VIII.] THE FEATHERS. 205 



meatus. It becomes divided internally into two parts by the 

 tympanic membrane. 



The structures which surround the mouth are beginning 

 to become avian in form, though the features are as yet not 

 very distinctly marked. The inferior maxillary processes 

 meet in front and form the lower boundary of the mouth ; 

 while, separated from these by only a narrow slit, the superior 

 maxillary processes and fronto-nasal process meet in a similar 

 way above, to form the upper boundary. The union of the 

 superior maxillary processes is not with the tip of the fronto- 

 nasal process, but with its sides, so that an angular space is 

 left on each side between them (vide Fig. 58). The nasal 

 grooves are however completely roofed over. 



The tongue has appeared on the floor of the mouth as a 

 bud of mesoblast covered by epiblast. 



7. 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. 



8. From 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 containing 

 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 allowing 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 

 nineteenth day when many of them are an inch in length. 



On the eighth day a chalky-looking patch is observable 

 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 toes. 

 These on the sixteenth day become harder and more horny, 

 as does also the beak. 



