I Ialler and Kuhlemannt never found an ovum in the uterus of the sheep till the seventeenth day after copulation, and our own observations on both the sheep and sow agree precisely with theirs. Hausmann never found the ova in the uterus of the sow before the period of four weeks after conception, and those of the bitch before three weeks ; but here we must caution the reader against the error of sup posing that in the sheep and some other animals, because the ova have not been ob served in the uterus, they do not actually exist there previous to a certain date ; for the large size of the ovum and its membranes, as well as the state of the flatus, which though small is already somewhat developed, entitle us to conclude that the ovum of the sheep must have been some time in the uterus. The recent interesting observations by Al. Coste have thrown great light upon this subject, he having detected the ova of the sheep so early as five days after conception. In the cow also, in which the period of gestation is nearly twice the length of that in the sheep, the ovum seems to arrive almost as early in the uterus, if we may judge from the state of advancement of the flatus at an early period.: With regard now to the time at which the ovum first enters the uterus in the human fe male, let us examine the facts which are before us. The greater number of observations of this kind arc made on aborted products ; many of these are malformed or diseased, in consequence of which very probably they have been thrown off by abortion ; others are injured by the violence of the action which causes the uterus to be emptied of its contents. Our knowledge of the time of conception is generally founded upon the cessation of the menstrual flow on the first occasion when it ought to have recurred after conception has taken place, and conception may in the greater number of instances have taken place at any period of the interval. In a very few cases only have we any means of determining the time of concep tion, and in still fewer instances has there been an opportunity of examining the uterus in situ at an early period after conception when the perind of sexual intercourse was known. In by far the greater number of instances, there fore, there may be an error in the calculation of ten days or a fortnight.
It is by no means rare to see specimens of the human ovum or foetus in anatomical col lections marked as being a fortnight or three or four weeks old ; but it is now generally ac knowledged that the greater number of these are incorrectly marked. We have seen, however, more than one such ovum, which, both from the history of the cases and from the structure and size of the parts of the ovum and foetus, we should be inclined to consider as dating between three and four weeks after concep tion.* There are some who describe the human fetus at less than a fortnight old, and even as early as the eighth day, as in the well-known and often-quoted example described by Sir E. Ilome. But there is some reason to think that Sir E. home was mistaken in the case alluded to. Either, supposing that conception had occurred eight days before death, the body in question was not the fetus, or if it was the foetus, it must have been considerably older than he supposed.
The earliest example of the human ovum with which we are acquainted is that mentioned by ? M. Velpeau in his work sur l'Embryologie
Illumine; which, if he was not deceived by the person who gave it to him, he had the best reason to believe was discharged on the four teenth day after sexual intercourse.
This ovum, the description and drawings of which are very meagre, is described as about the size of a pea; the foetus was already some what formed, though very small ; and all points of structure in the foetus and ovum appear to us (so far as we can judge from the description) to correspond with one another, and to shew that the product was quite natural. This ovum from its size and from the state of advancement of the fetus must have been in the uterus at least two or three days.
We possess also the recent record of two valuable observations made on the structure of the gravid uterus of females dying suddenly eight days after sexual intercourse ; the one by Weber, the other by Professor Baer. No ovum was detected in either of these instances either in the uterus or tubes. We feel inclined to place much reliance on these two observations as being made by persons well acquainted with the various circumstances necessary to be at tended to in such a delicate investigation, and with all the advantages of recent knowledge, and though they afford negative evidence only, yet we are disposed to found upon them as proofs that at the eighth day the ovum has not descended into the uterus.
On comparing the degree of advancement of the foetus in the ovum described by Velpeau and in others with that of the fetus in the dog, cat, and sheep, at known periods, we would hazard the opinion that the human ovum arrives in the uterus on the eleventh or twelfth day after conception. Valentin thinks the twelfth or four teenth day, but we are inclined to believe that it cannot be much later than in the dog.
Change of the uterus after conception.— Before the arrival of the ovum in the uterus, a change has already taken place in the interior of that organ preparatory to the reception of the foetus. An exudation of a substance having many of the characters of organizable lymph furnishes a soft flaky lining to the cavity of the uterus, and serves to form a covering of the ovum when it afterwards descends into the uterus. This newly formed substance is re flected over the ovum so as to give it a double covering, the two layers of which constitute the two folds of the decidual membrane. The decidua is filled with bloodvessels formed by a process of organization similar to that which occurs in inflammatory adhesion by coagu lable lymph. These bloodvessels are conti nuous with those of the uterus, and as the ovum advances in the progress of develop ment, they are much dilated in some parts so as to form sinuses, which are ultimately intermingled, though by no means continuous with the bloodvessels which pass out of the umbilicus of the fcetus. The placenta or or ganic connection between the female parent and child, by means of which the respiration and partly also the nutrition of the latter is car ried on, is in great part formed in the decidua with which the flocculent chorion is closely in corporated; but the description of these parts belongs to another place.