AMNION AND ALLANTOIS. 743 



it embraces more and more of the inferior hemisphere, its vessels con- 

 verging toward the opposite pole of the yolk. 



The function of the area vasculosa is to absorb nourishment from the 

 cavity of the vitelline sac. As the albumen liquefies during the process 

 of incubation, it passes by endosmosis into the vitelline cavity ; the 

 whole yolk growing constantly larger and more fluid in consistency. 

 The blood of the embryo, circulating in the vessels of the area vasculosa, 

 absorbs the oleagino-albuminous matters of the vitellus, and, carrying 

 them back to the embryo by the returning veins, supplies the tissues 

 and organs with appropriate nourishment. 



During this period the amnion and the allantois have been also in 

 process of formation. At first the body of 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 posterior portion of the abdominal cavity, turns upward over the 

 body of the embryo, and comes in contact with the shell membrane. It 

 then spreads out rapidly, extending toward the two 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. 



The umbilical vesicle is at the same time formed by the separation of 

 part of the yolk from the abdomen of the chick ; and the vessels of the 

 original area vasculosa, which were at first distributed over the yolk, 

 now ramify upon the surface of the umbilical vesicle. 



At last the allantois, by its continued growth, envelops nearly the 

 whole of the remaining contents of the egg ; so that toward the later 

 periods of incubation, at whatever point we open the egg, the internal 

 surface of the shell membrane is found to be lined with a vascular ex- 

 pansion. This expansion is the allantois, supplied by arteries emerging 

 from the body of the embryo. 



The allantois is accordingly adapted, by its structure and position, to 

 perform the office of a respiratory organ. The air penetrates from the 

 exterior through the porous shell and its lining membranes, and acts 

 upon the blood in the vessels of the allantois much in the same manner 

 that the air in the lungs of the adult animal acts upon the blood in the 

 pulmonary capillaries. Examination of the egg, at various periods of 

 incubation, shows that changes take place in it which are entirely anal- 

 ogous to those of respiration. 



The egg, in the first place, during the development of the embr} r o, 

 loses water by exhalation. This exhalation is not a simple effect of 

 evaporation, but is the result of the nutritive changes going on in the 

 interior of the egg ; since it does not take place, except in a compara- 

 tively slight degree, in unimpregated eggs, or in those which are not 

 incubated, though freely exposed to the air. The exhalation of fluid is 

 also essential to the processes of development ; since it has been found, 

 in hatching eggs ~by artificial warmth, that if the air of the hatching 

 chamber become unduly charged with moisture, so as to retard or pre- 

 vent further exhalation, the development of the embryo is arrested. The 

 loss of weight during natural incubation, mainly due to the exhalation 



