154 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



The cylinder cells are less elongated and their nuclei are also larger. 

 Their boundaries are not easily determined. 



At the stage shown in Figure 16 (Plate 3), the cylinder cells and the 

 intermediate cells are completely divided into ridges by the extensions 

 of the basal membrane. These ridges are destined to give rise to the 

 barbs and their barbules. 



Davies left undecided the question whether the formation of ridges 

 was brought about by the cylinder-cell layer invading the mass of inter- 

 mediate cells and dividing it up into ridges, or whether the intermediate 

 cells grouped themselves into ridges and thus made room for the 

 cylinder-cell layer to enter between successive ridges ; but he con- 

 sidered the latter view the more probable. 



I, too, believe that the initiative in the process of ridge formation is 

 taken by the intermediate cells (cl. i^m.), and for the following reasons : 

 (1) they are evidently changing position, as may be seen in Plate 2, 

 Figures 12-14; (2) a tendency to group themselves is manifested in 

 the formation of lateral plates, which are represented in cross-section 

 by rows of cells (Plate 3, Fig. 16, ser. cl.). 



Maurer ('95) has pointed out that there must be a very great pres- 

 sure upon the central pulp by the growing epidermal region with its 

 increasing need of space, and that this seems to result in the formation 

 of numerous small elevations and depressions (Plate 2, Fig. 12, crs".) 

 varying in size with the resistance at difteretit points. I agree with 

 him in considering this a factor also in the formation of ridges (Plate 2, 

 Fig. 14, crs.), especially in producing extensions of the basal membrane 

 into the epidermis of the feather germ. 



As was observed by Davies, the ridges do not arise simultaneously 

 at any given level, but are first seen on the sides of the feather germ. 

 The distal portion of a ridge is formed before the proximal part, where 

 it joins the shaft or rhachis ; the differentiation of the barb and its 

 barbules therefore begins at the distal tip of the ridge and gradually 

 approaches the proximal insertion on the rhachis. In a single cross- 

 section, there will be ridges cut at various distances from their point 

 of union with the shaft. The sections of the ridges most distant from 

 the I'hachis, i. e. of those on the ventral side of the feather germ, pass 

 through the distal ends of ridges which will appear successively nearer 

 to the shaft in sections taken at more proximal points in the germ. 

 These relations may be more easily understood by reference to Figure 4 

 where ridges (crs.) in various stages of differentiation are represented by 

 rows of pigment cells. 



