IJEXELOI'.MKNT Ol WING 34-5 



trumpeter at one time piobably owned at least two good, 

 fimetional, clawed fingers as does the young hoatzin of today. 

 'I'hat time, however, is passed, though the stage representing 

 the young hoatzin may possibly yet be found in the embryo. 

 The fact that trum])eters live ])i'iiK'ipally on the ground — 

 only roosting in the trees at night — and are essentially run- 

 ning birds, having lost most of their arboi'eal habits, may ac- 

 count for the reduction of these digits. J Jke the hoatzin they 

 nest in trees, but. unlike them, as soon as the egg is hatched, 

 the parent conveys the chick to the ground, where it soon 

 learns to run about and hide as well as any newly hatched 

 pheasant or ])artridge. It does not acquire its flight feathers 

 as soon as the partridge and this lack of wing exercise may 

 account in part for the relatively slow development of the 

 hand and forearm. 



The adult domestic pigeon has a longer arm than fore- 

 arm. Both are practically of the same length when the em- 

 bryo is fidly developed, but the arm rapidly lengthens while 

 the forearm relatively decreases in length when the egg- 

 hatches and the squab commences to grow. The develop- 

 ment of the two segments in this case is also in opposite di- 

 rections, but the directions are diff'erent from the preceding 

 examples. 



The development of the w^ing of the Guiana green heron 

 {Bidoridcs vircscciis) is peculiar because there is little of 

 the variation between the growth of the forearm and the 

 hand which is so characteristic of the other species mentioned. 



Butoridcs viresccns (Linn.) 



Arm Forearm Hand 



10 day embryo 70..).) mm. 02.7 mm. .'55.2 mm. 



14 "* "' 09. " 03. " 57. 



3 " nestling 70. " 05. " 55. 



14 " " 00.3 " 00.3 " 57.2 " 



Fully fledged )8.5 " 59.5 " 02.4 " 



Aduit 04. " 09.5 " 50. 



