116 THE THIRD DAY. [CHAP. 



with the embryo from the egg, as well as through the double 

 amniotic fold. 



We may repeat that, the view being from below, whatever is 

 described in the natural position as being to the right here 

 appears to be left, and vice versd. 



This important change of position at first affects 

 only the head (Fig. 37), but subsequently extends also to 

 the trunk. It is not usually completed till the fourth 

 day. At the same time the left vitelline vein, the one on 

 the side on which the embryo comes to lie, grows very 

 much larger than the right, which henceforward gradu- 

 ally dwindles and finally disappears. 



Coincidently with the change of position the whole 

 embryo begins to be curved on itself in a slightly 

 spiral manner. This curvature of the hody becomes 

 still more marked on the fourth day, Fig. 67. 



In the head very important changes take place. 

 One of these is the cranial flexure, Figs. 37, 38. This 

 (which must not be confounded with the curvature of 

 the body just referred to) we have already seen was 

 commenced in the course of the second day, by the 

 bending downwards of the head round a point which 

 may be considered as the extreme end either of the 

 notochord or of the alimentary canal. 



The flexure progresses rapidly, the front-brain being 

 more and more folded down till, at the end of the third 

 day, it is no longer the first vesicle or fore-brain, but 

 the second cerebral vesicle or mid-brain, which occupies 

 the extreme front of the long axis of the embryo. In 

 fact a straight line through the long axis of the embryo 

 would now pass through the mid-brain instead of, as at 

 the beginning of the second day, through the fore-brain. 



