"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.