166 bulletin: museum of COMrAUATIVE ZOOLOGY. 



I luive studied, but pigment rods when fully formed, i. c, at the stage 

 indicated in Figure 36 (Plate 6) ai"e of uniform size for each species. 

 The peculiar rod-like appearance and also the size ai'e indicated in Figure 

 36 (Plate 6), which was drawn with a magnification of 1500 diameters. 

 I have found the pigment rods of Sterna invarial)ly as near to 2 niicra 

 long as I could measure, and about one-third of a micron in diameter. 

 The shape does not seem to vary noticeably in different snecies. 



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 

 common dove (reddish-brown feather) the length is onlj'^ 0.9 p.. 



I iiud 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 alwaj's 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. Habl ('9*) 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 cjdinder-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 im{)ression 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) ; B, those shown in Figure 33 ; 

 C, those in Figure 3-t ; and D, those in Figure 35. The table gives 



