DEVELOPMENT OF ANI 315 



of the adult wing is used as a constant of measure and all 

 otlier lengths are |)r()])ortionally increased to it. 



We see, from tlie curve, that, in the lialf-developed 

 embryo, the palm is nearly twice as long as the index. A 

 few days later the rate of the index growth has increased so 

 that now it is about tliree-quarters as long as the palm. 

 Twenty-four hours before hatching they again diverge and 

 the palm makes the more ra])id growth. Forty-eiglit hours 

 later, or twenty-four hours after hatching, the index, having 

 increased its rate of growth, is practically the same lengtli 

 as the palm. From now on they continue to grow evenly 

 until the adult stage is reached. 



If embryonic characters are any indication of charac- 

 ters of past ages, the early preponderance of the palm would 

 indicate that at some time it played a more important part 

 than it does now. A glance at archaeopteryx would make 

 it appear as if this might be the case, for that ancient rep- 

 tile-bird bore most of the primaries on its palm. Archaeop- 

 teryx, however, cannot be taken as a criterion, for it was as 

 highly specialized along certain lines as our present day birds 

 and mav not necessarilv have been the true ancestral type of 

 the modern bird. The probabilities are that the ancestral 

 ani had a longer metacarpus than the modern representative, 

 which was a survival of the long metacarpals of the lizard, 

 and ])ossibly it held more remiges than at the present day. 



Figure 106b is a curve of the growth of the wing parts, 

 all measurements being increased to adult size as before. The 

 humerus makes a rapid growth until near hatching time when 

 it suddenly changes to comparative slowness, and later, to 

 fair rapidity, which keeps up until adult. The ulna at first 

 grows rapidly at the same rate as the humerus, but, after 

 the embryo is half developed, is delayed until after hatching. 

 Then it grows approximately at the same rate as the hu- 

 merus. The pinion is delayed, on the other hand, in the 

 embryo until it is about three-fifths developed. Its rate of 

 growth takes an intermediate path between the other two 



