176 Origin of the Paired Limbs of Vertebrates 
fins, paired and unpaired alike, are compelled to reach out of a direct 
position (7. e., out of the transverse plane of their myotomes) in order 
to get into the fin-fold. That is to say, the fin base is shorter at this 
period than the combined length of the myotomes concerned in supplying 
muscle-buds to the fins. This condition has been carefully discussed 
by Mollier, 93; Braus, 98, and Goodrich, 06, in the paired fins, and by 
Paul Mayer, 86, and Goodrich, 06, in the case of the unpaired fins. The 
writer has also observed it to be true for Cestracion and Spinax in both 
paired and median fins, o6b. 
VI. The nerves which supply the paired and unpaired fins arise seg- 
mentally in the same manner as external rami of the dorsal and ventral 
branches of the spinal nerves. (Balfour, 78, 81; P. Mayer, 86; Mollier, 
93; Braus, 98, 01; Osburn, 06b; Goodrich, 06.) 
VII. The blood-vessels which supply the various fins all arise in a 
similar manner. ‘They are all branches of body-wall blood-vessels which 
take their origin on the dorsal side of the dorsal aorta. 
VIII. The procartilaginous anlage of the fin skeleton arises in the 
same way in all fins, differentiating out of the mesenchyme plate; and, 
when basals are present, as in the paired fins of all sharks and the dorsal 
and anal fins of many, the rays and basals (and the girdle in the paired 
fins) form one continuous structure. (Balfour, 78; Mollier, 93; Ruge, 
02; Braus, 04a; Osburn, 06b; Goodrich, 06.) 
IX. As the continuous procartilaginous anlage of the fin skeleton be- 
comes differentiated into cartilage, the formation of joints takes place 
(Ruge, 02), with the result that the elements of the adult fin are defined. 
The process is identical in all the fins, but the amount of separation varies 
greatly according to the species, depending probably upon adaptation, 
1. €., aS to the special way in which the fin comes to be used. 
X. The sequence of development of the various structures entering into 
the formation of the fin is the same in all cases, viz.: 
(a) The epidermal fold is raised. 
(6) The mesenchyme pushes its way into the fold and forms the 
mesenchyme thickening. 
(c) The muscle-buds migrate into the fins accompanied by their re- 
spective branches of the spinal nerves. 
(d) The continuous procartilaginous anlage of the fin skeleton be- 
comes differentiated in situ out of the thickened mesenchyme (this in- 
cludes also the girdles of the paired fins). 
(e) The chondrification of the skeleton takes place, with the forma- 
tion of joints and the consequent separation into skeletal elements. 
