474 



The Bird 



of this is emphasized in the development of the embryo, 

 during which process a number of additional ribs dis- 

 appear. The abortive ribs of the neck-bones are especially 

 noticeable during the egg-life of the bird, so that in some 

 species we can make out traces of as many as fifteen ribs 

 all told. 



On page 97 a short account was given of the origin of 



the wings and feet, in the case 

 of the chick from a primi- 

 tive fin-fold in some general- 

 ized aquatic ancestor. About 

 the fourth day of incuba- 

 tion, sections of our embryo 

 chick will show a low, round- 

 ed ridge, extending the whole 

 length from the neck to the 

 tail. While we can never be 

 absolutely certain that perfect 

 homology exists between the 

 two, yet it is very significant 

 that soon after its develop- 

 ment it dwindles away, leav- 

 ing four conical, isolated 

 buds the beginnings of the limbs of the bird. Within 

 two or three days after the appearance of the limbs, faint 

 streaks become visible upon the tips of the extremities, 

 and these hints of the bones of fingers and toes, for such 

 they are, soon push out beyond the edge, still bound 

 together by their transparent membrane, and for some 

 time they present the appearance of webbed paws or 



FIG. 367. Early embryo of Canada 

 Goose, showing fin-like limbs. 



