THE HONEY-GUIDES 21 



Inasmuch as these hooks are temporary structures that are shed 

 considerably before the young bu-d is ready to leave the nest, it 

 follows that in cases where observers have stated that nesthngs did 

 not have any bill hooks it does not necessarily mean they never 

 had any. Even in Indicator indicator and /, minor, where we have 

 ample evidence of these bill hooks, there are cases where observers 

 saw no sign of them on nestling birds. Apparently the nestlings were 

 found after they had shed the bill hooks. Bearing in mind that the 

 absence of hooks in a half-grown chick is no proof that the bird 

 may not have had them, it may be noted that such a bookless con- 

 dition has been reported for Melichneutes robustus (one specimen), 

 and for the West African forest race of the lesser honey-guide, /. 

 minor conirostris. It is doubtful that a character of this sort would 

 be present in one race and absent in another of the lesser honey- 

 guide, and it seems probable that hooks are present in all the honey- 

 guides, except possibly Prodotiscus. 



In order to ascertain the structural nature of these hooks, the bill 

 of a two-daj^-old Indicator indicator, collected by Mr. Graham H. 

 Patten, was serially sectioned, stained, and studied microscopically. 

 As the photomicrograph of a sagittal section of the maxilla (pi. 1, 

 top) shows clearly, the hook is merely a continuation of the egg- 

 tooth material distally over the end of the maxilla and prolonged 

 into a decurved point. The mandibular hook showed the same type 

 of structure, but unfortunately it broke off from the mandible while 

 being sectioned and therefore the resulting slides, while anatomically 

 informative, do not lend themselves to illustrative purposes. 



With the photomicrograph of the honey-guide maxillary hook are 

 shown copies of some of Gardiner's (18S4) drawings of bill formation 

 in an embryo of the domestic chick (fig, 4:,d-f). When the bill first 

 develops the two jaws are originally one closed continuum, and the 

 egg-tooth is formed on the dorsal surface before the two are sepa- 

 rated by the invagination of the oral groove from the tip of the bill. 

 In the figure showing the early stages of this invagination (fig, 4i,dJ) 

 the future mandible is turned up at the tip and the path of the invag- 

 ination also causes an original downward deflection of the tip of the 

 future premaxilla. If these terminal curvatures were somewhat elon- 

 gated and retained we would get what we find in the honey-guides, 

 where they form the bill hooks. Furthermore, in the cross-section 

 of the bill of an embryo kite {Milvus milvus) (fig. 4,^), Gardiner 

 found a terminal papiUum, wliich, if it were recurved instead of 

 decurved, would be essentially the same as the mandibular hook of 

 a honey-guide. 



While no one hitherto had made a histological study of these hooks, 

 they were generally assumed to be direct derivatives from the egg- 



