1894.] Development of the Wing of Sterna wilsonii. 761 
THE DEVELOPMENT OF THE WING OF STERNA 
WILSONII. 
Bv VIRGIL L. LEIGHTON. 
Although various students have investigated the structure 
and the development of the wing of the bird, many points 
still remain unsettled, and prominent among them, the rela- 
tionsof the carpal elements, the number of digits present and 
the comparison ofthese digits with those of the normal penta- 
dactyl manus. Professor J. S. Kingsley suggested to me to 
attempt the solution ofsome of these problems and the studies 
detailed below were carried out in the Biological Laboratory 
of Tufts College under his direction. To him I owe the ma- 
terial—embryos of various stages of Wilson’s tern, Sterna wil- 
80nii from the Island of Penekese, Mass.—which formed the 
basis of my work. 
The alcoholic material was studied both in toto by clearing 
with oil of clove, and by means of serial sections. The latter 
proved far preferable and much more dependence can be placed 
upon results obtained in this way, especially with the younger 
embryos than by the more common methods of dissection and 
clearing in essential oils. The figures of structural details 
which illustrate the paper were obtained from reconstruction 
projections of the sections and are magnified twenty diameters. 
I am not able to state the ages of the various embryos, but 
this is a matter of little importance since the approximate 
development can readily be made out from the figures of the 
various stages, each natural size. The numbering of the sep- 
arate stages is entirely arbitrary. 
I might state here, incidentally, that I have also studied to 
some extent the foot of the tern and I find in it, as has already 
been pointed out by other observers, (Miss Johnson, Studer, 
W. K. Parker and others) a fifth metatarsal present. 
STAGE I, (Fra. 1). 
At this stage (fig. a) the principal elements of the wing are 
becoming differentiated. The radius and ulna are entirely 
