REPROD UCTION. 911 



With the latter instead of the unimpregnated ovum present in the Fallopian 

 tube, the hypertrophied uterine mucosa does not break away as in menstrua- 

 tion, but remains, and henceforth is called the deddua graviditatis, special 

 names being given to special parts. Entering the uterus, the ovum attaches 

 itself in an unknown manner to the wall of the womb. The part of the mucous 

 membrane that forms its bed is henceforth known as the decidua serotina ; as 

 the seat of the future placenta, it is physiologically the most interesting and 

 important portion of the uterine mucosa. The surrounding cells and tissues 

 are stimulated to active proliferation and grow around and over the ovum, 

 completely covering it with a layer, the decidua reflexa. The remainder of 

 the uterine lining membrane constitutes the deddua vera. Between the reflexa 

 and the vera is the uterine cavity. At first thickened, the reflexa later thins 

 away as the embryo grows, and approaches close to the vera ; finally it touches 

 the latter, and the original cavity of the body of the uterus becomes oblit- 

 erated. By the sixth month the reflexa disappears, either coalescing with the 

 vera or undergoing total degeneration (Minot). During the latter half of 

 gestation the vera itself thins markedly. This atrophy of the comparatively 

 unimportant reflexa and vera, in contrast to the placental hypertrophy of the 

 serotina, is interesting. The arrangement of the parts is well shown in the 

 accompanying illustration (Fig. 314). 



The Fetal Membranes. The segmented ovum absorbs nutriment at first 

 directly from its surrounding maternal tissues, and later through the mediation 

 of the placenta. Its growth and cell-division are active, and it increases in 

 size and complexity. It early takes the form of a generalized vertebrate em- 

 bryo, and by the fortieth day begins to assume distinctly human characteristics. 

 It becomes surrounded early by the fetal membranes, which are two in num- 

 ber, the amnion and the chorion or, as it is usually called in other vertebrates, 

 false amnion. The amnion is a thin, transparent, non-vascular membrane imme- 

 diately surrounding the embryo (Fig. 314). In origin a derivative of the embry- 

 onic somatopleure, later it becomes completely separated from the body of the 

 embryo. The space enclosed by the amnion, the amniotic cavity, within which 

 the embryo lies, is traversed by the umbilical cord and contains a serous fluid, 

 the liquor amnii. This fluid, highly variable in quantity, averages at full 

 term nearly a liter (If pints). It has in general the composition of a serous 

 fluid. It contains between 1 and 2 per cent, of solids, consisting of proteids 

 (0.06-0.7 per cent.), mucin, a minute and variable quantity of urea, and inor- 

 ganic salts. It is derived perhaps in part by exudation from the fetus, but 

 doubtless chiefly by transudation from the maternal fluids, as is indicated by 

 the ready appearance within the amniotic cavity of solutions injected into the 

 maternal veins. It bathes the entire surface of the embryonic body, and is, 

 moreover, apparently swallowed at times into the stomach, as the presence of 

 fetal hairs and epidermal scales within the alimentary canal attests. Its chief 

 functions appear to be those of protecting the fetus from sudden shocks and 

 from pressure, maintaining a constant temperature, and supplying the fetal 

 body with water. The proteid possibly confers upon it a slight nutritive 



