The different theories which the ingenuity of the physiologist has invented for the elucidation of this mysterious and wonderful process, have been supported with so much zeal and argument by the disciples of one school, and disputed with so much warmth and plausibility by those of another,[362] that to recite the merits and defects of each system would be a task as laborious in its execution, as it must be unsatisfactory and unprofitable in its results; we shall therefore not attempt to ascend into the scale of causes, but rest on the phenomenon of conception, as an ultimate fact, and confine our researches to the history of its œconomy. The series of changes which constitute the phenomena of conception and gestation are clearly proved by the experiments of _De Graaf_ to originate in the ovaries, and not in the uterus, as former physiologists had supposed. One or more of the vesicles, or _ova_, contained in the former of these organs, no sooner receive the vivifying impression communicated by the _coitus_ than they are loosened from their connection, and grasped by the fimbriæ of the Fallopian tube, by whose peristaltic contractions they are, in due time, conveyed into the uterus; the spot in the ovarium from which the ovum has been thus separated, when examined after death, exhibits a slightly lacerated appearance, as if the germ had been detached from a vesicle at the moment of conception, by the rupture of its parietes; to this structure, which from the colour that it assumes has been called by physiologists _corpus luteum_, we shall have frequent occasion to allude during the course of the present inquiry. While these actions are proceeding, the uterus passes through several contemporary changes, in order to prepare it for the reception of the _ovum_; its blood-vessels are increased in size, as seen in slight cases of inflammation; the texture of its internal surface becomes softer, and more spongy, and a white mucus is secreted, which, from the extreme delicacy of its arrangement, has been compared by _Harvey_[363] to the web of the spider; it soon, however, assumes a more solid form, becomes vascular, and adheres so as to form a lining to the whole cavity of the uterus, except at the orifices which lead to the Fallopian tubes, and the os uteri. _Dr. William Hunter_ considered it as the inner lamina of the uterus cast off, like the _exuviæ_ of some animals, after every conception, and he accordingly called it the _Decidua_, and from the manner of its passing over the ovum, the Decidua _Reflexa_.[364] It is not known what exact interval is required for the fœtal primordia to pass through the Fallopian tube, and descend into the cavity of the uterus. _Valisnieri_ and _Haller_ have never been able to find it distinctly in the latter viscus before the seventeenth day. As the mouth of the pregnant uterus is sealed up with gelatinous matter from the moment of conception, it is, under ordinary circumstances, incapable of