The Development of the Chick 117 



layers of similarly colored cloth which we have 

 called the mesoblast. 



The formation of the amnion (Fig. 38, am.f) 

 may be represented in our model by lifting 

 up with the fingers a small fold of the upper 



FIG. 38. A B DIAGRAMS ILLUSTRATING THE DEVELOPMENT OF 

 THE FCETAL MEMBRANES OF A BIRD. (After Parker and Haswell.) 



A, early stage in the formation of the amnion, sagittal section. B, slightly 

 later stage, transverse section. 



and second layers of cloth, and pulling these 

 two layers back over the head end of the em- 

 bryo ; this fold will correspond to the head-fold 

 of the amnion (Fig. 38, am./). Similar folds 

 might be lifted up at the posterior end and at 

 the sides of the embryo model, to represent 



