160 bulletin: museum of COMrARATIVE ZOOLOGY. 



Haecker also mentioned an outer cpitrichiura 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 RhacMs. 



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. Tiieir nuclei are 

 shrivelled. The deeper cells, including the cylinder cells, retain their 

 regular form and size until a later stage. 



