DEVELOPMENT OF ANI 317 



until soon after hatching and then parallels them to the time 

 when the bird is able to leave the nest. Both ulna and pin- 

 ion then show a slight decrease in growth as compared with 

 the humerus. 



A curious condition exists just before and after the 

 chicks emerge from their shells. A few hours before, the 

 growth of the three bones becomes retarded and remains so 

 until at least twenty-four hours after hatching. Then they 

 again commence to grow rapidly. The retardation is prob- 

 ably due to the immense amount of energy used up by the 

 chick in forcing its way out of the shell. Its wings play an 

 important part in this operation and are constantly in use, 

 thus consuming the energy which otherwise would have been 

 applied to their growth. The slight retardation of the ulna 

 and pinion after the young bird leaves the nest is due to the 

 more violent use they are put to in flying especially the 

 pinion and possibly to the fact that these bones hold the 

 chief flight feathers, which make a stronger growth than any 

 others on the humerus. To prove the former statement I 

 kept a young ani, taken just upon leaving the nest, in close 

 confinement for over a month, where he could not use his 

 wings. At the end of that time the ulna growth remained 

 about the same, but the pinion had increased at exactly the 

 same rate that it had been growing, up to the time the bird 

 was able to leave the nest. It was longer in actual measure- 

 ment than in the adult. The humerus, on the other hand, 

 was slightly retarded. 



Another point that may be worth touching upon is the 

 opposite or alternate rate of growth of the ulna and pinion 

 in the embryonic stages. The ulna slackens when the pinion 

 makes a rapid growth. Then seeming somehow to have 

 gained the ascendency, it commences a rapid growth, the 

 pinion immediately decreasing its rate in the same propor- 

 tion. They both decrease, about twenty-four hours before 

 hatching, when the delayed stage for the whole wing com- 

 mences. There seems to be a definite connection between 



