DEVELOPMENT OF ANI 313 



tened and slightly concave, the two parallel bone centers 

 clearly showing. Distally it gradually decreases in width 

 and the heavy portions run together until just before con- 

 nection with the upper carpal, it narrows suddenly to ex- 

 treme thinness so that it is flattened vertically at this point. 



A study of the embryonic wing shows the metacarpal 

 in question to be a strong, though somewhat smaller bone 

 than the one above it. It is round and shows no sign of flat- 

 tening. Upon examining an older embryo I found that, 

 when the bone begins to ossify, the ossification is weaker 

 above and stronger at the sides. The endochondral bone 

 takes the form of a scroll with the fissures running to the right 

 at the distal end. After the bird hatches and ossification of 

 the shaft approaches completion, the bone still remains 

 rounded, though the metacarpal has commenced to broaden 

 slightly and to become thinner at the time the bird leaves the 

 nest. When the adult state is attained we find the bone 

 flattened as above described. 



The second and more interesting point is the strange 

 T-shaped form of the third digit. It really is composed of 

 two elements at an early embryonic stage. The stem is sep- 

 arate from the cross piece. It is an irregular rounded body 

 resting against the cross piece, but not a part of it, as shown 

 by its dividing line. It appears in the embryo, twenty-four 

 hours before hatching and long after ossification has set in, 

 as a rounded knob partly ankylosed to the main digit. The 

 ankylosis is complete at hatching time, but actual ossification 

 does not commence until the bird is several hours old. Up 

 to this time it has only been in a weak cartilaginous state. 

 (Fig. 105.) 



Other evidence of a fourth digit has been found in the 

 early embryos of a tern and the hoatzin, while W. K. Parker, 

 in his two papers, "Fowl's Wing" * and "Morphology of 

 Opisthocomus," 2 states that there is a projection on the 



1 Phil. Trans. Zool. Soc., London, 1888. 



2 Trans. Zool. Soc., London, 1895, pp. 69-71. 



