160 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Haecker also mentioned an outer epitrichium covering the cortex. I 
have not been able to satisfy myself that such a layer actually exists. 
There are appearances suggesting an epitrichium, but these I regard as 
purely optical effects. 
Haecker’s figures of transverse sections of barbs are, with few excep- 
tions, the only ones that I have found approaching accuracy in detail, 
and even his are sometimes confusing. I have therefore prepared figures 
showing in detail cross-sections of barbs from different birds, though 
several of them have been figured before. The figures given by Jeffries 
(83) for transverse sections of barbs are almost worthless, but their 
crudity is probably largely explained by the lack of a suitable technique. 
The cortex in a cross-section of a barb from Megascops asio, which 
appeared in an otherwise beautiful plate published by Chadbourne (’97), 
is wholly erroneous. 
4. The Rhachis. 
The shaft, or rhachis, arises on the dorsal side of the feather germ 
and represents two or more combined ridges (Plate 1, Fig. 2; Plate 9, 
Fig. 42, rch.) ; its structure is, in general, like that of a barb with a 
central medulla of polygonal cells and an outer thickened cortex. It 
also bears barbules like those of the barb, between the points of inser- 
tion of the latter, on its sides. The development of the rhachis was 
carefully studied by Davies, to whose account I have nothing to add. 
5. The Residual Cells. 
As has already been stated, not all the cells of the ridge are employed 
in the formation of the barbules and barb. With the growth of the 
ridges, the layer of cylinder cells is pushed closely against the corre- 
sponding layer of the neighboring ridges, and these cells (Plate 3, Fig. 
16, cl. cyl.) still continue to be so crowded in the layer that their nuclei 
appear almost to touch each other; but with the great longitudinal ex- 
tension of the germ, due to the growth of the barbs and barbules, in 
which the lateral cylinder cells do not share, the cylinder cells become 
more and more spread out (Plate 4, Fig. 19, cl. cyl., Figs. 20-21). 
The inner-sheath cells also experience a contraction during the growth 
of the feather. In Figure 23, Plate 5, the elements of the feather 
proper have been shaded. Residual cells are scattered through the 
more superficial spaces not occupied by the barbules. Their nuclei are 
shrivelled. The deeper cells, including the cylinder cells, retain their 
regular form and size until a later stage. 
