STRONG: DEVELOPMENT OF COLOK IN DEFINITIVE FEATHER. 153 



ing properties, these cells are in no way distinguishable from the adjacent 

 cells in the deeper portion of the intermediate cell layer at this level. 



In his description of the cylindei'-cell layer, Davies ('89, p. 574) re- 

 marked tluit the typical cylindrical form is seldom seen in cells of this 

 layer. On the contrary, as will be seen in Figures 12-14 (Plate 2) and 

 21-24 (Plates 4, 5), I have found the cylindrical form a very common 

 characteristic of tliese cells in Sterna; however, it must be admitted 

 that in the region from 15 to 20, Figure 1, the cylindrical form is lost 

 (Plate 3, Fig. 15 ; Plate 4, Fig. 20). 



The intermediate cells (cl. i'm.) occupy about owe third of the thick- 

 ness of the epidermis. They ai"e undergoing active proliferation, which, 

 as far as I have observed, is always accomplished by mitotic division. 

 Their nuclei, like those of the cylinder cells, are elongated in tlie direc- 

 tion of the long axes of the cells. 



Outside the intermediate cells comes the layer of inner-sheath cells 

 (cl. tu. ?'.), which occupies about one half the thickness of the epider- 

 mis. The deeper cells of this layer are easily distinguishable from the 

 intermediate cells by their larger and more sphei'ical nuclei, their more 

 sharply defined cell boundaries, and their more or less polygonal form. 

 The more superficial inner-sheath cells are flattened, with their long 

 axes at right angles to those of the intermediate cells. Those most 

 superficial are cornifying to form the sheath, which at this point has 

 not attained to the full thickness shown in Figure 14. It is also not 

 separable from the follicular sheath at the level of this section. 



The sheath (tn.') consists of flattened cornified cells more or less fused 

 together. Its finer structure has been described by Lwoff ('84). All 

 layers appear thicker and the cells more elongated than they would in a 

 section strictly perpendicular to the epidermal walls (cf. 12, Fig. 1). 

 At the level of the section from which "Figure 13 was made some changes 

 are to be noticed. The intermediate-cell layer is now easily distinguish- 

 able from the cylinder-cell layer and the inner-sheath cells. Though it 

 was possible to demonstrate cell boundaries at the stage shown in Figure 

 12, this could not be done for the intermediate cells at this later stage. 

 The nuclei are larger and more spherical. They are also more numer- 

 ous. The whole thickness of the epidermis is much reduced from that 

 of the first stage described. 



A very short distance above this level we have, as seen in Figure 14, 

 the first evidence of the differentiation of ridges, in the form of exten- 

 sions of tlie basal membrane. The intermediate cells are in great con- 

 fusion and their nuclei are still larger than they appeared in Figure 13. 



