When the fœtus does descend into the uterus, it is contained in a double membrane. The internal one is the amnion, and possesses no distinct vessels. The external one is the chorion, and is from the first vascular, and soon becomes so much so, that its vessels have by some been described as forming a distinct tunic. From what has been said, it will be evident, that, when this vesicle does reach the uterus, it will be received amongst the primary vessels which will surround it, whilst the secondary vessels, or decidua interna, will lie before it, and prevent any communication betwixt it and the uterine cavity. But it cannot remain long thus; for, in proportion as the vesicular part of the ovum increases, it will push the decidua interna before it, and encroach upon the cavity of the uterus. This circumstance, together with the intimate connection formed betwixt the vessels of the chorion and those of the decidua, with the consequent production of a placenta, I have already very fully detailed in a former publication, to which I refer[2]. When the ovum descends, and the subsequent changes are beginning to take place, the body of the uterus comes also to form decidua; for I have formerly said, that, until the fœtus comes into the uterus, the fundus alone yields this production. At the same time, the processes of the decidua interna elongate still more, and, together with those which are now formed by the portion produced by the newly-formed decidua externa (for additional primary vessels imply additional secondary ones) at the lower part of the uterus, will come to fill up all the intervening space betwixt the bottom of the chorion or decidua reflexa and the cervical efflorescence; so that, by the end of the second or beginning of the third month, the cavity of the uterus is quite filled, and we have the ovum perfectly organized. We have the fœtus inclosed in its membranes, and swimming in water; we have the placenta thick and well formed, and large in proportion to the membranes; we have the decidua reflexa distinctly seen, and the lower part of the uterus filled with the two layers of decidua, and the processes of the internal one.