v.] CHANGE OF POSITION. 87 



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 absorbing nutritive matter from the yolk. 



4. The folding in of the embryo makes great progress 

 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 bona 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 alimentary 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. (Com- 

 pare Fig. 8. A and B, which may be taken as diagrammatic 

 representations of longitudinal and transverse sections of an 

 embryo of this period.) 



5. The embryo is almost completely covered by the 

 amnion. Before the close of the day the several amniotic 

 folds will have met along a line over the back of the embryo. 

 Their complete coalescence, and the obliteration of their line 

 of junction, will however not take place till the following 

 day. 



6. 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. 



This important change of position is almost invariably 

 completed on the third day. At the same time the left 

 omphalo-mesaraic vein, the one on the side on which the 

 embryo comes to lie, grows very much larger than the 

 right, which henceforward gradually dwindles and finally dis- 

 appears. 



7. Coincidently with the change of position the whole 

 embryo begins to be curved on itself. This curvature of the 

 body, Fig. 46, becomes still more marked on the fourth day. 



8. In the head very important changes take place. 

 One of these is the cranial flexure, Figs. 24, 25. This (which 



