OF THE PARTS OF FEATHERS. 5 



1. The STEM (Plate I, fig. 1, a), forms the main stalk of the feather, and bears all the other 

 external parts. It usually resembles a greatly elongated cone, or a spindle. At the lower part, 

 which is inserted in the skin, it is cylindrical, hollow, and transparent ; higher up, it is filled 

 with a cellular pith. But even the hollow part, which bears the name of the TUBE (calamus, 

 Spuhle), is not quite empty, but contains some large cells adhering to each other in a row, which 



made on the gradual development of these parts, with the view of completing NITZSCH'S description of 

 the constituents of feathers. 



A young feather, before it passes beyond the limits of the skin, forms a cylindrico-conical follicle 

 pointed above, which is completely closed, and contains the whole of the constituent parts of the 

 feather. If it be opened about this time, that is to say, before it has passed the outer surface of the 

 skin, the tolerably firm, leathery follicle is found to contain a second membranous follicle, filled with a 

 gelatinous fluid, the axis of which is permeated by blood-vessels, viz., a vein and an artery, of which 

 the latter is distinguished by its greater fineness and its lighter colour. These blood-vessels penetrate 

 from the fatty layer of the bird beneath the skin into the inferior extremity of the feather-follicle, where 

 they perforate both the outer and inner sacs. Between the two follicles there is a peculiar layer, 

 formed of a soft, pasty, finely granular substance, which coats the inner follicle, in the same way that 

 it lines the inner surface of the outer one. The whole three layers of the feather-germ may be 

 explained as follows : 



1. The outer follicle is the envelope within which the true feather is formed ; it does not enter 

 into the substance of the feather, and consists of large, thick, epithelial cells, so that it must be 

 regarded as a continuation of the epidermis, an inversion of the latter within the cavity which contains 

 the feather. In Plate I, fig. 17, I have represented a portion of this follicle under a magnifying power 

 of 500 diameters ; the large epithelial cells, and the cell-nuclei on their walls, are easily seen here. 

 From the fact that several (often from four to six) other similar nuclei of subjacent cells shimmer 

 through each cell, I think I may conclude, that the rather thick and firm follicle is formed of from five 

 to seven layers of such cells. For the sake of distinctness I have not represented the nuclei and cell- 

 walls seen through the others, but only those of the uppermost layer. 



2. The intermediate, finely granular layer is the formative material of the feather, and its 

 granules are nothing but cell-nuclei, which, in proportion as they are situated more towards the upper 

 part of this layer, become more distinct and acquire a clear space (the future cell), in the midst of 

 which they may be seen as oval granules of a yellowish colour. In many of these nuclei may also be 

 detected a dark point, and sometimes two ; these are the so-called nuclear corpuscles, which I regard 

 as a central cavity, only formed at a later period, when the nucleus itself has become enlarged to its 

 normal size. 



3. The interior or central follicle, which consists of a very thick, spongy membrane filled with 

 a gelatinous mass and also contains the blood-vessels already mentioned, is the focus for the 

 formative material of the feather, its so-called matrix, which evidently forms the gelatinous matter 

 (perhaps albumen) from the blood, and subsequently converts it into cell-nuclei. With a high 

 magnifying power, indeed, I found an innumerable quantity of granules of unequal size, precipitated 

 upon the outer surface of the sac ; these, when mixed with water, were in part dissolved, lay upon the 

 slide, and were of very unequal size. By far the greater part of them were smaller than the 

 cell-nuclei ; but many were larger. In the sac itself I could detect no cellular structure. 



In the progress of the formation of the feather the intermediate layer undergoes a very essential 

 alteration and transformation, whilst the other two remain unchanged, pretty much as I have just 

 described them. 



The first thing that takes place when the epithelial follicle (which I shall hereafter indicate 



