THE ORIGIN OF VERTEBRATE LIMBS 419 



is unquestionably a much older type of fish, the first anlage 

 of the skeleton undoubtedly makes its appearance in the region 

 of the base of the rays and the neighboring part of the primary 

 basal, we must conclude that this objection is at least not final. 

 On the contrary, it seems plain from the evidence at hand that 

 Spinax is the exception to the rule. 



III. Against this objection we must weigh the following 

 facts : 



(i) The writer finds that in Cestracion the rays and basalia 

 begin to appear before the girdle. Other investigators since 

 the time of Balfour ('78) have found the same to be true in 

 various species (Spinax excepted). 



(2) The basalia and rays do not "grow" out of any pre- 

 existing structure, but are differentiated, both in the same 

 manner, out of the same band or layer of mesenchyme. This 

 layer gives rise also to the rudiment of the girdle. The above 

 distinction is an important one since upon it depends the inter- 

 pretation that all of these skeletal structures of the fins are 

 developed in situ by differentiation instead of "growing out." 

 The latter term implies internal development and change of 

 location during the process, a condition contrary at least to 

 the present observations. 1 



IV. The fact that the dorsal end of the pectoral girdle has 

 relations with the visceral musculature (trapezius group, in- 

 nervated by viscero-motor nerves), has been interpreted by 

 the gill-arch theorists to show a primitive connection, — "die 

 alten Relikte der einstmaligen kopfmuskelversorgung des Schult- 

 ergurtels " (Furbringer '02) . The connection is beyond question, 

 but that the above conclusion is not final is evident from the 

 fact that in Cestracion the first anlage of the pectoral girdle 

 arises entirely ventral to that anlage of the trapezius, quite 

 separated from it, and comes into relation with the visceral 

 muscles only by its later dorsal growth. This is true also of 

 Spinax, as shown by my preparations of a 20 mm. embryo. 



1 By way of comparison, it is worthy of notice that the muscle-buds do 

 grow into the fins. They disarrange the mesenchyme cells and push 

 them out of the way during their progress. Nothing of this nature 

 takes place in connection with the development of the fin skeleton. 



