140 THE DEVELOPMENT OF THE CHICK 



fusion of the ectodermal thickenings of the opposite sides, 

 beginning at the posterior angle of the head-fold and proceeding 

 backwards. The energy of fusion is sufficient in itself to lift the 

 somatopleure up in the form of a fold around the body of the 

 embryo. Thus new parts of the ectodermal thickening are con- 

 stantly being brought together and the fusion progresses steadily, 

 and this in its turn prolongs the lateral amniotic folds. These 

 possess no independent power of elevation of any considerable 

 amount, for, when the initial fold of one side is destroyed by 

 cauterization, the fold of the opposite side remains as an insig- 

 nificant elevation in the somatopleure a long distance lateral to 

 the embryo. 



Fig. 77. ■ — Transverse section through the anterior angle of 



the eetamnion a few sections in front of the tip of the head. 



Stage of 14-15 s. 



b. c Extra-embryonic body-cavity, c, Cavity in the 

 entoderm, e. a., Eetamnion. 



The tail-fold arises in an analogous manner to the head-fold, 

 except that there is no proamnion here. The progress of the 

 various folds and their final fusion follows from what has already 

 been said. 



Practically all of the somatopleure of the pellucid area is 

 amniogenous with the exception, naturally, of that part internal 

 to the limiting sulci that forms the body-wall. What effect has 

 the turning of the embryo on its left side on the amniogenous 

 somatopleure? We will suppose that the latter is primitivelv 

 of equal width on both sides and that the notochord represents 

 approximately the axis of rotation. During the process of rota- 

 tion, the embr3'0 sinks and the lateral limiting sulci become deeper. 

 A direct consequence of the rotation must be, therefore, a strong 

 tension on the somatopleure belonging to the under (left) side, 

 a-h, and practically none on the upper (right) side, c-d. (See 

 Fig. 78 A, B). 



