282 THE SIXTH DAY. [CHAP. 



The tongue has appeared on the floor of the mouth 

 as a bud of mesoblast covered by epiblast. 



During the eighth, ninth, and tenth days the 

 embryo grows very rapidly, the head being still especially 

 large, and at the same time becoming more round, the 

 mid-brain not being so prominent. 



Prom the eleventh day onwards the embryo suc- 

 cessively puts on characters which are not only 

 avian, but even distinctive of the genus, species and 

 variety. 



So early as the ninth or tenth day the sacs con- 

 taining the feathers begin to protrude from the surface 

 of the skin as papillae, especially prominent at first along 

 the middle line of the back from the neck to the rump, 

 and over the thighs, the sacs of the tail feathers being 

 very conspicuous. On the thirteenth day these sacs, 

 generally distributed over the body, and acquiring the 

 length of a quarter of an inch or more, appear to the 

 naked eye as feathers, the thin walls of the sacs allow- 

 ing their contents, now coloured according to the variety 

 of the bird, to shine through. They are still however 

 closed sacs, and indeed remain such even on the nine- 

 teenth day, when many of them are an inch in length. 



Feathers are epidermal structures. They arise from an in- 

 duration of the epidermis of papillae containing a vascular core 



On the eighth day a chalky-looking patch is ob- 

 servable on the tip of the nose. This by the twelfth 

 day has become developed into a horny but still soft 

 beak. 



On the thirteenth day, nails are visible at the ex- 

 tremities, and scales on the remaining portions of the 



