CHAP. VII.] THE LIMBS. 175 



stalk and the flattened terminal expansion ; and the carti- 

 laginous precursors of the several bones have already 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 respectively. 



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 downwards, 

 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 even curved 

 edge of which they do not project. On the sixth or seventh 

 day the three digits of the wing (the median being the 

 longest), and the four (or in some fowls five) digits of the 

 foot may be distinguished, and on the eighth or ninth day 

 these begin to project from the edge of the expanded foot 

 and wing, the substance of which, thin and more or less 

 transparent, remains for some time as a kind of web between 

 them. By the tenth day, the fore and hind extremities, save 

 for the absence of feathers and nails, are already veritable 

 wings and feet. 



At an early period of development we find the following elements in the 

 avian manus, as separate masses of cartilage. 



In the carpus there are four elements. Two in the proximal row which 

 remain distinct through life, viz. (i) the radiale, (2) the united intermedium 

 and ulnare. In the distal row (according to some recent observations of 

 I)r Rosenberg, Zeitschrift fur Wiss. Zoologie, 1873, p. 139, etc.) there are also 

 two elements. One of these is the united first and second carpal which we 

 may call carpal l ~ u , and the other is the united third and fourth carpal which 

 we may call carpal in ~ lv . These subsequently unite with the metacarpal 

 bones, and form with them a united ' carpo-metacarpus ' comparable with the 

 tarso-metatarsus of the avian foot. 



Four metacarpals are present, viz. the first, second, third, and fourth. The 



