$y 2 FORMATION OF THE AMNION AND ALLANTOIC 



becomes filled with fluid the amniotic fluid is developed around the embryo 

 [so that the embryo really floats in the fluid of the amniotic sac]. In mammals 

 also, the amnion is developed very early, just as in birds (fig. 665, VII, A). 

 From the middle of pregnancy onwards, the amnion is applied directly to the 

 chorion; and united to it by a gelatinous layer of tissue, the tunica media of 

 Bischoff. 



Amniotic Fluid The amnion, and the allantois as well, are formed only in mammals, 

 birds, and reptiles, which have hence been called amniota, while the lower vertebrates, which 

 an- devoid of an amnion, are called anamnia. Composition. The amniotic fluid is a clear, 

 serous, alkaline fluid, specific gravity 1007 to 1011, containing, besides epithelium, lanugo 

 hairs, i to 2 per cent, of fixed solids. Amongst the latter are albumin ( T V to $ per cent. ), 

 mucin, globulin, a vitelline-like body, some grape-sugar, urea, ammonium carbonate, very 

 probably derived from the decomposition of urea, sometimes lactic acid and kreatinin, calcic 

 sulphate and phosphate, and common salt. About the middle of pregnancy, it amounts to 

 about 1-1-5 kilo. [2-2-33 lbs.], and at the end about 0'5 kilo. The amniotic fluid is of foetal 

 origin, as is shown by its occurrence in birds, and is, perhaps, a transudation through the 

 fcetal membranes. In mammals, the urine of the foetus forms part of it during the second 

 half of pregnancy (Gusscroic). In the pathological condition of hydramnion, the blood-vessels 

 of the uterine mucous membrane secrete a watery fluid. The fluid preserves. the foetus, and also 

 the vessels of the foetal membranes from mechanical injuries ; it permits the limbs to move 

 freely, and protects them from growing together ; and, lastly, it is important for dilating the 

 os uteri during labour. The amnion is capable of contraction at the 7th day in the chick; and 

 this is due to the smooth muscular fibres which are developed in the cutaneous plate in its 

 mesoblastic portion, but nerves have not been found. 



Allantois. From the anterior surface of the caudal end of the embryo, there 

 grows out a small double projection, which becomes hollowed out to form a sac 

 projecting into the cavity of the ccelom or pleuro-peritoneal cavity (fig. 665); it 

 constitutes the allantois, and is formed in the chick before the 5th day, and in 

 man, during the 2nd week. Being a true projection from the hind-gut, the 

 allantois has two layers, one from the hypoblast, and the other from the muscular 

 layer, so that it is an offshoot from the splanchnopleure. From both sides, there 

 pass on to the allantois the umbilical arteries from the hypogastric arteries, and 

 they ramify on the surface of the sac. The allantois grows, like a urinary bladder 

 gradually being distended, in front of the hind-gut in the pleuro-peritoneal cavity 

 towards the umbilicus ; and lastly, it grows out of the umbilicus, and projects 

 beyond it alongside the omphalo-mesenteric or vitelline duct, its vessels growing 

 with it (fig. 665, VII, a); but, after this stage, it behaves differently in birds and 

 mammals. 



In birds, after the allantois passes out of the umbilicus, it undergoes great development, so 

 that within a short time it lines the whole of the interior of the shell as a highly vascular and 

 saccular membrane. Its arteries are at first branches of the primitive aorta?, but with the 

 development of the posterior extremities they appear as branches of the hypogastric arteries. 

 Two allantoidal, or umbilical veins, proceed from the numerous capillaries of the allantois. 

 They pass backward through the umbilicus, and at first unite with the omphalo-mesenteric 

 veins to join the venous end. of the heart. In birds this circulation on the allantois, or second 

 circulation, is respiratory in function, as its vessels serve for the exchange of gases through the 

 porous shell. The circulation gradually assumes the respiratory functions of the umbilical 

 vesicle, as the latter gradually becomes smaller and smaller, and ceases to be a sufficient 

 respiratory organ. Towards the end of the period of incubation the chick may breathe and 

 cry within the shell {Aristotle) & proof that the respiratory function of the allantois is partly 

 taken over by the lungs. The allantois is also the excretory organ of the urinary constituents. 

 Into its cavity in mammals the ducts of the primitive kidneys, or the Wolffian ducts, open, but 

 m birds and reptiles, which possess a cloaca, these open into the posterior wall of the cloaca. 

 The primitive kidneys, or Wolffian bodies, consist of many glomeruli, and empty their secre- 

 tion through the Wolffian ducts into the allantois (in birds into the cloaca), and the secretion 

 passes through the allantois, per the umbilicus, into the peripheral part of the urinary sac. 

 Remak found ammonium and sodium urate, allantoin, grape-sugar, and salts in the contents of 

 the allantois. From the 8th day onwards, the allantois of the chick is contractile ( Vulpian), 

 owing to the presence of smooth fibres derived from the splanchnopleure. Lymphatics accom- 

 pany the branches of the arteries {A. Budge). 



Allantois in Mammals. In mammals and man, the relation of the allantois is 



