744 ACCESSORY ORGANS IN IMPREGNATED EGG. 



of water, has been found by Baudrimont and St. Ange 1 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 above mentioned, ascertained that during eighteen days' 

 incubation, the egg absorbs nearly 2 per cent, of its weight of oxygen, 

 while the quantity of carbonic acid exhaled from the sixteenth to the 

 nineteenth day amounts to nearly of a gramme in twenty-four hours. 

 It is also observed that in the egg during incubation, as well as in the 

 adult animal, more oxygen is absorbed than is returned by exhalation 

 under the form of carbonic acid. 



The allantois, however, is not simply an organ of respiration; it 

 takes part also in the absorption of nutritious matter. As the process 

 of development advances, the skeleton of the young chick, at first carti- 

 laginous, begins to ossify. The calcareous matter, necessary for ossifi- 

 cation, is in great part derived from the shell. The shell is perceptibly 

 lighter and more fragile toward the end of incubation than at first ; and, 

 at the same time, the calcareous ingredients of the bones increase in 

 quantity. The lime-salts, requisite for ossification, are apparently ab- 

 sorbed from the shell by the vessels of the allantois, and thus transferred 

 to the skeleton of the growing chick ; so that, in the same proportion 

 that the former becomes weaker, the latter grows stronger. The dimi- 

 nution in density of the shell is connected not only with the develop- 

 ment of the skeleton, but also with the final escape of the chick from 

 the egg. This deliverance is accomplished mainly by the movements 

 of the chick itself, which become, at a certain period, sufficiently vigor- 

 ous to break out an opening in the attenuated shell. The first fracture 

 is generally accomplished by a stroke from the end of the bill ; and it 

 is precisely at this point that the solidification of the skeleton is most 

 advanced. The egg-shell, therefore, which at first serves for the pro- 

 tection of the embryo, afterward furnishes the materials which are used 

 to accomplish its own demolition, and at the same time to effect the 

 escape of the fully developed chick. 



Toward the latter periods of incubation, the allantois becomes more 

 adherent to the internal surface of the shell-membrane. At last, when 

 the chick, arrived at the full period of development, escapes from its 

 confinement, the allantoic vessels are torn off at the umbilicus ; and the 

 allantois itself, cast off as an effete organ, is left behind in the cavity 

 of the abandoned shell. 



Both the amnion and the allantois are, therefore, formations belong- 

 ing to the embryo, and constituting, for a time, accessory but essential 

 parts of its organization. Developed from the peripheral portions of 

 the outer and inner blastodermic layers, they are important organs 

 during the middle and latter periods of incubation ; but when the chick 

 lias become fully developed, and is ready to carry on an independent 

 existence, they are thrown off as obsolete structures, their place being 

 afterward supplied b}' organs belonging to the adult condition. 



1 DSveloppement du Foetus. Paris, 1850, p. 143. 



