CHAPTER IX. 



FEOM THE SIXTH DAY TO THE END OF INCUBATION. 



The sixth day marks a new epoch in the develop- 

 ment of the chick, for distinctly avian characters then 

 first make their appearance. 



Striking and numerous as are the features, which 

 render the class Aves one of the most easily recognizable 

 in the whole animal kingdom, the embryo of a bird does 

 not materially differ in its early phases from that of a 

 reptile or a mammal, even in the points of structure 

 which are most distinctively avian. It may, it is true, 

 be possible to infer, even at a comparatively early stage, 

 from some subsidiary tokens, whether any given em- 

 bryo belongs to this class or that (and indeed the same 

 inference may be drawn from the ovum itself) ; but up 

 to a certain date it is impossible to point out, in the 

 embryo of the fowl, the presence of features which may 

 be taken as broadly characteristic of an avian organiza- 

 tion. This absence of any distinctive avian differen- 

 tiation lasts in the chick roughly speaking till the com- 

 mencement of the sixth day. 



18—2 



