ACCESSORY EMBRYONIC ORGANS. 643 



vitellus to rise toward the surface by virtue of its less specific gravity, 

 and bringing the blastoderm almost immediately beneath the lining 

 membrane of the shell. The embryo is thus placed in a favorable 

 position for the reception of warmth and other necessary external 

 influences. The liquefied albumen is absorbed by the vitelline mem- 

 brane, and the yolk becomes larger, softer, and more diffluent than 

 before incubation. 



In the earliest stages of the embryonic circulation the body of the 

 embryo is surrounded, in the adjacent parts of the blastoderm, by the 

 inosculating blood-vessels of the area vasculosa (page 637). As 

 development proceeds, this area increases in extent and its circulation 

 becomes more active. It covers the upper hemisphere of the yolk ; 

 and then, passing this level, embraces more and more of its inferior 

 hemisphere, extending nearly to its opposite pole. 



During this period the amnion and the allantois are in process of 

 formation. At first the embryo lies upon its abdomen, as heretofore 

 described ; but, as it increases in size, it alters its position so as to lie 

 upon its left side. The allantois, emerging from the abdominal cavity, 

 turns upward over the body of the embryo, and comes in contact with 

 the shell membrane. It then expands in every direction, toward the 

 extremities and down the sides of the egg, enveloping the embryo and 

 the vitelline sac, and taking the place of the albumen which has been 

 liquefied and absorbed. 



When the umbilical vesicle is formed, b/ f he partial separation of 

 the yolk from the abdomen of the embryo, th vessels of the area vas- 

 culosa, at first distributed over the yolk, then lamify upon the surface 

 of the umbilical vesicle. 



At last the allantois, by its continued growth, surrounds nearly all 

 the remaining parts ; so that, at whatever point the egg may be opened, 

 the internal surface of the shell membrane is found to be lined with a 

 vascular expansion. This expansion is the allantois, supplied by arteries 

 emerging from the body of the embryo. 



The allantois in the fowl's egg is accordingly adapted, by its structure 

 and position, to perform the office of a respiratory organ. The air 

 penetrates from without through the porous shell and its fining mem- 

 branes, and acts upon the blood in the vessels of the allantois in the 

 same way as in the pulmonary capillaries of the adult animal. Exam- 

 ination of the egg, at various periods of incubation, shows that it 

 undergoes changes similar to those of respiration. 



The egg, in the first place, during the development of the embryo, 

 loses water by exhalation. This is not the result of simple evaporation, 

 but depends upon nutritive changes in the interior of the egg ; since it 

 does not take place to the same degree in unimpregnated eggs, nor in 

 those which are not incubated, though freely exposed to the air. It 

 is also essential to development ; since in hatching eggs by artificial 

 warmth, if the air of the hatching chamber become charged with moist- 

 ure, so as to retard or prevent further exhalation, the development of 



