^ AND ALLANTOIS. 599 



CHAPTER IX. 



AMNION AND ALL ANTOIS. DE VELOPME NT OF 

 THE CHICK. 



WE shall now proceed to the description of two other accessory 

 organs, which are formed, during the development of the fecundated 

 egg, in all the higher classes of animals. These are the amnion and 

 the allantois ; two organs which are always found in company with 

 each other, since the object of the first is to provide for the forma- 

 tion of the second. The amnion is formed from the external layer 

 of the blastodermic membrane, the allantois from the internal layer. 



In the frog and in fish, as we have seen, the egg is abundantly 

 supplied with moisture, air, and nourishment, by the water with 

 which it is surrounded. It can absorb directly all the gaseous and 

 liquid substances, which it requires for the purposes of nutrition 

 and growth. The absorption of oxygen, the exhalation of carbonic 

 acid, and the imbibition of albuminous and other liquids, can all 

 take place without difficulty through the simple membranes of the 

 egg; particularly as the time required for the formation of the 

 embryo is very short, and as a great part of the process of develop- 

 ment remains to be accomplished after the young animal leaves 

 the egg. 



But in birds and quadrupeds, the time required for the develop- 

 ment of the foetus is longer. The young animal also acquires a 

 much more perfect organization during the time that it remains 

 inclosed within the egg ; and the processes of absorption and exha- 

 lation necessary for its growth, being increased in activity to a 

 corresponding degree, require a special organ for their accomplish- 

 ment. This special organ, destined to bring the blood of the fcetus 

 into relation with the atmosphere and external sources of nutrition, 

 is the allantois. 



In the frog and the fish, the internal blastodermic layer, forming 

 the intestinal mucous membrane, is inclosed everywhere, as above 

 described, by the external layer, forming the integument; and 



