DEVELOPMENT OF THE CHICK 127 



amnion, and an outer, the chorion — separated 

 from each other everywhere except at the 

 sero- amniotic connection, marking the last point 

 at which the various folds became continuous 

 with each other. 



The amnion is connected with the body- wall 

 of the embryo at the ever-narrowing umbilicus. 

 Between the embryo and the amnion the 

 amniotic fluid accumulates. The chorion is at 

 first surrounded by the albumen of the egg ; 

 but as this becomes absorbed the chorion 

 approaches and finally comes into contact 

 with the inner shell-membrane. 



It is clear from their mode of formation 

 that the chorion must consist of ectoderm on 

 the outside with an inner fining of mesoblast, 

 while the amnion must be formed by mesoblast 

 on the outside with ectoderm on the side 

 looking towards the embryo. 



The allantois has an altogether different 

 origin inasmuch as it springs originally from 

 the embryo itself. Soon after the production 

 of the tail-fold of the embryo a hollow bud 

 makes its appearance in the ventral wall of the 

 primitive gut (Fig. 69). During the fourth day 

 of incubation this bud, the rudiment of the 

 allantois, grows out into the extra-embryonic 



