166 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
I have studied, but pigment rods when fully formed, i. e., at the stage 
indicated in Figure 36 (Plate 6) are of uniform size for each species. 
The peculiar rod-like appearance and also the size are indicated in Figure 
36 (Plate 6), which was drawn with a magnification of 1500 diameters. 
I have found the pigment rods of Sterna invariably as near to 2 micra 
long as I could measure, and about one-third of a micron in diameter. 
The shape does not seem to vary noticeably in different species. 
In the following species the rods are of practically the same size as in 
Sterna: Passerina ciris, P. cyanea, and the “homer” pigeon. In the 
coramon dove (reddish-brown feather) the length is only 0.9 pu. 
I find myself in entire agreement with Post (’94) as to the origin of 
melanin in feathers. At no time have I found pigment in the pulp. 
The pigment cells, moreover, have always been separated from the pulp 
by the cylinder-cell layer and the basal membrane, so that there could 
be no question of misinterpretation as to the place of the pigment 
granules. Rabl (’94) has made the same observation on the down: 
feathers of the chick. 
I have examined many preparations, at stages both preceding and 
accompanying the formation of pigment cells, for evidence that leuco- 
cytes enter the epidermis. Although leucocytes are to be found in the 
blood capillaries close to the basal membrane, I have not seen a single 
case suggesting actual invasion of the epithelium by them or by any 
other form of cell. It may be objected that because my preparations 
did not catch wandering cells at the moment of their entering the 
epithelium, I have not sufficient ground for denying that they ever pen- 
etrate. Even granting the force of this contention, we still should 
have a right to expect transition stages in the form of the nuclei from 
that of typical leucocytes to that of pigment cells, but such intermediate 
stages I have never been able to find. Furthermore, if there were an 
immigration of prospective pigment cells, or melanoblasts, from the pulp, 
it is reasonable to suppose that at the earlier stages of the development 
of pigment the cell would be comparatively near to the cylinder-cell 
layer ; but there is no evidence that such is at any time the condition. 
In order to have something more definite than a general impression on 
this point, I have noted the distances of pigment cells from the pulp at 
various stages in their development, and for this purpose have divided 
the cells into four groups. The following table gives the results of 
these measurements. Group A includes the youngest stages, those 
represented in Figures 30-32 (Plate 6); 3, those shown in Figure 33; 
C’, those in Figure 34; and D, those in Figure 35. The table gives 
