62^ t*ftOF. J. COSSAft EWAteT ON 'mt 



lengtli and consist of two distinct bundles of barbs — a large outer 

 bundle in a line with the shaft of the preplumula and a smaller 

 bundle in a line with the afterslmft. 



In the plumula from a five weeks' duckling represented in 

 PI. YI. fig. 21, the outer bundle already consists of a rhachis and 

 eight pairs of barbs, but the inner or aftershaft bundle only 

 consists of five pairs of barbs. By the end of the sixth week the 

 development of both the shaft and aftershaft is complete, and 

 the unsplit portion of the epidei'mic tube is giving rise to a 

 calamus. In the completed plumula (PI. Y. fig. 18) the shaft 

 consists of a rhachis and over twenty pairs of bai'bs ; and the 

 aftershaft of a rhachis with about a dozen pairs of barbs. 



From PI. VI. fig. 21 it is evident that the aftershaft of a 

 plumula in its development and structure agrees with the shaft ; 

 as it is developed before the calamus it cannot be regarded 

 as a secondarily acquired appendage produced from a forward 

 elongation of the calamus. 



2. 27ie Develojmient of the Aftershaft of Emu Feathers. — It 

 has hitherto been often assumed that in the Emvi the proto- 

 ptile has " no distinct aftershaft." That there is an aftershaft in 

 the first, as well as in the second and third Emu feathers, is indi- 

 cated by the diagram, text-fig. 11. In the young Emus I have 

 examined the protoptile aftershaft has, with rare exceptions, 

 consisted of seven or more barbs longer than, or at least as long 

 as, the barbs of the shaft. In the Emu protoptile given in 

 PL YI. fig. 22, the aftershaft has a short rhachis. It is especially 

 noteworthy that in the Emu the rhachis of the protoptile shaft, 

 instead of ending in a hollow calamus containing "cones," is 

 connected with the relatively thick rhachis of the shaft of the 

 feather of the second generation by a solid band (PI. YI. fig. 23). 



In a like manner the rhachis of the protoptile aftershaft is 

 connected with the aftershaft rhachis of the second feather. 

 When, as sometimes happens, the connecting bands occupying 

 the position of a calamus separate from each other (PI. YI. fig. 23), 

 it becomes obvious that the tip of the aftershaft of the second 

 feather is not free as generally assumed, but from the outset 

 continuous with the protoptile aftershaft. 



Though as in plumulse the rhachis of the aftershaft is formed 

 by the fusion of two barbs, the distal poi^tion of the rhachis of the 

 shaft is simply a continuation of the band proceeding backwards 

 from the protoptile shaft. That the aftershaft of the feathers of 

 the second generation is not " developed as a ventral elongation 

 of the calamus," but develops from apex to base like the shaft, 

 is made clear by Pis. YI. and YII. figs. 23, 24, & 25, which show 

 the second feather at various stages of growth. 



As the destined length of the second-generation feather is 

 reached the rhachis of the shaft expands, gives off six or more 

 pairs of relatively long barbs, and thereafter becomes continuous 

 with the rhachis of the shaft of the feather of the third generation 

 <'P1. YII. fig. 26). In a like manner the proximal end of the 



