8 PTERYLOGRAPHY. 



tube. Moreover, in all the larger feathers, the shaft is nearly quadrangular, although sometimes 

 quite flat, as in Aptcnodytes. 



2. The AFTERSHAFT (Plate I, fig. 1,6), originates from the underside of the feather beneath 

 the umbiliciform pit, and, indeed, pretty nearly at the same place where this penetrates the main 



stria approaches the superior extremity of the barb. But if the feather be coloured, an accumulation 

 of pigment is formed on each of the oblique striae above each of its individual cells, and this is larger 

 the nearer the cell is to the main stem of the barb. This accumulation of pigment is the cause of the 

 spotted appearance of the barbules. It is, however, unequal on the two series of barbules, and 

 is stronger on that which subsequently becomes the anterior series and bears the booklets, but weaker 

 on the opposite one, which is shown to the left in our figure. The portion here represented is, 

 moreover, from a portion of the barb in which the separation of the barbules from each other has not yet 

 taken place, and the different series of cells are still attached together by their walls. Higher up this 

 union is dissolved, and the striae become perfectly isolated barbules. Each barbule has now a necklace- 

 like appearance, each joint of which, however, is not round, but much compressed, and consequently 

 possesses narrow upper and lower sides, and broad right and left sides. On the former the barbule 

 very soon undergoes its principal change, which consists in the gradual formation by one-sided 

 thickening of the cell-membrane on the narrow sides, of elevated combs, which soon increase into 

 large teeth. Each cell forms its own processes, which only become amalgamated with those of 

 the neighbouring cells at a later period. At the basal portion of the barbule this amalgamation is 

 complete, but towards the middle it is only partial, so that here the teeth may be seen still separate 

 even on nearly perfect barbules (fig. 12, from the left, and fig. 13, from the right side). This, 

 however, is the case especially at the lower edge, where the barbule forms a very thin lamella ; on the 

 upper edge, such excrescences never occur in the barbules of the left side, nor in those of the right 

 side at the base ; but the barbule becomes thickened here, and forms a ridge, which is the strongest 

 part of the barbule. Where this ridge ceases, the teeth commence. Fig. 13 shows, that at first each 

 cell still furnished with a nucleus, only forms a tooth beneath, and that this becomes elongated, 

 and curved and hooked at the end ; and also, that a remarkable abbreviation of the cells in a 

 longitudinal direction, is associated with this formation of hooks. Where the hook-formation ceases, 

 excrescences at the ends of the cells make their appearance on the upper edge of the barbule also, and 

 these become larger in proportion as those of the opposite side are less hooked. Simultaneously with this 

 the cells again increase in length, and afterwards form thin, cylindrical joints, which emit from their 

 extremities a little point above and below, the barbicels of the perfect barbule (fig. 4). The barbs of 

 all down-feathers and filoplurnes are formed in the same way ; all exhibit a very distinct knotted 

 structure, which indicates their origin from series of cells, and that each cell has formed peculiar and 

 very differently formed excrescences at its extremity. If the barbules be still young, the original cells 

 are distinctly seen in it (figs. 9 and 12) ; but if it be older, the cells disappear, and it appears 

 homogeneous (figs. 4-5, 19-25). I must also remark, that the dark streaks seen in figs. 12 

 and 13, are enlargements of the pigment spots situated immediately above the cells, and are 

 wanting in all colourless barbules (figs. 4 and 5). The downy barbule also, when it is coloured, 

 does not usually show a homogeneous, but most frequently an interrupted coloration (figs. 21, 22, and 24.) 

 In this way the exceedingly multifarious barbules, with, their appendages, knots, barbicels and 

 hooks, are formed from the simple series of cells ; but at the same time the barb itself which bears these 

 barbules, undergoes a gradual alteration, which consists especially in a transformation of its cells. 

 Even in fig. 15, when compared with fig. 14, we find these more angular and less regular, and ou the 

 whole somewhat larger. In this mode of development they gradually advanca, becoming still larger, 



