VI.] THE LIMBS. 143 



folds, that the alimentary canal may be said to be connected 

 with the yolk sac by a very narrow neck only. This rem- 

 nant of the splanchnic stalk we may now call the umbilical 

 duct; though narrow, it is as yet quite open, affording 

 still free communication between the inside of the yolk sac 

 and the interior of the alimentary canal. 



The somatic stalk, though narrowing somewhat, is much 

 wider than the splanchnic stalk, so that a considerable 

 ring-shaped space exists between the two. 



4. Another very prominent feature is the increase in the 

 cranial flexure. During the third day, the axis of the front 

 part of the head was about at right angles to the long axis of 

 the body ; the whole embryo being still somewhat retort- 

 shaped. On this day, however, the flexure has so much 

 increased that the angle between the long axis of the body 

 and that of the front segment of the head is an acute one, 

 and the mouth is turned so as completely to face the chest. 



The tail-fold, which commenced to be noticeable during 

 the third day, has during this day increased very much, and 

 the somewhat curved tail (Fig. 46) forms quite a conspicuous 

 feature of the embryo. The general curvature of the body has 

 also gone on increasing, and as the result of these various 

 flexures, the embryo has very much the appearance of being 

 curled up on itself (Fig. 46). 



5. The distinct appearance of the limbs must be reckoned 

 as one of the most important events of the fourth day. 



Owing to the continued greater increase of depth than of 

 breadth, the body of the embryo appears in section (Fig. 47) 

 higher and relatively narrower than even on the third day, and 

 the muscle-plates, instead of simply slanting downwards, come 

 to be nearly vertical in position. Not far from the line 

 which marks their lower ends, the somatopleure, almost 

 immediately after it diverges from the splanchnopleure, is 

 raised up (Fig. 47, W.E.) into a low rounded ridge which runs 

 along nearly the whole length of the embryo from the neck 

 to the tail. 



It is on this ridge, which is known as the Wolffian ridge, 

 that the limbs first appear as flattened conical buds project- 

 ing outwards. They seem to be local developments of the 

 ridge, the rest of which becomes less and less prominent as 

 they increase in size. Each bud, roughly triangular in sec- 



