CHAPTER VIII. 



FKOM THE SIXTH DAT TO THE END OF INCUBATION". 



1. The sixth day marks a new epoch in the development 

 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 com- 

 paratively early stage, from some subsidiary tokens, whether 

 any given embryo 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 organization. This absence 

 of any distinctive avian differentiation lasts in the chick 

 roughly speaking till the commencement of the sixth day. 



We do not mean that on the sixth day all the organs 

 suddenly commence to exhibit peculiarities which mark 

 them as avian. There are no strongly marked breaks in the 

 history of development ; its course is perfectly gradual, and 

 one stage passes continuously into the next. The sixth and 

 seventh days do however mark the commencement of the 

 period in which the specialization of the bird begins to be 

 apparent. Then for the first time there become visible 

 the main features of the characteristic manus and pes ; the 

 crop and the intestinal caeca make their appearance ; the 

 stomach takes on the form of a gizzard; the nose begins 



