NISTLIXG FEATHERS OF TEE MALLARD. 621 



beginning of the third to the end of the fifth week the true tail- 

 quills take an ever increasing part in forming the " nestling tail."' 

 Hence nestling feathers only function during three of the six 

 weeks that Mallard ducklings feed and disport themselves like 

 diving ducks. When the protoptiles are shed, the tail-quills, 

 owing to the rhachis being sihort and incomplete, look unfinished^ 

 Perhaps for this reason the first set of tail-quills is ere long 

 superseded by larger and moi'e perfect quills in which the rhachis, 

 as in adult ducks, ends in a fine point. Usually the first set of 

 tail-quills begins to drop out during the fourteenth week, and a 

 new tail consisting of feathers of the adult type is eventimlly 

 developed. 



The Structure of the Tail Protoptiles. — The calamus of the tail- 

 quill, like that of the wing-quill, protoptile, continues to grow 

 after hatching. In the case of the protoptiles at each side of the 

 middle line the calamus only reaches a length of 4 or 5 mm. and 

 may have only three or four " cones'' ; biit the calamus of the 

 outer tail protoptiles may eventually measure 15 mm. and contain^ 

 over twenty " cones." I expected the entire protoptile calamus: 

 to be directly continuous with the expanded tip of the tail-quill 

 rhachis, but I found that, as text-fig. 7 shows, the calamus splits, 

 into an outer segment continuous with the tail-quill rhachis and 

 an inner segment continuous with the rhachis of a vestigial 

 aftershaft, the presence of this unexpected vestige of an after- 

 shaft indicates that even the highly specialized tail quills- 

 originally consisted of two shafts, and hence were constructed on 

 the same plan as the double feathers of the Emu. 



The shaft of the tail-quill protoptiles of a Mallard duckling 

 as a rule consists of fourteen or more pairs of barbs, each pro- 

 vided with barbules. As already mentioned, the majority of the 

 barbs of the wing-quill protoptiles are long and pointed and have 

 only a limited number of slender barbules (PI. II, fig. 8), but some 

 of the barbs of the tail-quill protoptiles are ribbon-shaped 

 (text-fig. 9), and they all have barbules along their whole length,. 

 The majority of the barbules have simple cilia along both 

 margins; but the six or seven pairs of libbon-shaped baibules at 

 the tip of the protoptile have large curved cilia along one margin, 

 which differ but little from the booklets on the barbules forming 

 the tip of the true tail-quills. The difference between the 

 specialized bai"bules with hook-like cilia at the tip of the pro- 

 toptile and the slender proximal barbules with small cilia will be- 

 evident if text-fig. 8 is compared with text-fig, 9. When the 

 broad barbules with hook-like cilia on one edge cross each other 

 the approximation to a true teleoptile is especially marked.. 

 What Mallard ducklings gain by having the tips of the tail-quill 

 protoptiles highly specialized is not very obvious. The after- 

 shaft of the Mallard's tail quill consists of eight long barbs 

 (Plate III. fig. 11) bearing slender almost straight barbules, each 

 with two rows of small cilia. 



It may be mentioned that in the case of the goslings of Chinese- 



