92 BULLETIN OF THE 
to form the rudimentary limb ; and it is well known that, in some forms 
at least, in the further development of this rudimentary limb, or limb- 
bud, there are contributed by the myotomes secondary mesodermal 
elements which are destined to form its skeleton and musculature. 
It has been my purpose in this paper to determine, if possible, the 
source of the mesoderm which enters into the formation of the paired 
fins of Teleosts, and to ascertain to what extent Fundulus conforms to 
this general rule. This problem involves, to a greater extent than I 
had anticipated, the early differentiation of the mesoderm itself, and 
indeed the origin of the mesoderm; but the latter subject, although 
very interesting and important, I have been obliged to omit entirely 
from my present study. 
The protovertebrie, the lateral layers, and the structures known as 
“head mesoderm” and “intermediate cell-mass” are intimately related 
in their origin, and the source of the mesoderm in the pectoral fin has 
been referred by various authors to one or more of these structures. I 
therefore begin my observations with the formation of these parts, and 
trace their development and relations through the early stages, with a 
view to ascertaining their connection with the origin of the pectoral fin. 
The very high degree of specialization which characterizes the ontogeny 
of Teleosts renders the investigation of certain questions quite difficult, 
and problems concerning points of homology and phylogenetic signifi- 
cance are beset with limitations and uncertainty. The extreme variation 
in the development of different forms of this vertebrate phylum also ren- 
ders hazardous any attempt to draw general conclusions from the inves- 
tigation of a single or only a few forms. Hence I cannot hope to place 
beyond the reach of criticism the conclusions which I have ventured to 
draw from observations made almost exclusively upon a single form. 
This paper is the result of studies carried on during the year 1889-90 
in the Zodlogical Laboratory of Harvard University, under the direction 
of Dr. E. L. Mark, at whose suggestion this problem was taken up. I 
owe grateful acknowledgment of my obligations to Dr. Mark for the uni- 
formly kind and valuable assistance rendered to me in this work. I am 
also under great obligations to Prof. Alexander Agassiz, through whose 
courtesy I enjoyed the advantages of pursuing my studies at the U. S. 
Fish Commission Laboratory at Wood’s Holl during the summer of 
1889, where I was enabled to secure the material upon which the fol- 
lowing observations were made. I am also indebted to Colonel Marshall 
McDonald, U. S. Commissioner of Fish and Fisheries, for various favors 
received through his department, and especially for many courtesies while 
