152 bulletin: museum of comparative zoology. 



more distal in position, a single feather presents at successive levels con- 

 ditions which are identical with those of a given region of a feather in 

 successive stages of its growth. The conditions shown in Figures 12-23 

 were taken from sections marked in the diagram, Figure 1, by the num- 

 bers 12-23, which are successively more and more distal in position. 

 They correspond to successively older stages in the development of a 

 feather germ. I begin my account of the conditions presented by the 

 remiges of Sterna hirundo with a description of the conditions nearer 

 the inferior umbilicus (12, Fig. 1). 



In Figure 12 (Plate 2) is shown a portion of a cross-section just above 

 the umbilicus. A peripheral portion of the pulp (drm.) is shown at the 

 bottom of the figure. It consists of closely packed connective-tissue cells, 

 whose long axes are cut at right angles. Blood vessels are especially 

 numerous at the periphery of the pulp. 



Between the pulp and the epidermis lies the so-called basal mem- 

 brane. This is seen most favoi'ably in preparations where decolorization 

 was not carried very far. I have also recognized this structure in picro- 

 sulphuric material, but far less clearly. Studer ('73) described as 

 structureless a membrane lying between the dermis and epidermis of 

 the feather, but later ('78, p. 425) noticed that it was cellular. Davies 

 ('89) noted Studer's observations of a basal membrane in liis review of 

 Studer's work, but, in his own account, does not mention the basal 

 membi-ane as a separate structure. He treats of it as a part of the 

 connective-tissue pulp, without, however, discussing the subject. 



That this structure is cellular in Sterna hirundo, is evident from the 

 presence of the nuclei which ai-e inclosed in it (Plate 2, Fig. 14, nl.). 

 There can be no doubt, moreover, that it is of dermal origin, for the 

 nuclei have the characteristic smaller size of dermal nuclei ; besides, a 

 sharper line of demarcation exists between the membrane and the cylin- 

 der-cell lavcr than between it and the dermal cells. The nuclei are not 

 abundant, but where they do occur they leave no doubt as to the cellu- 

 lar nature of the structure. 



Proceeding distally along the fundament of the feather, the basal 

 membrane becomes thinner and therefore less conspicuous (Figs. 

 15-21). 



The epidermis of the feather germ, including the feather sheath, 

 comprises four ftxirly well marked layers : The deepest layer, that next 

 the pulp, consists of a single row of spindle-shaped cells (d. ci/l.) elon- 

 gated in the direction of the radii of the cylindrical germ, and called 

 cylinder cells. Except for their blunt deep ends and their weaker stain- 



