182 Origin of the Paired Limbs of Vertebrates 
from the region of the axial skeleton. The latter, moreover, has not at 
this stage become noticeable in the region of the hemal spines. The 
mesenchyme of the fin continues to develop toward the axial region while 
that of the latter becomes more concentrated and at last the two- regions 
are united. When the rays become evident by the differentiation of the 
mesenchyme they arise in situ and do not grow out* of the hemal spines. 
The same statement holds good for the development of the rays and basals 
of all fins, paired and unpaired,—all arise by differentiation in their 
proper position from the mesenchyme plate. 
The other unpaired fins not only resemble the inferior caudal in 
having an external origin for the mesenchyme plate, but they never 
originate in contact, much less in continuity, with the axial skeleton. In 
case spines are present, as in Spinax, Cestracion, etc., the spine second- 
arily comes into contact with the vertebral column for support. That 
this attachment is secondary is admitted even by the gill-arch theorists. 
Species, among the lower sharks, which do not possess fin spines usually 
have the fin skeleton widely separated from the axial skeleton (Noti- 
danus, Chlamydoselachus, Mustelus, Scyllium, Lamna, etc.). Figs. 19 
and 16 are camera drawings from Van Wijhe preparations of the dorsal 
and anal fins of Chlamydoselachus showing the relation to the vertebral 
column in an embryo of 225 mm. In Cestracion the large basale of the 
anal fin comes nearly or quite into contact with the ends of the haemal 
spines. A study of the development of this form, however, shows plainly 
that the basale in question arises earlier than, and entirely independent 
of, the hemal spines. Figs. 12 and 13 are camera drawings of vertical 
sections through the anal fin of a 40 mm. embryo, illustrating the inde- 
pendent origin of the fin skeleton. 
PAIRED FINS CONTRASTED WITH GILLS. 
The followers of Gegenbaur have always asserted the similarity of the 
paired fins to the gills, and recently K. Fiirbringer, 03, and Braus, oga, 
have renewed the attempt to compare these structures, the former ana- 
* As I have elsewhere (o6b) pointed out, the terms “ grow out,” ‘“ auswach- 
sen,’ etc., are entirely misleading when applied to the development of the 
fin skeleton. The muscle-buds may be rightly said to grow out into the fin, 
since by internal development the tip of the bud is moved forward. Its 
progress is marked by the disarrangement of the connective tissue and cells 
of the mesenchyme plate as the bud forces its way by such growth into its 
position in the fin. Figs. 5 and 6 illustrate this disarrangement and show 
how the cells are pushed out of the way. There is nothing comparable to this 
in the development of the fin skeleton since the rays elongate by the continued 
differentiation of mesenchyme cells at their ends. 
