156 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



clefts (Langsfurchen) between the lateral plates and the axial plates, 

 lie described these clefts as being filled ultimately by tlie growth of the 

 cells of tlie barbule fundaments. They would thus provide room for 

 the expansion. 



B. The Differentiation of the Feather. 



1. The Barbules. 



Each barbule is composed of a single series of "intermediate cells" 

 placed end to end, thus forming a column of cells (Plate 7, Fig. 38, 

 col. cL), which comes to lie nearly parallel to the feather germ, with its 

 own axis forming a feeble spiral. The columns of cells are so closely 

 arranged as to be in contact with each other by their edges. Accord- 

 ingly, in cross-sections of the germ many columns are cut cross- 

 wise, each being represented by a single cell. Tliose cells form, 

 in any given series, a row (Plate 3, Figs. 16, 18, ser. cL) ; those nearest 

 the pulp in the row are also nearest the cells destined to form the 

 barb. They are cut nearer the base, or attached end, of the prospective 

 barbules than cells which lie farther from the pulp in the row. Those 

 at the extreme periphery, next to the inner-sheath cells, are the ones 

 which are destined to form the tips of the barbules. A single row 

 of tliese cells in a cross-section (Figs. 16-21, set', cl.) therefore shows 

 conditions of development for various portions of different barbules. 



By a comparison of the stages shown in Figures 16-21 and 24, it may 

 be seen that the deeper cells in a row undergo a great metamorphosis in 

 shape and size to form the broad flattened portion of tlie future barbule 

 (Plate 5, Figs. 25 and 26). The more superficial, and tlierefore more 

 distal, barbule cells become elongated to form the attenuated portion 

 of tlie barbule. They appear, consequently, much smaller in cross- 

 section than the proximal cells. 



In the broad flattened cells the nuclei come to occupy a ventral 

 position (Plate 5, Figs. 23, 27). The boundaries between contiguous 

 proximal cells of a single barbule run obliquely forward from the dorsal 

 n)argin to a point near the ventral margin just proximal to the nuclei, 

 where they turn slightly backwards towards the proximal end of the 

 barbule (Plate 5, Figs. 26 and 27). In the region of transition from the 

 bi-oad flattened form to the slender distal portion (Fig. 27), the outline 

 of these inter-cell boundaries changes to a form presenting a convexity 

 in an opposite direction, ^. e. towards tlie proximal end of the barbules; 

 the sides of the convexity being likewise more symmetrical. 



