644 REPRODUCTION. 



the embryo is arrested. The loss of substance during natural incuba- 

 tion, mainly from 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 above mentioned ascertained that during eighteen days' 

 incubation> the fowl's egg absorbs nearly two per cent, of its weight of 

 oxygen, while the carbonic acid, exhaled from the sixteenth to the nine- 

 teenth 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 also 

 takes part in the absorption of nutritious matter. As development 

 advances, the skeleton of the chick, at first cartilaginous, begins to 

 ossify. The calcareous matter necessary for this process 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 mineral constituents of the embryonic skeleton increase in 

 quantity. The lime-salts, requisite for ossification, are absorbed from 

 the shell by the vessels of the allantois, and transferred to the bones 

 of the chick ; so that, as the former becomes weaker, the latter grow 

 stronger. The diminution in density of the shell is connected not only 

 with the development of the skeleton, but also with the final release of 

 the chick from the egg. This deliverance is accomplished mainly by 

 the chick's movements, which become, at a certain period, sufficiently 

 vigorous to break 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 deposit of osseous matter is most abundant. The 

 egg-shell, therefore, which at first serves for the protection of the em- 

 bryo, afterward furnishes the materials which are to accomplish its own 

 demolition, and thus allow the escape of the fully developed chick. 



Toward the end of incubation, the allantois becomes more adherent 

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

 at its full period of development leaves the egg, the allantoic vessels 

 are torn off at the umbilicus ; and the allantois is left behind as an 

 effete organ 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 parts of its 

 organization. Developed from the peripheral portions of the blasto- 

 derm, they are important structures during the middle and latter periods 

 of incubation ; but when the young animal has become sufficiently devel- 

 oped to carry on an independent existence, they are detached from 

 their connections, and replaced by other organs belonging to the adult 

 condition. 



* Developpement du Foetus. Paris, 1850, p. 143. 



