198 THE FOURTH DAY. [CHAP. 



another to tlie first visceral arch. Yll. seventh nerve passing 

 to the second visceral arch. O.Ph. glossopharyngeal nerve 

 passing towards the third visceral arch. Pg. pneumogastric 

 nerve passing towards the fourth visceral arch. iv. investing 

 mass (basilar plate). No attempt has been made in the figure 

 to indicate the position of the dorsal wall of the throat, which 

 cannot be easily made out in the living embryo, ch. noto- 

 chord. The front end of this cannot be seen in the living 

 embryo. It does not end however as shewn in the figure, 

 but takes a sudden bend downwards and then terminates in 

 a point. Ht. heart seen through the walls of the chest. M.P. 

 muscle-plates. W. wing. H.L. hind limb. Beneath the 

 hind limb is seen the curved tail. 



curvature of the body has also gone on increasing, and 

 as the result of these various flexures, the embryo has 

 become somewhat spirally curled up on itself (Fig. 67). 



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 iacrease of depth 

 than of breadth, the body of the embryo appears in 

 section (Fig. 68) 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. 

 68, TT.ii.) 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 projecting outwards. They seem to be local de- 



