36 SUMMARY OF CURRENT RESEARCHES RELATING TO 



means of wliicli the cells become ntiit(Ml with one another ; a striation 

 then becomes aj)pareut iu these prolongations, which is tlie lirst in- 

 dication of tlie librcs. Finally, similar striaa appear in the middle of 

 the cell around the niiclovis, and when the end of the cell is free the 

 strife become distinct fibrils, which appear as uncommonly fine fila- 

 ments. It is difiicult to say certainly whether, as is probable, each 

 fibril extends along the whole length of the cell. It is clear, however, 

 that the fibrils are formed from the protoplasm of the cell, by its 

 differentiation ; and the greater part of the cell is so used. The 

 fibrils run parallel to the cell. 



In a longitudinal section through the bristle of a pig the papilla 

 was seen to have the form of a high cone with an elongated tip, which 

 projected into the cells of the medullary substance ; in transverse 

 section this papilla had the appearance of a rounded corpuscle with 

 some processes radiating round it. Around the papilla the epithelial 

 cells lie in folds, which correspond to notches in the papilla. This 

 is an arrangement unlike that which obtains in other hairs, and calls 

 to mind ratlicr the si)ines of the hedgehog. The cortical siibstance 

 forms a ring with irregular processes, which project into the interior 

 of the medullary substance; bristles seem, then, to be intermediate 

 between spines and hairs. 



The hedgehog's spines are next described, and it is pointed out 

 there are two kinds of them ; the majority arc completely formed 

 spines, are cylindrical in form, taper at either end, and end quite 

 sharply above. Inferiorly there is a bulb-shaped enlargement which 

 is fixed in the skin, and prevents the spine from being drawn out 

 complete. Among the comijleted spines we find others which are 

 younger, and which can be easily drawn out from the skin as they 

 have no terminal bulb ; these two stages may be paralleled in the 

 history of a hair. The bulb is largely composed of cortical sub- 

 stance, the elements of which have a direction parallel to the axis of 

 the spine. 



After describing the development of feathers, the author compares 

 the four structures which he has examined, and comes to the con- 

 clusion that the greater development of the papilla is always accom- 

 panied by a more complicated arrangement and greater differentiation 

 of the parts which are adjacent to it. This is due not only to the 

 elongation of the papilla, but to the concomitant ingrowth of it into 

 the surrounding epithelial cells. Of hairs, the least developed are 

 bristles, and in them the papillae are longer than in other hairs. At 

 the same time the bristle presents an indication of that regular 

 differentiation which is seen in the spine ; and, in corresi^ndence 

 with this, the papilla is still better developed in the spine, being both 

 higher and more deejdy set among the epithelial cells than is the case 

 with that of the bristle. In the embryonic feather the papilla extends 

 to the end of the rudiment, passes more deeply among the surrounding 

 cells, and breaks up into as many groups as there are to be rays. In 

 the completed feather, in which the papilla is still more developed, 

 there is a still greater complication in structure. The author thinks 

 that the importance of the papilla for all hair-like structures must 



