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. 
ia. 77. — Transverse section through the anterior angle of 
the ectamnion a few sections in front of the tip of the head. 
Stage of 14-15 s. 
b. e., Extra-embryonie 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 sulei 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 embryo 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—b, and practically none on the upper (right) side, c-d. (See 
Fig. 78 A, B). 
