FROM TWELVE TO THIRTY-SIX SOMITES 141 



Even though the difference may be partly compensated by 

 drawing of the embryo to tlie left, the tendency ^YOuld be to 

 stretch a-h. If there were no such compensation and a and h 

 were practically fixed points, the length of a-h at the conclusion 

 of the rotation would much exceed that of c-d (Fig. 78 B), and 



J^ 



a 



C 



Fig 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, b and c repre- 

 sent the junction of amnion and body-wall on left and right 

 sides respectively. In Fig. A, a-h 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-h and c-d, the latter would of necessity be thrown into folds, 

 but not the former. Finally, if the amniotic folds were forming 

 at the same time (as is actually the case), the right one would 



