1 68 



OSCAR RIDDLE. 



Fig. 2. A section of the proximal plumulaceous portion of a body covert from a 

 chick, showing a modified region with "downy" formations//, the much-modified 

 bases of which were produced during a "starving" period of two days, m, the 

 horny covering (quill) of a bundle of six barbs ; a, the abnormal area (fault-bar) pro- 

 duced at another point ; c, the pigmentless part of the shaft which was grown during 

 this period. Drawing made with camera lucida. Actual length of section shown 2 cm. 



cell boundaries lost in a mass of fibrous tissue. Only the row 

 of cells next to the pulp, representing the cylinder cell layer, 

 retains its characteristic shape. At a still later stage in develop- 

 ment, represented by Fig. 46, in which the epitrichial sheath is 

 not shown, cornification of the outer rows of cells, representing 

 the region of the sheath cells, has taken place, only suggestions 

 of its original fibrous structure remaining. The outermost inter- 

 mediate cells have become almost wholly fibrous, narrow spaces 

 representing the position of the nuclei. The process of cornifi- 

 cation now proceeds rapidly until practically all of the intermedi- 

 ate cells become cornified, and the cylinder cell layer becomes 



