HUMAN FETAL MEMBRANES. PLACENTA. FETAL CIRCULATION. 975 



of the posterior extremities, as branches of the hypogastric artery. From the 

 rich capillary network of the allantois there originate two allantoic or umbilical 

 veins. These enter the navel and pass, at first in association with the omphalomes- 

 enteric veins, into the venous portion of the heart. In birds this allantoic circu- 

 lation (or second circulation) subserves the purpose of respiration, as its vessels 

 maintain an interchange of gases through the porous egg-shell. This circulation 

 gradually assumes the respiratory function of the yolk-circulation; this is neces- 

 sary, because the yolk-sac, steadily decreasing in size, no longer presents a suffi- 

 ciently large respiratory surface. Toward the end of the period of hatching the 

 bird can breathe and cry within the shell, a sign that the respiratory function of 

 the allantois is taken up, at least in part, by the lungs. The allantois is further- 

 more the excretory organ for the urinary constituents. Especially in mammals 

 the excretory ducts of the primitive kidneys, the Wolffian or Oken ducts, empty 

 into the cavity of the allantois (in birds and snakes, which possess a cloaca, they 

 empty into the posterior wall of the cloaca). The primitive kidney, consisting 

 of many glomeruli, discharges its secretion through the Wolffian duct into the 

 allantois (in birds into the cloaca) ; and the secretion passes by way of the allan- 

 tois through the navel into the peripheral portion of the urinary sac. Remak 

 found in the allantoic contents, ammonium and sodium urates, urea, allantoin, 

 grape-sugar, and salts. From the eighth day on, the allantois of the chick is 

 contractile from the presence of fibrillar cells that are derived from the splanch- 

 nopleure. Lymphatic vessels accompany the arterial branches. 



In mammals and in man, the relation of the allantois is somewhat 

 different. From the first part, the urinary bladder is formed; from the 

 vertex of this the urachus, at first still open, passes as a tube out through 

 the navel (Fig. 376, VIII, a). 



The blind sac of the allantois, which is situated outside the abdomen, 

 is in some animals filled with a fluid resembling urine. In man, how- 

 ever, this sac atrophies in the course of the second month. The vessels 

 alone remain and these apparently lie in the splanchnopleural portion 

 of the allantois. In some animals the allantoic sac continues to grow, 

 without undergoing atrophy, and then conveys from the bladder through 

 the urachus an alkaline, cloudy fluid that contains some albumin, sugar, 

 urea, and allantoin. The relations of the allantoic vessels will be 

 described in connection with the fetal membranes. 



HUMAN FETAL MEMBRANES. PLACENTA. FETAL CIRCULATION. 



When the fecundated ovum gains entrance into the uterus, it be- 

 comes surrounded by a particular membrane, which William Hunter 

 described as the deciduous membrane, because it is expelled in the act 

 of parturition. A distinction is made with regard to the basilar or true 

 decidua (Fig. 376, VIII, p), which is nothing else than the thickened, hy- 

 peremic, spongy endometrium, loosely attached to the uterine wall. From 

 this there develops an especial formation around the ovum, which 

 receives the latter as in the pocket of a swallow's nest; this thinner 

 membrane is known as the capsular decidua or decidua reflexa (VIII, r). 

 Between the second and the third month there is still a space in the 

 uterus outside of the decidua reflexa, but in the fourth month the entire 

 cavity is occupied by the ovum and the decidua. At one point the 

 ovum is thus applied directly to the endometrium (basal or true de- 

 cidua}; but in its greatest extent, however, it is in contact with the 

 decidua reflexa. The first layer forms later the placenta. 



The decidual swelling and softening of the endometrium begins in the mucosa 

 of the tubes of the cervical canal ; in the third month the membrane is from 4 to 7 

 mm. thick, in the fourth month only from i to 3 mm., and it is devoid of epithelium, 

 rich in blood- vessels, and has lymph-spaces around the glands and vessels; its spongy 



