FROM TWELVE TO THIRTY-SIX SOMITES 141 
Even though the difference may be partly compensated by 
drawing of the embryo to the left, the tendency would be to 
stretch a—b. If there were no such compensation and a and b 
were practically fixed points, the length of a—b at the conclusion 
of the rotation would much exceed that of c-d (Fig. 78 B), and 
Fia 78. A, B, and C. Diagrams to represent the effect of 
rotation of the embryo on the amniogenous somatopleure. 
a represents in all figures the position of the ectamnion on 
the left (lower) side; d represents in all figures the position 
of the ectamnion on the right (upper) side. 6 and c¢ repre- 
sent the junction of amnion and body-wall on left and right 
sides respectively. In Fig. A, a-b and c-d are equal. In 
Fig. B, rotation of the embryo is assumed to have taken 
place without formation of the amnion; the distance a—b has 
become greater than c-d. In Fig. C is represented rotation 
of the embryo with synchronous formation of the amniotic 
folds, as is actually the case; c-d is inevitably thrown into 
secondary folds. The vertical lines at the extreme right 
and left represent the margins of the pellucid area. 
if, during this process, there were actual independent growth 
of a-b and c-d, the latter would of necessity be thrown into folds, 
but not the former. Finally, if the amniotie folds were forming 
at the same time (as is actually the case), the right one would 
