472 AN AMERICAN TEXT-HOOK OF PHYSIOLOGY. 



membrane that forms its bed is henceforth known as the decM.ua 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 rejiexa. The remainder of 

 the uterine lining membrane constitutes the decidua 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. 227). 



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. 227). 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 liquid, 

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

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

 liquid, 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. Its origin, whether from the fetus, especially from the fetal 

 kidneys, or from the mother, has been much discussed. It may possibly 

 come in small part from the former, but its chief origin is doubtless by trans- 

 udation from the maternal blood, as is indicated by the ready appearance 

 within the amniotic cavity of solutions injected into the maternal veins, and 

 the fact that the amniotic Liquid of diabetic women contains sugar. It bathes 

 the entire surface of the embryonic body, and is, moreover, apparently swal- 

 lowed 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 pro- 

 teid possibly confers upon it a very slight nutritive value, and the minute 

 quantity of urea is perhaps indicative of an unimportant excretory function 

 of the fital kidneys. As growth proceeds, the amnion expands and becomes 

 loosely attached to the outer fetal membrane, the chorion. 



