According to Mr. John Hunter, the decidua vera is continued some little way into the Fallopian tubes, more especially, on that side where the corpus luteum has been formed; it is perforated at the points where the Fallopian tubes enter, as well as at the os uteri, a fact which is beautifully shown in Dr. Hunter's last plate: but this does not continue long, for, as Mr. John Hunter observes, the inferiour opening becomes closed in the first month, and, according to Lobstein's observations, the openings of the Fallopian tubes are closed after the second month. "Where the decidua reflexa is beginning to pass over the chorion, there is, at an early period of pregnancy, an angle formed between it and the decidua, which lines the uterus; and here the decidua is often extremely thin and perforated with small openings so as to look like a piece of lace. "In proportion as pregnancy advances, the decidua reflexa becomes gradually thinner and thinner, so that at the fourth month it forms an extremely fine layer covering the chorion; it comes at the same time more and more closely in contact with the decidua, which lines that part of the uterus to which the placenta is not fixed, till at length they adhere together."[20] That portion of the decidua which passes between the placenta and uterus during the latter half of gestation, is called the _placental decidua_, the description of which will be given with that of the placenta. To Dr. W. Hunter are we indebted for the first correct description of the decidua; indeed, so excellent is it, that the membrane has been called after him, the _decidua of Hunter_. Although he was the undoubted discoverer of the reflexa, the existence of the decidua was distinctly noticed by Burton, in 1751. In stating the _post mortem_ examination of a woman, who died undelivered at the full time of pregnancy, he says, "Upon wiping the inside of the uterus very gently with a sponge, there seemed to be pieces of a very tender thin transparent membrane adhering to it in such parts of the uterus where the placenta did not stick to it; but as the womb was somewhat corrupted, and the membrane so very tender, we could not raise any bulk of it so as to be certain what it was." (Burton's _Midwifery_.) The decidua seems chiefly intended to form the maternal part of the placenta: (see _Placenta_:) hence in all those quadrupeds when the maternal part of the placenta is permanently appended to the internal surface of the uterus, no decidua is found. Having described the maternal membranes of the ovum, we come now to the membranes which form the parietes of the ovum. These are called the _foetal membranes_, for they are essentially connected with the origin of the foetus itself. They are the _chorion_ and the _amnion_; besides which, there are two others that require notice, viz. the _vesicula umbilicalis_ and _allantois_.