148 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
Professor Mark for the encouraging interest he has shown in my inves- 
tigations, for helpful suggestions, and for invaluable training in precision 
of method. 
In the course of my histological studies on the developing feather I 
have naturally examined the literature of the subject, and believe that 
a more elaborate analysis and description of the various stages in the 
development of the complex structure of the feather, especially of those 
elements producing color, is highly desirable. This work therefore deals 
mainly with the histological side of the subject of color in the definitive 
feather with some contributions to the general knowledge of the 
development of the feather. 
II. Methods and Material. 
My principal material has been obtained from the remiges of Sterna 
hirundo Linn. During the summer of 1899 while occupying a table in 
the laboratory of the United States Fish Commission Station at Wood’s 
Hole, Mass., I obtained two young birds of S. hirundo with feather 
germs (“pin feathers”), some of which had begun to expose fully corni- 
fied portions at their ruptured distal ends. 
Immediately after killing the birds, the wings and strips of skin 
bearing feathers were placed either in Kleinenberg’s picro-sulphuric 
mixture, or saturated aqueous solution of corrosive sublimate. 
In the summer of 1900 I put up some more material of S. hirundo, 
this time using Kleinenberg’s picro-sulphuric fluid and the fixing 
mixtures of both Hermann and Flemming. I found that better pene- 
tration was secured when the feather was simply pulled from the 
feather follicle and dropped into the fluid, without the superfluous 
tissue of the follicle and the connective tissue below the inferior 
umbilicus. One soon learns to perform this operation easily and 
without injury to the tissues, in spite of the fact that the latter are 
very delicate at the proximal end of the feather germ. 
I have found Kleinenberg’s picro-sulphuric mixture and Hermann’s 
fluid the most satisfactory fixing agents; the latter gives by far the 
best preservation. Kleinenberg’s picro-sulphuric is especially advanta- 
geous for the study of developing pigment cells, in that it leaves no stain 
after proper washing, whereas osmic-acid fluids produce a blackening of 
the cytoplasm that is very objectionable in the study of early stages 
of the pigment cell. 
