CHAP. VII.

 

Comparison of my own Experiments with those of Leeuwenhoek.

 

            THOUGH my experiments were made with all the attention of which I was capable, and though I often repeated them, I am satisfied that many thing must have escaped me.  I have only described what I saw, and what every man may see, at the expence [sic] of a little art and patience.  To free myself from prejudice, I even attempted to forget what other observers pretended to have seen, endeavouring, by this means, to be certain of seeing nothing but what really appeared; and it was not till I had digested my experiments, that I wished to compare them with those of former writers, and particularly with those of Leeuwenhoek, who had occupied himself more than 60 years in experiments of this kind.

 

            Whatever authority may be due to this acute observer, it is certainly allowable to institute a comparison between a man’s own observations, and those of the most respectable writer on the same subject.  By an examination of this kind, [193] truth may be established, and errors may be detected, especially when the only object of inquiry is to ascertain the genuine nature of those moving bodies which appear in the seminal fluids of all animals.

 

            In the month of November 1677, Leeuwenhoek, who had formerly communicated many microscopic observations to the Royal Society of London, concerning the juices of plants, the texture of trees, the optic nerve, rain-water, &c. writes to Lord Brouncker, president of the Society, in the following terms:  “Postquam* Esxc. Dominus Professor Cranen me visitatione sua saepius honorarat, litteris rogavis Domino Ham cognate suo, quadsam observationum mearum videndas darem.  Hic Dominus Ham me secundo invisens, secum in laguncula vitrea semen viri, gonorrhoea laborantis, sponte destillatum, attulit, dicens, se post paucissiman tempris minuitias (cum material illa jam in tantum effet resoluta ut sistulae vitreae immitti posset) animalcula viva in eo observasse, quae caudam et ultra 24 horas non viventia judicabat:  Idem referebat se animalcula observasse mortua post sumptam ab aegroto therebintinam.  Materiam praedicatam sistulae vitreae immissam, praesente Domino Ham, observavi, quasdamque in ea creaturs viventes, ac post decursum 2 au 3 horarum eamdem solus materiam observans, mortuas vidi.”  [194]

 

            “Eamdem materiam (semen virile) non aegroti alicujus, non diuturna conservatione corruptam, vel post aliquot momenta fluidiorem factam, sed sani viri statim post ejectionem, ne interlabentibus quidem sex arteriae pulsibus, saepiuscule observavi, tantamque in ea viventium animalculorum muyltitudinem vidi, ut interdum plura quam 1000 in magnitudine arenae sese moverent; non in toto femine, sed in material fluida crassiori adhaerente, ingentem illam animalculorum multitudinem observavi; in crassiori vero seminis material quasi fine motu jacebant, quod inde provenire mihi imaginabar, quod material illa crassa ex tam variis cohaereat partibus, ut animalcula in ea se ruborem adferentibus haec animalcula errant, out judicem, millena millia arenam grandiorem magnitudine non aequatura.  Corpora eorum rotunda, anteriora obtuse, posteriora ferme in aculeum definentia habebant; cauda tenui longitudine corpus quinquies sexiesve excedente, et pellucida, crassitiem vero ad 25 partem corporis habente, praedita errant, adeo ut ea quo ad figuram cum cyclaminis minoribus, longam caudam habentibus, optime comparare quaem:  Motu caudae serpentino, aut ut angguillae in aqua natantis, progrediebantur; in material vero aliquantulum crassiori caudam octies deciesve quidem evibrabant antequam latitudinem capilli procedebant.  Interdum imaginabar me inter- [195] noscere posse adhuc varias in corpore horum animalculorum partes, quia vero continuo eas videre nequibam, de iis tacebo.  His animalcules minor adhuc animalcula, quibus non nisi globuli figuram attribuere possum, permista errant.”

 

            “Memini me ante tres aut quatuoro annos, rogatu Domini Oldenburg B.M. semen virile observasse, et praedicta animalia pro globulis habuisse; sed quia fastidiebam ab ulteriori inquisitione, et magis quidem a descriptionoe, tunc temporis eam omisi.  Jam quoad partes ipsas, ex quibus crassam seminis materiam, quoad majorem sui partem consistere saepius cum admiratioone observavi, ea sunt tam varia ac multa vasa, imo in tanta multitudine haec vasa vidi, ut credam me in unica seminis gutta plura observasse quam anatomico per integrum diem subjectum aliquod secanti occurrant.  Quibus visit, firmiter credebam nulla in corpore humano jam formato esse vasa, quae in semine virili bene constituto non reperiantur.  Cum material haec per momenta quaedam aëri fuisset exposita, praedicta vasorum multitude in aquosam magnis oleaginosis globulis permistam materiam mutabatur,” &c.

 

            The secretary of the Royal Society replied to this letter of Leeuwenhoek, that it would be proper to make similar experiments on the seminal fluids of other animals, not only to support the original discovery, but to distinguish whatever differences might appear in the number and [196] figure of the animalcules:  And, with regard to the vascular texture of the thick part of the seminal fluid, he suspected that it was only a congeries of filaments, without any regular organization.  “Quae tibi videbatur vasorum congeries, fortassis seminis sunt quaedam filamenta, haud organice constructa, sed dum permearunt vasa generationi inservientia in istiusmodi figuram elongate.  Non dissimili modo ac saepius notatus sum salivam crassiorem ex glandularum faucium foraminibus editam, quasi e convolutes fibrillis constatem.”*

 

            Leeuwenhoek replied, 18th March 1678, in the following words:  “Si quando canes coeunt marem a foemina statim sepoonas, ateria quaedam tenuis et aquosa (lympha scilicet spermatica) a pene solet paulatim exsillare; hanc materiam numerosissimis animalcules repletam aliquoties vidi, eorum magnitudine quae in semine virili conspiciuntur, quibus particulae globulares aliquot quinquagie majores permiscebantur.”

 

            “Quod ad vasorum 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 pertissimo anatomico haud unquam magis perspicue observatas, quam mihi vasa in seminin virili, ope perspicilli, in conspectum venere.”  [197]

 

            “Cum mini praedicta vasa primum innotuere, statim etiam pituitam, tum et salivam perspicillo applicavi; verum hic minime existential animalia frustra quaesivi.”

 

            “A cuniculorum coitu lymphae spermaticae guttulam unam et alteram, e semella extillantem, examini subjeci, ubi animalia praedictorum similia, sed longe pauciora, comparuere.  Globuli item quam plurimi, plerique magnitudine animalium, iisdem permisti sunt.”

 

            “Horum animalium aliquot etiam delineations transmisi; figura 1. [pl. VI. Fig. 1.] exprimit eorum aliquot vivum (in semine cuniculi arbitror) eaqueforma qua videbatur, dum aspicientem me versus tendit.  A B C, capitulum cum trunco indicant; C D, ejusdem caudam, quam partier ut suam anguilla inter natandum vibrat.  Horum millena millia, quantum conjecture est, arenulae majoris molem vix superant.  [Pl. VI. Fig. 2. 3. 4.] sunt ejusdem generic animalia, sed jam emortua.”

 

            [Pl. VI. Fig. 5.] delineator vivum animalculum quemadmodum in semine canino sese aliquoties mihi attentius intuenti exhibuit.  E F G, caput cum trunco indigitant, G H, ejusdem caudam.  [Pl. VI. Fig. 6. 7. 8.] alia sunt in semine canino quae motu et vita privantur, qualium etiam vivorum numerum adea ingentem vidi, ut judicarem portionem lymphae spermaticae arenulae mediocri respondentem, ceorum ut minimum decena millia continere.”  [198]

 

            In another letter to the Royal Society, dated May 31. 1678, Leeuwenhoek adds, “Seminis canini tantillum microscopio applicatum iterum contemplatus sum, in eoque antea descripta animalia numerosissime conspexi.  Aqua pluvialis part quantitate adjecta, iisdem confestim mortem accerit.  Ejusdem seminis canini portiuncula in vitreo tubulo unciae partem duodecimalem crasso servata, sex et triginta horarum spatio contenta animalia vita destitute pleraque, reliqua moribunda videbantur.”

 

            “Quo de vasorum in semine genitali existential magis constaret, delineationem aliqualem mitto, ut in figura A B C D E [pl. VI. fig. 9.] quibus literis circumscriptum spatium arenulam mediocrem vix superat.”

 

            I have transcribed these passages from the Philosophical Transactions, because they first appeared in that work, before Leeuwenhoek had formed any theory; and, therefore, they must be more agreeable to truth.  After the ingenious author had formed a system of generation, his account of the spermatic animals varies, even in matters that are essential.*

 

            In the first place, what he says concerning the number of these pretended animalcules is true; but the figure of their bodies corresponds not always to his description.  Sometimes the end next the tail is globular and sometimes cylindri- [199] cal; sometimes it is flat, and at other times it is broader than long, &c.  With regard to the tail, it is often thicker and shorter than he makes it.  The vibratory motion, he ascribes to the tail, and by which he alledges the animals are enabled to proceed forward, I never could distinguish in the manner he describes it.  I have seen these moving bodies vibrate eight or ten times, from right to left, or from left to right, without advancing the breadth of a single hair; and I have observed many of them which never could proceed forward, because this tail, in place of assisting them to swim, was, on the contrary, a small thread attached to the filaments or mucilaginous part of the liquor, and, of course, totally prevented the progressive motion of the body.  Even when the tail appeared to have any motion, it only resembled the small bendings of a thread at the end of a vibration.  I have seen these threads or tails fixed to the filaments, which Leeuwenhoek calls vessels:  I have seem them separate from the filaments, after repeated efforts made by the moving bodies; I have seem them first long, then diminish, and at last disappear.  Thus these tails ought to be regarded as accidental appendages, and not as real members of the moving bodies.  But, what is more remarkable, Leeuwenhoek expressly affirms, in this letter to Lord Brouncker, that, besides the tailed animals, he observed in this liquor animalcules still more minute that had no tails, and were perfectly globular:  “His [200] [PLATE VIII here] animalcules (caudatis scilicet) minora adhuc animalcula, quibus non nisi globuli figuram attribuere possum, permista errant.”  This is the truth.  After Leeuwenhoek, however, had maintained that these animalcules were the only efficient principle of generation, and that they were transformed into men, he regards as real animals only those which had tails; and, accordingly, as it was necessary that animalcules, to be transformed into men, should have a constant and invariable figure, he never afterwards mentions the round animals without tails.  I was struck with the difference between the original composition of this letter, and the form in which it appeared twenty years afterwards in his third volume:  For, instead of the words which we have just now quoted, we meet with the following in pag. 63.  “Animalculis hisce permistae jacebant aliae minutiores particulae, quibus non aliam quam globulorum seu sphaericam figuram assignare queo.”  This is a very different account of the matter.  A particle of matter, to which he ascribes no motion, is extremely different from an animalcule.  It is astonishing that Leeuwenhoek, in copying his own letter, should have changed an article of so much consequence.  What he immediately subjoins likewise merits attention.  He says, that, at the intreaty of Mr Oldenburg, he had examined this liquor three or four years ago; and that he then imagined these animalcules to be globules.  Thus, these [201] pretended animalcules are sometimes globules without any sensible motion; and sometimes they are globules which move with great activity; sometimes they have tails, and sometimes no tails.  Speaking of spermatic animals in general, he remarks,* “Ex hisce meis observationibus cogitare coepi, quamvis anthehac, de animalcules in seminibus masculinis agens, scripserim me in illis caudas non detexisse, fieri tamen posse ut illa animalcula aeque caudis fuerint instructa, ac nunc comperi de animalcules in gallorum gallinaceorum semine masculine:”  Another proof that he has often seem spermatic animals of all kinds, without tails.

 

            Secondly, It is worthy of remark, that Leeuwenhoek had very early discovered the filaments which appear in the semen before it be liquified; and that, at that time, when he had not framed his hypothesis concerning the spermatic animals, he conceived the filaments to be veins, nerves, and arteries.  He firmly believed, that all the parts and vessels of the human body might be clearly distinguished in the seminal fluid.  He even persisted in this opinion, notwithstanding the representations made to him by Mr Oldenburg, in name of the Royal Society.  But, after he conceived the notion of transforming his spermatic animals into men, he never again takes any notice of these vessels.  Instead of regarding them as the nerves and blood-ves- [202] sels of the hyuman body already formed in the semen, he does not even ascribe to them their real function, which is the production of the moving bodies.  He observes,* “Quid fiet de omnibus illis particulis sue corpusculis praeter illa animalcula semini virili hominum inhaerentibus!  Olim et priusquam haec scriberem, in ea sentential fui praedictas strias vel vasa ex testiculis principium secum ducere,” &c.  And, in another place, he says, that what he had formerly remarked concerning vessels in the semen deserved no attention.

 

            Thirdly, If we compare the figures 1. 2. 3. and 4. Pl. VI. and VII. Which we have represented exactly as they appear in the Philosophical Transaction, with those which Leeuwenhoek caused to be engraved several years after, we shall find very great differences, especially in those of the dead animalcules of the rabbit, 1.3. and 4. and in those of the dog, which I have also delineated, in order to give a distinct idea of the matter.  From all this, it may fairly be concluded, that Leeuwenhoek has not always seen the same phaenomena; that the moving bodies, which he regards as animals, have appeared to him under different forms; and that he has contradicted himself with a view to make the species of men and of animals uniform and consistent.  He not only varies as to the fundamental part of these experiments, but also as to the manner of ma- [203] king them; for he expressly tells us, that he always diluted the semen with water, to separate its parts, and to give more freedom of motion to the animalcules;* and yet, in his first letter to Lord Brouncker, he says, that, when he mixed the semen of dogs, in which he before had seen innumerable animals, with water, they all instantly died.  Thus Leeuwenhoek’s first experiments were made, like mine, without any mixture; and, it appears, that he was not in use to mix the liquor with water till long after he began his experiments, and till he conceived the idea of water’s killing the animalcules; which, however, is not true:  I imagine that the addition of water only dissolves the filaments too suddenly; for, in all my experiments, I have seen but very few filaments in the liquor, after its being mixed with water.

 

            Leeuwenhoek was no sooner persuaded that the spermatic animalcules were transformed into men and other animals, than he imagined that he saw two distinct kinds in the semen of every animal, the one male, and the other female.  Without this difference of sex in the spermatic animalcules, it was difficult, he says, to conceive the possibility of producing males and females by simple transformation.  He mentions these male and female animalcules in his letter published in the Philosophical Transactions, No. 145. and in several other places.+  But he attempts not to [204] describe the differences between male and female animalcules, which never existed but in his own imagination.

 

            The famous Boerhaave having asked Leeuwenhoek, whether he had observed any differences in the growth and size of spermatic animals?  Leeuwenhoek replied, that, in the semen of a rabbit which he had opened, he saw an infinite number of animalcules:  “Incredibilem,” says he,” viventium animalculorum numerum conspexerunt, cum haec animalcula scypho imposita vitreo et illic emortua, in rariores ordinies disparassent, et per continuos aliquot dies saepius visu examinassem, quadam ad justam magnitudinem nondum excrevisse adverti.  Ad haec quasdam observavi particulas perexiles et oblongas, alias aliis majores, et, quantum aculis apparebat, cauda destitutas; quas quidem partiulas non nisi animalcula esse credidi, quae ad justam magnitudinem non excrevissent.”*  Here we have animalcules of different sizes, and some with tails, and others that had no tails, which better corresponds with my experiments than with Leeuwenhoek’s system.  We only differ in a single article.  He considers the oblong bodies without tails as young animalcules which have not yet arrived at their full growth: But I, on the contrary, have seen these pretended animals originally spring from the filaments with their tails or threads, which they gradually lost.  [205

           

            In the same letter to Boerhaave he says,* “that, in the semen of a ram he observed the animalcules marching in flocks like sheep:  “A tribus circiter annis testes arietis, adhuc calentes, ad aedes meas deferri curaveram; cum igitur materiam ex epididymibus eductam, ope microscopii contemplarer, non fine ingenti voluptate advertebam animalcula omnia, quotquot innatabant semini masculine, eundem natando cursum tenere, ita nimirum ut quo itinere priora praenatarent, eodem posteriora subsequerentur, adeo ut hisce animalcules quasi sit ingenitum quo doves facitare videmus, scilicet ut praecedentium vestigiis grex universus incedat.”  This observation, made by Leeuwenhoek in the 1713, and which he regarded as new and singular, is a sufficient proof that he had never so attentively examined the seminal fluids of animals as to enable him to give exact descriptions of them.  In the 1713, he was 71 years of age:  He had been in the constant practice of making experiments with the microscope for 45 years:  He continued to publish his observations during 36 years:  And yet, after all this practice, he now, for the first time, observed a phaenomenon which is exhibited in every semen, and which I have described, Exper. IX. In the human semen, Exper. XII. In the semen of a dog, and Exper. XXIX in the semen of a bitch.  To explain the moving of the animalcules of the ram in flocks, [206] [PLATE IX here] therefore, it is unnecessary to suppose them endowed with the natural instinct peculiar to sheep; since those of man, of the dog, and of the bitch, move in the very same manner; and since this motion depends upon particular circumstances, the principal of which is, that the fluid part of the semen should be on one side, and the thick or filamentous part on the other; for then the whole moving bodies disengage themselves from the filaments, and proceed, in the same direction, into the more fluid part of the liquor.

 

            In another letter, written the same year, and addressed to Boerhaave,* he relates some farther observations concerning the semen of the ram:  He tells us, that, when the liquor was put into separate glasses and examined, he observed flocks of animalcules moving all in the same direction, and other flocks returning the contrary way.  He adds: “Neque illud in unica epididymum parte, sed et in aliis quas praecideram partibus, observavi.  Ad haec, in quadam parastatarum resecta portione complura vidi animalcula, quae necdum in justam magnitudinem adoleverant; nam et corpuscular illis exiliora et caudae triplo breviores errant quam adultis.  Ad haec, caudas non habebant definentes in mucronem, quales tamen adultis esse passim comperio.  Praeterea, in quadam parastatarum portionem incidi, animalcules, quantum discerner potui, destitutam, tantum illi quaedam perexiguae inerrant [207] particulae, partim longiores partim breviores; sed altera sui extremitate crassiunculae; istas particularlas in animalcula transituras esses non dubitabam.”  From this passage, it is apparent that Leeuwenhoek had seen in this seminal liquor, what I have found in the semen of all the animals which I examined, moving bodies that differed in size, figure, and motion; and these circumstances, it is obvious, correspond better with the notion of organic particles in motion, than with that of real animals.

 

            It appears, therefore, that Leeuwenhoek’s observations, though he draws very different conclusions from them, perfectly correspond with mine:  And, though there be some opposition in the facts, I am fully persuaded, that, whoever shall take the trouble of repeating the experiments, will easily discover the source of these differences, and find that I have related nothing but the truth.  To enable the reader to decide in this matter, I shall add a few remarks.

 

            We do not always see, in the human semen, the filaments I have mentioned: For this purpose, the liquor must be examined the moment it is extracted from the body; and even then they do not uniformly appear.  When the liquor is too thick, it presents nothing but large globules, which may be distinguished with a common lens.  When examined with the microscope they have the appearance of small oranges; they are very opaque, and one of them oc- [208] cupies the whole field of the microscope.  The first time I observed these globules, I imaged them to be foreign bodies which had fallen into the liquor.  But, after examining different drops, I found that the whole liquor was composed of these large globules.  I observed one of the largest and roundest of them for a long time.  At first it was perfectly opaque:  A little after, I perceived on its surface, about half way between the centre and circumference, a beautiful coloured luminous ring, which continued more than half an hour, then gradually approached the centre, which became clear and coloured, while the rest of the globule remained opaque.  This light which illuminated the centre, resembled that which appears in large air-bubbles.  The globule now began to grow flat, and to have a small degree of transparency:  And, after observing it for three hours, I could perceive no other change, no appearance of motion, either internal or external.  I imagined that some change might happen by mixing the liquor with water.  The globules were indeed changed into a transparent homogeneous fluid, which presented nothing worthy of remark.  I left the semen to liquefy of its own accord, and examined it at the distance of 6, 12, and 24 hours; but found nothing like life or motion.  I relate this experiment to show, that the ordinary phaenomena are not always to be expected in seminal fluids, though they be apparently similar.  [209]

 

            Sometimes all the moving bodies have tails, especially in the semen of man, and of the dog; their motion is not then very rapid, and appears to be performed with difficulty.  If the liquor be allowed to dry, the tails or threads are first deprived of motion; the anterior extremity continues to vibrate for some time, and then all motion ceases.  These bodies may be long preserved in this state; and, if a small drop of water be then poured upon them, their figure changes; they fall down into several minute globules, which appear to have a small degree of motion, sometimes approaching each other, and sometimes trembling, and turning round their centres.

 

            The moving bodies in the human semen, and in those of the dog and bitch, resemble each other so strongly, that it is not easy to distinguish them, especially when examined immediately after they are taken from the body of the animal.  Those of the rabbit appear to be smaller and more active.  But these differences and resemblances proceed more from the different states of the fluids during the time of examination, than from the nature of the fluids themselves, which ought indeed to be different in different species of animals:  For example, in the human fluid, I have remarked large filaments, as represented in Pl. I. fig. 3. &c. and I have seen the moving bodies separate from these filaments, from which they appeared to derive their origin.  But I could perceive nothing of this kind in the [210] semen of the dog.  In place of distinct filaments, it is generally composed of a compact mucilage, in which we with difficulty perceive some filamentous parts; and yet this mucilage gives birth to moving bodies similar to those in the human semen.

 

            The motion of these bodies continues longer in the fluid of the dog, than in that of man, which enables us more easily to distinguish the change of form above taken notice of.  The moment the fluid issues from the body of the animal, we find most of the animalcules possessed of tails.  In 12, 24, or 36 hours afterwards, almost the whole tails disappear; we then perceive only oval bodies moving about, and generally with more rapidity than at first.

 

            The moving bodies are always below the surface of the liquor.  Several large transparent air-bubbles commonly appear on the surface; but they have no motion, unless when the liquor is agitated.  Below the moving bodies we often perceive others still more minute:  These have no tails; but most of them move:  And, in general, I have remarked, that, of the numberless globules in all these liquors, the smallest are generally blacker and more obscure than the others; and that those which are extremely minute and transparent have little or no motion.  They seem likewise to have more specific gravity; for they are always sunk deepest in the fluid.  [211]

Notes

 *  See phil. trans. No. 141. p. 1041 [back to page 194].

 

*  See phil. trans. No 141. p. 1043 [back to page 197].

 

*  Here the author attempts a formal proof that Leeuwenhoek invented the single microscope, and the existence of spermatic animals, before Hartsoeker, which interrupts the argument, is noways interesting to the reader, and is here omitted in the translation [Smellie’s note.  Back to page 199].

 

*  Tom. 3. p. 371 [back to page 202].

 

*  Tom. 1. p. 7 [back to page 203].

 

*  Tom 3. p. 92. 93 [back to page 204].

+  See tom. 1. p. 163. and tom. 3. p. 101. of his works [back to page 204].

 

*  See tom. 4. p. 280. 281 [back to page 205].

 

*  See tom. 4. p. 304 [back to page 207].