27. While the process or metamorphosis of the ovulum noticed by Boer takes place in the ovarium, in consequence of fecundation, the cavity of the womb does not remain idle, but forthwith sets about weaving for itself a general lining—a sort of pseudo-textile membrane—which extends all over the cavity, descends partly into the cervix, and is _often_, (not necessarily always,) projected even into a great portion of the fallopian tubes. 28. This adventitious lining of the cavity of the womb is formed quite independently of the presence of the ovum, for it has been found in most cases of devious gestation, where the fœtus was _extra muros uteri_ (Plate VIII. IX. X. Fig. 1 and 2, and page 35), and has been found advanced in its progress of formation, while the ovulum was, as yet, on its way through the fallopian tube after fecundation. (Haller, Lobstein, Velpeau, Meckel, Pockels.) 29. To this adventitious lining described by many ancient authors, but by none better than by Harvey, the name of decidua already mentioned (24, 26) has been given by Hunter, because, viewed as a production of the uterus it is caducous and not permanent like its own natural lining; as, however, it is no more caducous than any other of the involucra of the ovum, the denomination is not strictly correct. It is, nevertheless, more generally adopted than that of epi-chorion, given to the same pseudo-membrane by Chaussier. 30. Its mode of formation and its structure have been differently explained by different authors. The explanation offered by Burns runs counter to facts and realities. That author speaks of all the minute steps of generation, with the same confidence as if all had been demonstrated. His imaginings respecting primary and secondary vessels shooting out from the inner surface of the womb to form the decidua must be gratuitous, as no other anatomist or physiologist, either before or after him, has been able to ascertain the reality of such a process. 31. It is probable that the decidua consists of two laminæ, inasmuch as we always find it with one surface perfectly smooth and the other rough. If so, they are most intimately connected. It is at least one-twentieth of an inch in thickness during the first five or six weeks of uterofœtation, when its tissue is found to be more knotty, coarse, and full of short threads, (not unlike a very ordinary mat,) than a purely membranaceous or cloth-like lining would be. It is not until a more advanced period of gestation that the decidua becomes distinctly membranaceous, in which state it lines the entire cavity of the uterus. (Plate IX. and prep. 73, 75, 76, of Sir Charles Clarke’s Collection: also Dr. Agar’s beautiful specimen of impregnated uterus, Mus. Coll. Reg. Lond. Med., and several specimens in the College of Surgeons, particularly Sir W. Blizard’s case.)[3]