70 ON CLASSIFICATION. 



these appendages, which are termed " feathers," do not take the 

 form of mere plates developed upon the surface of the skin, but 

 are evolved within sacs from the surfaces of conical papilla of 

 the dermis. The external surface of the dermal papilla, whence 

 a feather is to be developed, is provided upon its dorsal surface 

 with a median groove, which becomes shallower towards the 

 apex of the papilla. From this median groove lateral furrows 

 proceed at an open angle, and passing round upon the under 

 surface of the papilla, become shallower, until, in the middle line, 

 opposite the dorsal median groove, they become obsolete. Minor 

 grooves run at right angles to the lateral furrows. Hence the 

 surface of the papilla has the character of a kind of mould, and 

 if it were repeatedly dipped in such a substance as a solution of 

 gelatine, and withdrawn to cool until its whole surface was covered 

 with an even coat of that substance, it is clear that the gelatinous 

 coat would be thickest at the basal or anterior end of the median 

 groove, at the median ends of the lateral furrows, and at those ends 

 of the minor grooves which open into them ; while it would be very 

 thin at the apices of the median and lateral grooves, and between 

 the ends of the minor grooves. If, therefore, the hollow cone of 

 gelatine, removed from its mould, were stretched from within ; or 

 if its thinnest parts became weak by drying ; it would tend to give 

 way, along the inferior median line, opposite the rod-like cast of 

 the median groove and between the ends of the casts of the 

 lateral furrows, as well, as between each of the minor grooves, 

 and the hollow cone would expand into a flat feather-like struc- 

 ture with a median shaft, as a " vane " formed of " barbs " and 

 " barbules." In point of fact, in the development of a feather 

 such a cast of the dermal papilla is formed, though not in 

 gelatine, but in the horny epidermic layer developed upon the 

 mould, and, as this is thrust outwards, it opens out in the 

 manner just described. After a certain period of growth tli; 1 

 papilla of the feather ceases to be grooved, and a continuous 

 horny cylinder is formed, which constitutes the " quill." 



Between Aves and Mammalia there is a hiatus, not perhaps, 

 in some respects, quite so wide as that between Amphibia and 

 Reptilia, but still very considerable. 



All Mammals possess an amnion of an essentially similar 



