156 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
clefts (Liingsfurchen) between the lateral plates and the axial plates. 
He described these clefts as being filled ultimately by the growth of the 
cells of the barbule fundaments. They would thus provide room for 
the expansion. 
B. The Differentiation of the Feather. 
l. THe BaARBULES. 
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. These 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 these cells in a cross-section (Figs. 16-21, ser. el.) 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 the future barbule 
(Plate 5, Figs. 25 and 26). The more superficial, and therefore more 
distal, barbule cells become elongated to form the attenuated portion 
of the 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 
margin 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 
broad 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, 7. e. towards the proximal end of the barbules ; 
the sides of the convexity being likewise more symmetrical. 
