1883.] 231 [Jeffries. 



Bei starkerer Vergrosserung sieht man, dass dieselben von den 

 Embryonalfedern, welche die Korperoberflache des Thierchens in 

 diesem Stadium bedecken, nur dadurch abweichen, dass sie etwas 

 kiirzer sind." 



Fraisse's statement of the setae in adults is correct as far as it 

 goes, but by no means gives a complete idea of the setae of a 

 duck. 



In adults the setae are of all shapes from simple conical papillae 

 to branched or brush-like, as in many of the ducks. Whatever 

 their shape they are all reducible to simple papillae, with a meso- 

 dermal core, and an epidermal coat of the usual layers (fig. 27). 

 The brushes like those found on the sides of the tongue in ducks 

 are the result of peculiar modifications of the horn layer. The 

 horn cells from certain patches of the mucous layer do not reach 

 maturity but remain soft and loose (fig. 32 a). Thus, since the 

 horn layer slowly slides over the mucous layer towards the tip of 

 the papilla, streaks of soft tissue are left between the harder 

 horn cells. The hard layers (fig. 32 b) accordingly split apart 

 from the tip of the seta down towards the base, and thus divide 

 the horn layer into a brush. The cell elements are the same as 

 in the other appendages, with the exception of the cells in the 

 soft streaks. These, while they dry up and become more or less 

 horny, keep the irregular form of the deeper transitional cells. 



The skin of the mouth follows exactly the same course in 

 development as that of the rest of the body. First, the epitri- 

 chial layer is formed, and below this, in young birds with soft 

 mouths, mucous cells of a polyhedral form, no granular layer 

 intervening between the epitrichial layer and the mucous cells. 

 These mucous cells keep on growing and wearing away during 

 the life of the bird. 



The development of parts covered with horn at birth is precisely 

 like that of the claws or bill. 



Up to the twelfth or thirteenth day the epiderm is composed 

 of the mucous layer, the transitional cells and the epitrichial 

 layer (fig. 29). About this time horn-cells begin to form from 

 this layer, and thus separate the first transitional cells from the 

 mucous layer. These outer transitional cells, dying or becoming 

 partly converted into horn, form a granular layer of varying 

 thickness. The horn cells continue to form during the life of the 

 bird. At or about birth the outer layers are shed. 



