652 TEGUMENTARY ORGANS. 



pith are successively derived from around the apex of the con- 

 tained central organized pulp, they are most developed towards 

 the outer dorsal convex part of the shaft, and are smallest near 

 the central cavity, which is indicated only by an inferior longi- 

 tudinal groove in the adult expanded feather. More nearly 

 in contact with the formative matrix, indeed, are found mere 

 granular nuclei, contained in a fluid cytoblastema ; and these 

 organized, though extravascular, independent nuclei, pass 

 through the ordinary phases of development and growth, 

 seen in other epidermic and epithelial cytoblasts. These 

 epidermic cells composing the white friable corky pith of the 

 shaft, were accurately described by Hooke in 1667, and 

 Leuwenhoek described and figured the cytoblasts, or glo- 

 bules, composing the barbs. 



The formation of a distinct, thin, dense, interior epidermic 

 layer, in the concavity of the growing shaft, and around the 

 exterior surface of the secreting matrix (147- H. I. a. b.) 

 completes the inferior concave surface of this part of the 

 feather, and prepares for its gradual protrusion, with the per- 

 fected and unfolding barbs continuous with its sides. On 

 opening the. organized pulp (147. D. b. b.), innumerable ves- 

 sels, turgid with red blood, are seen forming a continuous net- 

 work over every part of its interior parietes, and their trunks 

 are observed entering the convex conical part of its base where 

 a terminal opening or inferior umbilicus is left in the adult 

 quill (147. A. e.). The outer strong elastic layer forming the 

 dorsal fibrous covering of the shaft, between the outer ends of 

 the septa, or barbiferous cells, is early developed, and origi- 

 nates from cytoblasts, which undergo changes very similar to 

 those which give origin to the minute fibres of cellular tissue. 

 From their primitive rounded form, these cells are observed 

 to elongate, to become flattened, and gradually to subdivide 

 each into numerous longitudinal fibres ; their nuclei disap- 

 pear, their parietes become absorbed or fissured, and they at 

 length constitute the compact horny fibrous covering of the 

 dorsum of the shaft, and the entire parietes of the quill, 

 which is merely a cylindrical continuation of that portion of 

 the feather. The inferior grooved surface of the shaft is 

 covered by a similar deposition of granular independent 

 animated cells, with their nutrient fluid cytoblastema between 

 the lower ends of the grooved barbiferous septa, or compli- 



