THE ALLANTOIS AND ITS CHANGES. 957 



part of the alimentary canal. The allantois soon becomes a hollow pro- 

 trusion of the intestinal wall, growing out, in the form of a sac or 

 bladder, beyond the body of the embryo, and being placed like the 

 umbilical vessel, outside the sac of the amnion (Fig. 121). It carries 

 out with it numerous bloodvessels, which are developed in the middle 

 germinal layer, and are connected with the great vascular trunks of 

 the hinder part of the embryo. It quickly extends itself till it reaches 

 the inner surface of the shell-membrane of the egg, over the interior 

 of which it spreads, with its walls closely applied to each other, so as 

 to form a double membrane, the outer layer of which, in contact with 

 the shell-membrane, retains its bloodvessels, whilst those of the inner 

 layer next to the white of the egg become shrunken. The hollow stalk 

 or stem of the allantois, which is situated within the embryo, opens into 

 a small cavity, developed in connection with the lower end of the intes- 

 tine, named the urogenital sinus, which forms the rudimentary urfaiary 

 bladder. Between this cavity and the opening in the walls of the 

 embryo, through which the vitelline duct passes to the umbilical sac, 

 hence called the umbilical opening, the allantoid canal closes, and is 

 converted into the urachus or superior ligament of the bladder. Out- 

 side the umbilical opening, the bloodvessels of the allantois, now named 

 the umbilical vessels, ramify in the outer layer of the allantois, next to 

 the shell-membrane, forming a densely vascular structure applied to the 

 inner surface of the whole shell, separated from it only by the shell- 

 membrane. In an early stage of development the allantois of the 

 Bird is contractile, and acts as a sort of urinary bladder, its fluid con- 

 taining urea, allantoin, sugar, certain salts corresponding with those 

 of the blood, with slight traces of albumen. It receives, indeed, the 

 secretions of certain organs known as the Wolffian bodies, or primordial 

 kidneys, and also those of the rudimentary kidneys themselves. Its 

 superficial vascular layer next to the shell-membrane, which has been 

 named the endochorion, is the active respiratory organ of the embryo 

 bird, after the vessels upon the yolk-sac have ceased to be sufficient for 

 this purpose. The blood to be aerated passes from the embryo through 

 the umbilical arteries on to the allantois, and returns to the embryo 

 by the umbilical vein. As the period of hatching approaches, the ves- 

 sels of the allantois and this membrane itself become partially dried; 

 the young bird chips the egg, and begins to breathe by its lungs. By 

 the time it escapes from the shell, the allantois and its vessels are quite 

 desiccated. 



The allantois is present also in Reptiles, the shell of the eggs of 

 which is soft and thin, and the oxygenation of the blood easily per- 

 formed. In Amphibia and Fishes there is no allantois, the respiration 

 of their embryos, which are entirely aquatic, being at once accom- 

 plished by means of gills, so that no allantois is needed. 



In the Mammalia, however, the allantois is invariably present, and 

 fulfils a most important office ; far it is the means of conveying out- 

 wards from the embryo to the maternal structures the vessels which 

 connect the two and serve in the functions of nutrition and respiration. 

 Its inner part forms, as usual, the urachus, and joins the apex of the 

 future urinary bladder ; and its outer part for a time constitutes the 



