The Bird in the Egg 475 



radiate fins. But as early as the tenth day, except for 

 the absence of feathers and claws, the limbs are, in appear- 

 ance, very perfect wings and feet. The most interesting 

 fact in connection with the limbs is that their develop- 

 ment begins superficially and works inward, not, as would 

 be thought, starting at the shoulder and ending at the 

 digits. 



Even the deep-seated shoulder- and thigh-girdles of bone 

 (pp. 85 and 89) are not derived from the axial skeleton. 

 The former, in the long ago, was gradually pushed inward 

 from the surface by the deep-reaching rays of the fin-like 

 fore limbs, and it is believed that the pelvic girdle had 

 its origin in the spliced scales of some fish-like ancestor 

 of old, which had scales like those of some of the fossil 

 ganoids. These probably covered over the cartilage girdle 

 and then sunk in. 



An example of one out of many reptilian structures 

 which appear for a time and then vanish, is found in the 

 procoracoid bone which has apparently much to do with 

 the development of the typical coracoids, but which is 

 absent or reduced to a mere process in the adult bird.* 

 Strangely enough, in the embryo of the common chick 

 the coracoid and scapula fuse together at an earh^ stage, 

 being then in a condition comparable only to that found 

 in the full-grown ostrich. Later this inexplicable fusion 

 is dissolved and the bones complete their development as 

 they began, — two wholly independent structures. 



Again, in the embryo of a tern, faint vestiges of teeth 



* This process is quite pronounced in the case of the Ostrich. 



