606 AMN10N AND ALLANTOIS. 



teen 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 of incubation amounts to no less 

 than 3 grains in the twenty -four hours. 1 It is curious to observe, 

 also, 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. 



It is evident, therefore, that a true respiration takes place, by 

 means of the allantois, through the membranes of the shell. 



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

 takes part also in the absorption of nutritious matter. As the pro- 

 cess of development advances, the skeleton of the young chick, at 

 first entirely cartilaginous, begins to ossify. The calcareous mat- 

 ter, necessary for this ossification, is, in all probability, derived from 

 the shell. The shell is certainly lighter and more fragile toward 

 the end of incubation than at first ; and, at the same time, the cal- 

 careous ingredients of the bones increase in quantity. The lime- 

 salts, requisite for the process of ossification, are apparently ab- 

 sorbed from the shell by the vessels of the allantois, and by them 

 transferred to the skeleton of the growing chick ; so that, in the 

 same proportion that the former becomes weaker, the latter grows 

 stronger. This diminution in density of the shell is connected not 

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

 escape of the chick from the egg. This deliverance is accomplished 

 mostly by the movements of the chick itself, which become, at a 

 certain period, sufficiently vigorous to break out an opening in the 

 attenuated and weakened egg-shell. The first fracture is generally 

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

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

 advanced. The egg-shell itself, therefore, which at first only serves 

 for the protection of the imperfectly-formed embryo, afterward 

 furnishes the materials which are used to accomplish its own demo- 

 lition, and at the same time to effect the escape of the fully deve- 

 loped foetus. 



Toward the latter periods of incubation, the allantois becomes 

 more and more adherent to the internal surface of the shell- mem- 

 brane. At last, when the chick, arrived at the full period of de- 

 velopment, escapes from its confinement, the allantoic vessels are 

 torn off at the umbilicus ; and the allantois itself, cast off as a use- 



1 Op. cit., pp. 138 and 149. 



