CHAP. VIII.J THE LIMBS. 233 



cord, and will undergo no further change till near the 

 time of hatching. The space between it and the so- 

 matic stalk is still considerable, though the latter is 

 narrower than it was on the fourth day. 



The embryo remains excessively curved, so much 

 so indeed that the head and the tail are nearly in 

 contact. 



The limbs have increased, especially in length; in 

 each a distinction is now apparent between the more 

 cylindrical stalk and the flattened terminal expansion ; 

 and the cartilaginous precursors of the several bones 

 have abeady become visible. 



The fore and hind limbs are still exceedingly alike, 

 and in both the stalk is already beginning to be bent 

 about its middle to form the elbow and knee respec- 

 tively. 



The angles of both knee and elbow are in the first 

 instance alike directed outwards and somewhat back- 

 wards. By the eighth day, however, the elbow has 

 come to look directly backwards and the knee forwards. 

 In consequence of this change, the digits of the fore 

 limb point directly forwards, those of the hind limb 

 directly backwards. This state of things is altered by 

 a subsequent rotation of the hand and foot on the arm 

 and leg, so that by the tenth day the toes are directed 

 straight forwards, and the digits of the wing backwards 

 and somewhat ventralwards, the elbow and knee almost 

 touching each other. 



While these changes are taking place the differences 

 between wing and foot become more and more distinct. 

 The cartilages of the digits appear on the fifth day as 

 streaks in the broad flat terminal expansions, from the 



