Anutomij of the Houtzin. 



289 



tlic adult bird, hence tlie effect of wear during the lifetime 

 is obviously not sufficient to account for its presence. On 

 the dorsal surface the feathering appeared to be quite uniform^ 



Fijr. 3. 



Opistliocomus cristatns. 

 Young Chick, showing dorsal and ventral feather-tracts : y, yolk-sac. 



though sparse ; there was no distinct spinal apterion to be 

 made out ; in these young chicks the ventral feathering was 

 much closer than the dorsal. 



Now the continuous feathering of the adult in the Ostrich * 

 and Apteryxf is not met with in the chick, which shows in 

 both cases distinct apteria ; this fact is further evidence in 

 favour of regarding the flightless Struthious birds as the im- 

 mediate descendants of birds with wings developed for flying. 

 Nitzsch X explains the presence of apteria as an assistance to 

 the free movements of the limbs, which would be hampered 

 by a dense and uninterrupted plumage, and also considers 

 them to be partly due to the large size of the contour-feathers. 

 It seems likely, however, that in the primitive bird the fea- 

 thering was uniform and probably of a downy character ; 



* Miss B. Lindsay, "On the Avian Sternum," P. Z. S. 1885, p. 684, 

 pi. xliii. figs. 6, 7. 



t T. J. Parker, " On the Development of Aj)tert/.v,'^ Proc. Roy. Soc. 

 vol. xliii. p. 482. 



X ' Pterylography,' Engl, ed., p. 17. 



