Another and very different explanation of this remarkable circumstance of the impreg nated ovum and corresponding corpus luteurn being found on opposite sides, has been given by Dr. Tyler Smith f, who believes that the ovum, after descending the Fallopian tube of one side, traverses the upper part of the uterine cavity, and ascends the opposite ovi duct, where it becomes developed. I might also furnish the advocates of this doctrine with an argument founded upon a most in teresting and curious observation of Bischoff, which appears to have been overlooked, but which would at first sight seem to support this view. Bischoff, in his essay on the de velopment of the ovum in the dog and rabbit, frequently noticed a remarkable apportioning of the ova between the two cornua of the uterus, so as to equalise their number on the two sides, when these had been ori ginally unequal, as shown by the number of corpora lutea found in the ovaries. Thus, in the case of a bitch whose right ovary ex hibited one, and the left ovary five corpora lutea, each half of the uterus contained three ova, so that two of the ova must have tra velled across from the right to the left side. But it must be observed, that in the cases recorded by Bischoff the ova never ascended the Fallopian tube, but only travelled from one cornu of the uterus to the other.
When, therefore, we take into considera tion the great difference between the solid uterus of man and the intestine-like organ of the mammalia, on which these observations were made, there appears to be great diffi culty in supposing that the ovum could after once arriving at the uterus again enter an oviduct, especially when also it is remembered that while the conical form of the Fallopian tube, whose smallest aperture is towards the uterus, constitutes a provision for ensuring the arrival of the ovum there, this arrange ment would greatly diminish the possibility of a retrograde tnovement taking place in the human subject, if indeed it would not alto gether prevent it.
But to those cases of tubal gestation in which the corpus luteum is found in the cor responding ovary, neither of these explanations would apply. Here it is only necessary to suppose, that either the developmental changes already described as occurring normally to the ovum in the tube, have proceeded more rapidly than usual, or else, that the ovutn, having been accidentally delayed for a longer time than ordinary in transitu, had acquired too great a magnitude to admit of its passage by the ute rine orifice, even admitting, as some have supposed, that this orifice may, to a certain extent, dilate, for the purpose of allowing the ovum to pass, just as the os uteri dilates at the time of labour.