180 Origin of the Paired Limbs of Vertebrates 
and the inferior caudal of most sharks,—there is a very close correspond- 
ence or concordance throughout the fin. In certain cases,—e. g., the 
pelvic of Spinax (Braus) and the dorsals of Cestracion,—there is a con- 
cordance over a large portion of the fin and discrepancy over: the re- 
mainder, while in the anal fin of Cestracion it would be a difficult matter 
to trace any exact concordance between the cartilage plates of the fin 
skeleton and the broad sheet of muscle derived from the intimate fusion 
of the muscle-buds. Thus, instead of weighing against the fin-fold 
theory, the facts of discrepancy only point more certainly to the close 
relationship between the paired and median fins. 
Mope oF ORIGIN OF THE FIN SKELETON. 
The origin of the mesenchyme cells which first occupy regions of the 
body in which the fins are later to be developed is already well known, 
thanks to the researches of Balfour, 78, 81; Boyer, 92; Rabl, 91, 97, 
and Molher, 93. The concentration or thickening of this mesenchyme 
which gives rise to the fin skeleton begins in all fins just beneath the 
ectoderm at about the same time as the uplifting of the ectoderm to form 
the fin-fold. Figs. 1, 3, 4, and 5, show it in various fins. From this 
position the concentration extends outward with the growing fin-fold and 
inward until it covers all the region which the fin skeleton later occu- 
pies. The skeleton is derived from this concentrated mesenchyme,— 
not only the more externally situated fin-rays but also the basalia of the 
median and paired fins and the girdles of the latter as well. The change 
takes place by the direct differentiation of the mesenchyme tissue into 
the continuous procartilaginous anlage of the skeleton, and this later, 
by chondrification, becomes the definitive skeleton. In this process there 
are no topographic changes, the formation of joints taking place in situ. 
The concentration of the mesenchyme is from the first easily distinguish- 
able from that which develops into the ordinary connective tissue. Figs. 
1 to 6, inclusive, illustrate the’ difference in the appearance of the two 
kinds of cells. There can be no doubt that, ontogenetically, the earliest 
support of the fins in all cases is this concentrated mesenchyme which 
makes its appearance in and beneath the fin-fold long before the muscle- 
buds and nerves enter, and which becomes directly differentiated into the 
cartilaginous skeleton of the adult fin. There seems, therefore, no good 
reason for thinking that the evolution of the fin has followed any other 
course than this during its phylogeny. 
