VI.] CHANGE OF POSITION OF THE EMBRYO. 113 



all but encompasses the yolk, the prominence of the 

 sinus terminalis will become less and less in proportion 

 as the respiratory work of the vascular area is shifted 

 on to the allantois, and its activities confined to absorb- 

 ing nutritive matter from the yolk. 



The folding-in of the embryo makes great pro- 

 gress during this day. Both head and tail have become 

 most distinct, and the side folds which are to constitute 

 the lateral walls have advanced so rapidly that the 

 embryo is now a bond fide tubular sac, connected with 

 the rest of the yolk by a broad stalk. This stalk, 

 as was explained in Chap. II, is double, and consists of 

 an inner splanchnic stalk continuous with the alimen- 

 tary canal, which is now a tube closed at both ends and 

 open to the stalk along its middle third only, and an 

 outer somatic stalk continuous with the body-walls of 

 the embryo, which have not closed nearly to the same 

 extent as the walls of the alimentary canal. (Compare 

 Fig. 9, A and J5, which may be taken as diagrammatic 

 representations of longitudinal and transverse sections 

 of an embryo of this period.) 



The embryo is almost completely covered by the 

 amnion. Early in this day the several amniotic folds 

 will have met and completely coalesced along a line 

 over the back of the embryo in the manner already 

 explained in the last chapter. 



During this day a most remarkable change takes 

 place in the position of the embryo. Up to this 

 time it has been lying symmetrically upon the yolk 

 with the part which will be its mouth directed straight 

 downwards. It now turns round so as to lie on its left 

 side. 



F. & B. 8 



