iSa DISCOVERY REPORTS 



unsettled. It would seem sufficient, and certainly it would be in accord with the facts, 

 to regard the granular type of skin reproduced in the earliest feather papillae as being a 

 likely characteristic both of the ancestors of birds and of the scaly reptiles. Just such a 

 skin as this is still found in some of the more primitive lizards, e.g. chameleons. 



The papillae give rise to filaments by a process of exaggerated growth (Fig. 6 d). 

 The dermal element retains its granular appearance peripherally but has now in addition 

 a central pulp cavity in which there are numerous blood islands (Fig. 6 e, pc). 



For the next step in development the skin of an older embryo is necessary. A suitable 

 fragment is illustrated in Fig. 6 f. 



When the longitudinal and transverse sections of one of these filaments are compared 

 (Fig. 6^, //), it is at once evident that the further increase in length of the feather filament 

 has been accompanied by changes both external and internal. The external changes 

 result in the formation of a horny sheath, the internal in the isolation of eight solid epi- 

 dermal masses which are the rudiments of the barbs of the down feather. The dermis 

 plays no part in the construction of the feather other than that of supplying the vascular 

 requirements of the early stages and a formative zone for the epidermis. The boundary 

 between the two layers, as Wohlauer showed, is set up in the later stages of development 

 by the cylindrical epidermal cells. The middle layer of intermediate cells form the barbs, 

 and the outer cells make up the sheath. 



Just about the time when the embryo is due to hatch the sheath of the feather filament 

 disintegrates. Its loss releases eight barbs, which thereupon fluff out producing the 

 complete feather. In preparation for this event each of the barbs pushes out lateral 

 processes, or barbules, which therefore also arise in the intermediate cell layer. In 

 section they are seen intervening between the solid cell masses forming the barbs and 

 the surrounding sheath. Fig. 6 i was drawn from a fragment of the skin of the oldest of 

 the series of Ring penguin embryos. In this one of the feather filaments has lost the 

 outer sheath so as to free the barbs. Fig. 6j shows a longitudinal section through the 

 base of such a filament, and k a transverse section at the same stage cut far enough away 

 from the surface of the skin to show barbules as well as barbs. 



A further point concerning feather development deserves attention. It will be seen 

 that the filaments become deeply embedded in the dermal tissue and reach down to the 

 level of the subcutaneous musculature. This ensures a considerable degree of muscular 

 control, an obvious necessity for a homoiothermic animal which relies largely on such 

 outgrowths of the epidermis for the conservation of heat. It will also be observed that 

 each filament has, on one side of it or the other, an attendant surface papilla (Fig. 6f,fp). 

 These have exactly the same composition as the original feather papillae, but though so 

 closely connected with the filament they are not included in its sheath. It seems probable 

 that these are the rudiments of the filoplumes (Ewart, 1921, p. 626), which are retarded 

 in development amongst the down feathers but are nevertheless known to be present 

 in the adult (Pycraft, 1907, p. i,). 



In addition to these papillae each feather filament is encircled by still smaller papillae 

 of the same kind, which are less regularly distributed (Fig. 6/, pi). Exactly comparable 



