THE ALLANTOIS. 705 



iu the periphery of the ovum, yet in its origin it differs from the other 

 membranes now under consideration in its close connection with one of 

 the internal organs of the embryo. As ah-eady stated, this membrane 

 does not exist as a foetal structure in fishes or amphibia. 



In reptiles, birds and mammals, it is formed in connection with the 

 hinder part of the primitive intestine, is the bearer of an extended 

 capillary distribution of the umbilical or hypogastric vessels, and in 

 combination with them performs important functions connected with 

 the nutrition of the foetus and the aeration of the foetal blood. 



The recent observations of His and of Dobrynin have shown that 

 it springs from splanchno]:)leure elements of the mesoblast and hypoblast, 

 below and in front of the caudal extremity of the embryo close to 

 the place of division of the mesoblast into its somatopleural and 

 splanchnopleurallaminfe. The former of these is reflected in the caudal 

 fold of the amnion already described ; the latter buds out from the 

 end of the primitive intestine into the pleuro-peritoneal space, and 

 receives within it an evolution or outfolded process of the hypoblastic 

 lining of the alimentary canal. It is placed at first rather behind the 

 part which later becomes the cloaca, the orifice of which is still closed : 

 but very soon it is doubled forwards upon the cloaca, so as to lie below 

 it, and when this orifice is afterwards opened it forms the common 

 outlet of the intestine and the allantois (fig. 510, and fig. 512, 3 and 4). 



The blood-vessels, which are developed with great rapidity in the 

 outer layer of the allantois, are formed in connection with those which 

 become the two umbilical arteries and the corresponding umbilical 

 veins, which last, however, do not run entirely in the same course as the 

 arteries, but join the omphalo-mesenteric and pass towards the liver; 

 one of the original veins very frequently becoming obliterated, as occurs 

 in the human subject. The capillary network spread over the sur- 

 face of the allantois appears almost as soon as the first prominence 

 of the membrane begins to bud out from the wall of the primitive 

 intestine, and the vessels appear at first to be in direct connection with 

 the terminations of the two primitive aortas ; but subsequently, when 

 the two aortge coalesce, the umbilical arteries appear as branches of the 

 iliac arteries (see the Development of the Vascular System). 



The allantois in expanding takes the shape of a pediculated flask-like 

 vesicle, extends into the pleuro-peritoneal space, and is filled with 

 fluid like the other membranes of the ovum. It is usually directed 

 towards the right side of the embryo, or the opposite from that on 

 which the yolk-sac is laid. In its subsequent great expansion iu the 

 egg of birds the allantois spreads out in a flattened form over the whole 

 internal surface of the membrane of the shell, thus coming to occupy 

 more and more of the space previously held by the albumen, the rapid 

 liquefaction and disappearance of which are coincident with the greatest 

 expansion of the allantois and other membranes. 



The allantois, though greatly flattened out in its most advanced state, 

 still consists of an outer and an inner wall, separated by the fluid, and 

 both bearing the finely ramified blood-vessels, which, however, are most 

 richly distributed on the outer division ; and in these last it is easy to 

 see, on opening an egg during incubation from the eighth day onwards, 

 the marked difl'erence of colour of the blood in the outgoing and 

 returning' vessels fi'om the action of the surrounding air on the blood 

 which has passed through the capillaries. 



