Those physiologists who deny the possibility of superfœtation, among whom we find some of the most celebrated names, assert that one conception can never supervene another in the same woman, because _the os uteri is closed by coaguable lymph, and the entrance to the fallopian tubes is obstructed by the Decidua Uteri, soon after conception_, and therefore that the semen can never find its way to the internal organs of generation, so as to impregnate a second ovum; this opinion is fortified by the well known aphorism of _Hippocrates_,[435] “οκοσαὶ εν γαστρὶ εχουσὶ; τουτεων δε στομα των υστερων ξυμμεμυκεν.” _Galen_[436] also quoting _Herophilus_ says, “_Ne specilli quidem mucronem admittere uteros antequam mulier pariat; prœterea ne vel minimum quidem hiscere ubi conceperint_.” Neither _Galen_, however, nor _Ætius_, nor _Paulus Ægineta_, make any mention of superfœtation, a circumstance upon which the opponents of the doctrine lay considerable stress. _Avicenna_ alludes to it, but for the purpose of expressing his disbelief in its possibility. _Hebenstreit_[437] and _Ludwig_,[438] have also expressed very strong opinions upon the subject; the former of whom observes, “_Nullæ fere observationes extra omnem dubitationem positæ superfœtationem confirmant_.” _Baudelocque_[439] is equally hostile to such a belief. But it may be said that the argument founded on the entire closure of the uterus is quite gratuitous, many authorities might be cited who disavow the fact, we have already adduced the opinion of _Haller_ upon this point; besides, are we sufficiently acquainted with the manner in which impregnation is effected to authorise any deductions from our hypothesis? We are completely ignorant in what way the male semen arrives at the internal organs,[440] nay, we are not even convinced that its direct transmission to the ovaria is essential to fecundation; it is possible that these organs may be stimulated by sympathy with the vagina. _Parsons_ opposes another argument to the doctrine of superfœtation; it is, says he, impossible, because the fallopian tubes become after conception too short to embrace the ovaria, but this opinion is successfully combated by _Haller_. The cases which have been cited to illustrate the phenomenon of superfœtation, are regarded by those who oppose the doctrine as instances in which a plurality of children has existed, and in which one of the following circumstances have occurred, viz. 1. The fœtus has prematurely died, but has remained in utero with the living child, to the full period of utero-gestation. 2. The descent of the ova into the uterus from the ovarium, has not observed the same order of time, one being more slowly evolved than another, although both might have been fecundated by the same coitus. This latter was the favourite idea of _Celoni_:[441] “I am therefore decidedly of opinion,” says he, “that this superfœtation is no other than a later developement of a fœtus contemporaneously generated.”