1883.] 215 [Jeffries. 



and finally meeting complete the wall of the shaft on the inner 

 side. In those cases where the shaft is hollow a column of the 

 pulp is enclosed which upon drying up leaves a space ; in solid 

 shafted feathers the sides are simply flattened against each other 

 and do not enclose any of the pulp. 



The direction of the folds destined to form the lateral barbs 

 is also changed. They now run from a line (fig. 12 e) on the 

 inner side of the papilla downwards and shaftwards, so as to join 

 the shaft at an angle of about forty-five degrees. 



Studer, who seems to have overlooked the direction of these 

 folds, states that those on the inner side are the youngest and 

 least developed, and that the folds run from the tip towards the 

 base of the papilla. This is a slight error ; the folds on the inner 

 side are smaller because they are destined to form the tips of the 

 barbs, which are much smaller than the bases. As to their his- 

 tology the folds are as well advanced on the ventral side, though 

 they belong to barbs nearer the base of the feather. This is 

 owing to specialization advancing from the tips, or ends, of the 

 feather-parts to their bases. So the tip of any barb is complete 

 before the base of the barb distal to it. 



Thus far I have only spoken of the grosser structure, in fact, 

 what may be seen by the unaided eye. We have still to consider 

 the histology of the shaft and the formation of the barbs and 

 their appendages. 



The back of the shaft is formed by the transitional cells extend- 

 ing down the middle of the outer surface of the papilla. These 

 become converted into long fibres of horn, as so admirably shown 

 by Burmeister. The same is true of the other surfaces of the 

 shaft. The sack has no part in the formation of the shaft as 

 commonly stated. The cells in the body of the shaft, those des- 

 tined to form the pith, undergo no change of form but remain 

 more or less spherical, and by a process of drying finally form 

 little, horny sacks. In these may be found a trace of the proto- 

 plasm and nuclei. The cells forming the walls of the cavity in 

 hollow-shafted feathers form a simple, horny film. Here it is 

 worthy of note that the cells lining the cavity and covering the 

 inner side are true mucous cells. 



The formation of the barbs and barbules is quite complicated, 

 owing to the complexity of the final structure, but in plan is the 



