DEVELOPMENT OF THE CHICK. 605 



the foetus and the vitelline sac, and taking the place of the albumen 

 which has been liquefied and absorbed. 



It will also be seen, by reference to the figure, that the umbilical 

 vesicle is at the same time formed by the separation of part of the 

 vitellus from the abdomen, of the chick ; and the vessels of the area 

 vasculosa, which were at first distributed over the vitellus, now 

 ramify, of course, upon the surface of the umbilical vesicle. 



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

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

 later periods of incubation, at whatever point we break open the 

 egg, we find the internal surface of the shell-membrane lined with 

 a vascular membranous expansion, supplied by arteries which 

 emerge from the abdomen of the foetus. 



It is easy to see, accordingly, with what readiness the absorption 

 and exhalation of gases may take place by means of the allantois. 

 The air penetrates from the exterior through the minute pores of 

 the calcareous shell, and then acts upon the blood in the vessels of 

 the allantois very much in the same manner that the air in the minute 

 bronchial tubes and air- vesicles of the lungs acts upon the blood in 

 the pulmonary capillaries. Examination of the egg, furthermore, 

 at various periods of incubation, shows that changes take place in 

 it which are entirely analogous to those of respiration. 



The egg, in the first place, during its development, 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 comparatively 

 slight degree, in unimpregnated eggs, or in those which are not 

 incubated, though they may be freely exposed to the air. The 

 exhalation of fluid is also essential to the processes of development, 

 for it has often been found, in hatching eggs by artificial warmth, 

 that if the air of the chamber in which they are inclosed become 

 unduly charged with moisture, so as to retard or prevent further 

 exhalation, the eggs readily become spoiled, and the development 

 of the embryo is arrested. The loss of weight during natural incu- 

 bation, principally due to the exhalation of water, has been found 

 by Baudrimont and St. Ange' to be over 15 per cent, of the entire 

 weight of the egg. 



Secondly, the egg absorbs oxygen and exhales carbonic acid. 

 The two observers mentioned above, ascertained that during eigh- 



1 Du Developpement du Foetus. Paris, 1850, p. 143. 



