IO2 



FRANK R. LILLIE. 



area is shown by the fact that the splanchnopleure is often drawn 

 up with the outer limb of the amniotic fold, making a fold of the 

 splanchnopleure at this place) (Fig. 5). Finally let us assume 

 that the notochord represents approximately the axis of rotation. 

 During the process of rotation the embryo sinks and the lateral 

 limiting sulci become deeper. A direct consequence of the rota- 

 tion must be therefore a strong tension on the somatopleure be- 

 longing to the under (left) side, a-b, and practically none on the 

 upper (right) side, c-d, (see Fig. 4, A, B, C}. 



a 



Hi* 



Or 



a 



d 



C 



FIG. 4. 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 represent 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 repre- 

 sented 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. 



Even though the difference may be partly compensated for 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 



