WD 2 
2] 
however, preened away by the bird, but it can be seen enclosed in the 
middle of the ripe quill as a series of horny cones fitting over one another. 
Next the medulla sheath is a series of cells arranged for the most part 
in wedge-shaped groups, as seen in transverse section. J¢ is from these cells 
that the entire feather is formed, the pith taking no part in producing the 
feather beyond serving to nourish the growing cells. One of the groups of 
cells is much larger than the others, and this forms the shaft of the feather, 
while the numerous smaller groups of cells on each side give rise to the 
barbs and barbules of the fiue. The barbs, however, are formed from the 
cells nearest the pith, that is, from those which are nearest the supply of 
nutrition, while the barbules are formed from the cells further away. No 
blood-vessels occur among these feather-forming cells, consequently they 
have to depend for their nutrition upon the fluid or plasma part of the 
Fig. 2.—Transverse section of a feather at a very early stage of its development 
while soft and p astic within the socket (diagrammatic). In the middle is the pith or 
medulla containing the blood-vessels, through the walls of which the blood-plasma exudes 
to nourish the growing feather cells outside. Around the medulla is the medullary sheath, 
which remains after the blood ha- receded and the pith dried up. ‘The cells outside the 
medullary sheath give rise to the feather and are without blood-vessels. The large group 
at the top of the figure will give rise to the shaft of the feather, and the wedge-shaped 
groups along each side to the flue, the barb-forming cells being inside and the barbule- 
forming cells nearer the outside. Surroundng the entire feather is the feather-sheath, 
which is preened away to allow the plume to open out as it ripens. «, shaft-formin + 
cells ; >, medulla or pith ; ¢, blood-vessel ; d, medullary sheath ; e, feather sheath; /, 
barbule-forming cells ; y, barb-forming cells. 
blood which oozes out through the thin walls of the vessels in the pith; 
spaces, known as lymph-spaces, are found among the cells, and these are 
filled with the nutrient blood-plasma or lymph, which thereby bathes and 
nourishes the cells. 
Outside the wedge-shaped groups of cells which form the flue and shaft 
is a layer of cells which gives rise to the horny sheath of the feather. This 
sheath completely covers the feather before it expands, but 1s preened away 
