158 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



The two most proximal of the ventral barbicels (Plate 5, Fig. 27) are 

 smaller and without hooks. 



The bai'bicels of the proximal barbules (Fig. 25, hrhc.^ are rudimen- 

 tary except for the two most proximal on the ventral side, which are 

 similar in form and size to the corresponding barbicels of the distal bar- 

 bules. They may be absent altogether from both sets of barbules, as is 

 frequently the case in the more distal portions of body coverts. 



In a cross-section of the feather germ at the level of 21, Figure l,the 

 barbicels appear as loose irregular fragments. I have found teased prep- 

 arations most favorable for studying their origin. 



3. The Barb. 



^Between the two rows of barbule cells for each ridge, as seen in cross- 

 section, there is a group of cells which I have called the axial plate (la. 

 ax., Plate 3, Fig. IG). The cells of this plate never acquire a regular 

 arrangement like those of the lateral rows. At the same time it is to 

 be noticed that the rows of barbule cells do not extend quite to the apex 

 of the ridge, the apex being occupied by a group of cells (Plate 4, Fig. 

 20f fnd. brb.) wdiich is continuous with the axial plate. Differentiation 

 begins at a rather late stage. 



The cells in the deeper portions of the axial plate, near the cylinder- 

 cell layer, become large and conspicuous and have a more or less polyg- 

 onal form (Plate 4, Fig. 21, vied.). They are destined to form the 

 medulla of the future barb. 



The number of cells entering into the formation of the medulla at any 

 given place depends on the size of the barb at that region. Around 

 these medullary cells, as around an axis, other cells become applied and 

 flattened, so that, in cross-section, they appear spindle-shaped. These 

 form the cortex of the barb. In a region where the barb is large, i. e., 

 near its proximal end, almost all of the axial-plate cells enter into its for- 

 mation. 



With this differentiation the ridge experiences an extension in the 

 direction of a radius of the feather germ, and the diameter of the cen- 

 tral pulp decreases correspondingly. Before this differentiation began, 

 the region corresponding to the prospective barb occupied a compara- 

 tively small area in the cross-section (Plate 4, Fig. 19); but after the 

 differentiation, it occupies a large portion of the ridge (Plate 5, Fig. 

 23). The barbules are thereby pushed farther and farther away from 

 the pulp. 



The structure of the medulla and cortex was early studied by 



