Exc.[S] &c. Dominus Professor Cranen me visitatione sua sæpius honorarat, litteris rogavis, Domino Ham concrato suo, quasdam observationum mearum, videndas darem. Hic dominus Ham me secundo invisens, secum in laguncula, vitrea semen viri, gonorrhæa laborantis, sponte destillatum, attulit, dicens, se post paucissimas temporis minutias (cum materia ilia jam in tantum esset resoluta ut fistulæ vitreæ immitti posset) animalcula viva in eo observasse, quæ caudam & ultra 24 horas non viventia judicabat; idem referebat se animalcula observasse mortua post sumptam ab ægroto therebintinam. Materiam prædicatam fistulæ vitreæ immissam, præsente Domino Ham, observavi, quasdamque in ea creaturas viventes, at post decursum 2 aut 3 horarum eamdem solus materiam observans, mortuas vidi.

[S] See Phil. Trans. No. 141, page 1041.

"Eamdem materiam (semen virile) non ægroti alicujus, non diuturna conservatione corruptam, vel post aliquot momenta fluidiorem factam, sed sani viri statim post ejectionem, ne interlabentibus quidem sex arteriæ pulsibus, sæpiuscule observavi, tantamque in ea viventium animalculorum multitudinem vidi, ut interdum plura quam 1000 in magnitudine arenæ sese moverent; non in toto semine, sed in materia fluida crassiori adhærente, ingentem illam animalculorum multitudinem observavi; in crassiori vero seminis materia quasi sine motu jacebant, quod inde provenire mihi imaginabar, quod materia illa crassa ex tam variis cohæreat partibus, ut animalcula in ea se movere nequirent; minora globulis sanguini ruborem adferentibus hæc animalcula erant, ut judicem, millena millia arenam grandiorem magnitudine non æquatura. Corpora corum rotunda, anteriora obtusa, posteriora ferme in aculeum desinentia habebant; cauda tenui longitudine corpus quinquies sexiesve excedente, & pellucida crassitiem vero ad 25 partem corporis habente prædita erant, adeo ut ea quoad figuram cum cyclaminis minoribus, longam caudam habentibus, optime, comparare queam; motu caudæ serpentino, aut ut anguillæ in aqua natantis progrediebantur; in materia vero aliquantulum crassiori caudam octies deciesve quidem evibrabant antequam latitudinem capilli procedebant. Interdum mihi imaginabar me internoscere posse adhuc varias in corpore horum animalculorum partes, quia vero continuo eas videre nequibam, de iis tacebo. His animalculis minora adhuc animalcula, quibus non nisi globuli figuram attribuere possum, permissa erant.

"Memini me ante tres aut quatuor annos, rogatu Domini Oldenburg, B. M. semen virile observasse, & prædicta animalia pro globulis habuisse; sed quia fastidiebam ab ulteriori inquisitione, & magis quidem a descriptione, tunc temporis eam omisi. Jam quoad partes ipsas, ex quibus crassam seminis materiam, quoad majorem sui partem consistere sæpius cum admiratione observavi, ea sunt tam varia ac multa vasa, imo in tanta multitudine hæc vasa vidi, ut credam me in unica seminis gutta plura observasse quam anatomico per integrum diem subjectum aliquod secanti occurrant. Quibus visis, firmiter credebam nulla in corpore humano jam formato esse vasa, quæ in semine virili bene constituto non reperiantur. Cum materia hæc per momenta quædam aëri fuisset exposita, prædicta vasorum multitudo in aquosam magnis oleaginosis globulis permistam materiam mutabatur, &c."

The Secretary of the Royal Society, in answer to this letter, says, that it would be proper to make the like experiments on the seed of other animals, as dogs, horses, &c. not only to form a better judgment on the first discovery, but to know the differences which might be found in the number, and the figure of those animalcules. And with relation to the vessels of the thickest part of the seminal liquors, he greatly doubts they were only filaments without any organization, "quæ tibi videbatur vasorum congeries, fortassis seminis sunt quædam filamenta, haud organice constructa, sed dum permearunt vasa generationi inservientia in istiusmodi figuram elongata. Non dissimili modo ac sæpius notatus sum salivam crassiorem ex glandularum faucium foraminibus editam quasi e convolutis fibrilis constantem."[T]

[T] See the Secretary's answer to Leeuwenhoek's Letter in the Phil. Trans. No. 141, page 1043.

Leeuwenhoek answered him on the 18th of March, 1678, in the following words: "Si quando canes coeunt marem a foemina statim seponas materia quædam tenuis & aquosa (lympha scilicet spermatica) e pene solet paulatim exstillare; hanc materiam numerosissimis animalculis repletam aliquoties vidi, eorum magnitudine quæ in semine virili conspiciuntur, quibus particulæ globulares aliquot quinquagies majores permiscebantur.

"Quod ad vasorem in crassiori seminis virilis portione spectabilium observationem attinet, denuo non semel iteratam, saltem mihimetipsi comprobasse videor; meque omnino persuasum habeo, cuniculi, canis, felis, arterias venasve fuisse a peritissimo anatomico haud unquam magis perspicue observatas, quam mihi rasa in semine virili, ope perspicilli, in confectum venere.

"Cum mihi prædicta vasa primum innotuere, statim etiam pituitam, tum & salivam perspicillo applicavi; verum his minime existentia animalia frustra quæsivi.

"A cuniculorum coitu lymphæ spermaticæ guttulam, unam et alteram, e femella exstillantem, examini subjeci, ubi animalia prædictorum similia, sed longe pauciora, comparuere. Globuli item quam plurimi, plerique magnitudine animalium, iisdem permisti sunt.

"Horum animalium aliquot etiam delineationes transmisi, figura _a_ (_plate 3._) exprimit corum aliquot vivum (in semine cuniculi arbitror) eaque forma qua videbatur, dum aspicientem me versus tendit. A B C, capitulum cum trunco indicant; C D, ejusdem caudam, quam pariter ut suam anguilla inter natandum vibrat. Horum millena millia, quantum conjectare est, arenulæ majoris molem vix superant, (_fig. b, c, d,_) sunt ejusdem generis animalia, sed jam mortua.

"(_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 coepi, 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.

We shall observe in the third place, that if we compare the figures _a, b, c, d_, (PLATE III.) copied from the Philosophical Transactions, with those which Leeuwenhoek had engraved many years after, (PLATE IV.) we shall find considerable difference, especially in the figures of the dead animals, of a rabbit and in those of a dog, (which plate we have also copied for the satisfaction of our readers) from all which we may conclude, that Leeuwenhoek has not always observed objects entirely alive: that the moving bodies, which he looked upon as animals, appeared to him under different forms; and that he has varied in his assertions, with a view of making the species of men and animals perfectly consistent; he has not only varied in the basis of these experiments, but even in the manner of making them, for he expressly says, that he always diluted the liquor with water, in order to separate, and to give more motion to these animalcules: nevertheless, in his letter to Lord Brouncker, he says, that having mingled an equal quantity of rain water with the seminal liquor of a dog, in which he had before perceived an infinity of living animalcules, yet the mixing of this water killed them. The first experiment of Leeuwenhoek's therefore was made, like mine, without any mixture; and it even seems, that he was not of opinion to mix any water with the liquor till a long time after; because he thought he had discovered, by his first essay, that water caused the death of the animalculæ; which however is not the fact. I think that the mixture of the water only dissolves the filaments very suddenly; for I have seen but very few filaments in all the experiments I have made after mixing the water with the seminal liquor.

As soon as Leeuwenhoek was persuaded that spermatic animals were transformed into men, and other animals, he imagined he saw two sorts in the seminal liquor of every animal, the one male, and the other female; and this difference, according to him, served not only for the generation of themselves, but for the production of males and females, which was very difficult to conceive by a simple transformation. He speaks of the male and female animalcule, in his letter printed in the Philosophical Transactions, No. 145, and in many parts of his works,[U] but he does not describe the difference of these male and female animalcules, and which in fact never existed but in his own imagination.

[U] See vol I. page 163, and vol. III. page 101, of his works.

The famous Boerhaave having asked Leeuwenhoek, if he had not observed in spermatic animals different degrees of growth and size? Leeuwenhoek answered, that having dissected a rabbit, he observed in the semen an infinite number of living animals. "Incredibilem, says he, viventium animalculorum, numerum conspexerunt, cum hæc animalcula scypho imposita vitreo & illic emortua, in rariores ordines disparassent, & per continuos aliquot dies sæpius visu examinassem, quædam ad justam magnitudinem nondum excrevisse adverti. Ad hæc quasdam observavi particulas perexiles & oblongas, alias aliis majores, &, quantum oculis apparebat, cauda destitutas; quas quidem particulas non nisi animalcula esse credidi, quæ ad justam magnitudinem non excrevissent."[V] Here then are animalcules of different sizes, some with tails and others without, which much better agrees with my experiments, than with Leeuwenhoek's own system. We differ only in one particular; he says, that those without tails were young animalculæ, which were not arrived at their full growth; while I, on the contrary, have seen these pretended animals quit the filaments with tails or threads, and afterwards lose them by degrees.

[V] See vol. IV. pages 280 and 281.

In the same letter to Boerhaave, he says, in the semen of a ram, he perceived animalcules following each other in swarms like a flock of sheep. "A tribus circiter annis testes arietis, adhuc calentes, ad ædes meas deferri curaveram, cum igitur materiam ex epididymibus eductam, ope microscopii contemplarer, non sine ingenti voluptate advertebam animalcula omnia, quotquot innatabant semini masculino, eundem natando cursum tenere, ita nimirum ut quo itinere priora prinatarent eodem posteriora subsequerentur, adeo ut hisce animalculis quasis sit ingenitum, quod oves factitare vidimus, scilicet ut precedentium vestigiis grex universus incedat." This observation, which Leeuwenhoek made in 1713, and which he looks upon as singular and novel, proves to me, that he had never examined the seminal liquors of animals with attention, at least sufficient to give very exact descriptions of them. Leeuwenhoek was sixty-one years old in 1713, had made microscopical observations for more than forty-five years, had published the discovery of spermatic animals for about thirty-six years, and then, for the first time, saw in the seminal liquor of a ram, what is seen in all seminal liquors, and what I have described in Experiment IX. in the seed of a man; Experiment XII. in the seed of a dog; and in Experiment XXIX. in that of a bitch. It is not necessary to suppose the spermatic animals of the ram are endowed with instinct, to explain the floating of these animals, in flocks like sheep, since those of a man, dog, or bitch, does the same; and which motion depends solely on particular circumstances, whose principle is, that all the fluid matter of the seed is on one side, while the thick matter is on the other; for then all the bodies in motion will be disengaged from the mucilage, and follow the same road into the most fluid part of the liquor.

In another letter, written the same year, to Boerhaave, he relates some further observations he made on rams, and says, that he has seen, in the _vasa deferentia_, flocks of animals which float all on one side, and others which go in a contrary direction; and he adds, "Neque illud in unica epididymum parte, sed & in aliis quas præcideram partibus, observavi. Ad hæc, in quadam parastatarum resecta portione complura vidi animalcula, quæ necdum in justam magnitudinem adoleverant, nam et corpuscula illis exiliora & caudæ triplo breviores erant quam adultis. Ad hæc, caudas non habebant desinentes in mucronem, quales tamen adultis esse passim comperio. Præterea in quandam parastatarum portionem incidi, animalculis quantum discernere potui, destitutam, tantum illi quædam perexiguæ inerant particulæ, partim longiores, partim breviores, sed altera sui extremitate crassiunculæ; istas particulas in animalcula transituras esse non dubitabam." It is easy to see, by this passage, that Leeuwenhoek had seen, in this seminal liquor, what I found in all; that is to say, moving bodies of different sizes, figures, and motions; and which agrees much better with the idea of organic particles in motion than of that with real animals.

It appears, therefore, that Leeuwenhoek's observations are not contrary to mine, although he has drawn very different conclusions from them. I am persuaded that if any person would take the trouble of making the like experiments they would not have any difficulty in discovering from whence these differences proceed, and would find that I have advanced nothing which is not conformable to truth; and to enable the reader to decide thereon, I shall subjoin a few remarks.

The filaments I have spoken of are not always to be perceived in the seminal liquor of a man. To discover them it must be examined the moment it is taken from the body, and even then it will sometimes happen that there is not one to be seen. Sometimes the seminal liquor presents, especially when it is very thick, only large globules, which may be even distinguished with a common lens. By inspecting them with the microscope they appear like young oranges; they are very opaque, and a single one often fills up the whole table of the microscope. The first time I saw these globules I thought they were some foreign matters fallen into the liquor, but having examined different drops I discovered that the whole was composed of these thick globules. I selected one of the roundest, and whose size was such that, its centre being in the middle of the table of the microscope, I could at the same time observe the whole circumference; at first it was absolutely opaque; a short time afterwards I perceived a bright luminous ring to form on its surface, which remained about half an hour, and then approached by degrees towards the centre, which became clear, and of different colours, while the remainder of the globule continued opaque. This light, which brightened in the centre of the globule, resembled those seen in the great air bubbles. The globule then began to get a little flat, and acquire a small degree of transparency. Having examined it more than three hours I perceived no more alteration, nor any appearance of motion, either internally or externally. I then imagined, that by mixing this liquor with water, these globules might be changed; in fact they did change, but they presented only a transparent and homogeneous liquor, wherein was nothing remarkable. I suffered the seminal liquor to liquefy of itself, and examined it at the end of six, twelve, and twenty-four hours, but saw nothing more than a fluid; without the smallest resemblance of life or motion. I only relate this observation to shew that there are times when the common phenomena are not to be seen in the seminal liquor.

At times all the moving bodies appear to have tails, especially in the semen of a man and a dog; the motion is then the least brisk, and performed with difficulty. If this liquor is suffered to dry, the tails or threads are deprived of motion the first; the anterior extremity continues to vibrate for some time, and then all motion entirely ceases. These substances may be preserved in this state of dryness for a long time: if a small drop of water is mixed therewith, their figure changes, they are reduced into many globules, which sometimes appear to be in motion, as well by their approximation to each other, as by the trepidation and twirling round their centres.

These moving bodies in the seminal liquor of a man, dog, or bitch, so nearly resemble each other, as to admit of mistaking one for the other, especially if they are examined the moment the liquor is drawn from the animal. Those of the rabbit appear smaller and brisker; but these differences proceed more from the different states in which the liquor is at the time of observation, than from the nature of the liquor itself, which ought to be different in different kinds of animals; for example, in that of a man I have seen streaks of thick filaments, (_fig. 3._) and have perceived the moving bodies separate themselves from these filaments from whence they appeared to proceed; but I have never seen any thing like it in the semen of a dog; where, instead of filaments, or separated streaks, it is commonly a mucilage whose texture is more compact, and in which we with difficulty discern any filamentary parts; yet this mucilage gives birth to moving bodies like those in the semen of men.

The motions of these bodies remain a longer time in the liquor of a dog, than in that of a man; from which it is more easy to be certain of the alteration of form above mentioned. The moment the liquor issues from the body of the animal we perceive the animalcules to have tails; in twelve, twenty-four, or thirty-six hours after, we shall find they have lost those tails, and are then no more than ovals in motion, often much brisker than at first.

The moving bodies are always a little below the surface of the liquor. On the surface some large transparent air bubbles, which have no motion, generally appear, though sometimes these bubbles stir and seem to have a progressive motion, but which is nothing more than the agitation of the air. Below the moving substances we often see others much smaller, and which only appear like globules, having no tails, but the greatest number of which are oftentimes in motion. I have also generally remarked, that in the infinite number of globules, in all those liquors, those which are very small, are commonly black, or darker than the rest; and that those which are extremely minute and transparent, have but little or no motion; they appear also to weigh specifically heavier, for they are always the deepest in the liquor.