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 fœmina 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.