THE ORIGIN OF VERTEBRATE LIMBS 431 



from the oldest fossil sharks ( Pleuropterygidae, Acanthodidas, 

 and Diplacanthidae) and we have a clear case. 



II. The muscle-buds supplying the fins on the dorsal side 

 of the body arise from the dorsal ends of the myotomes, while 

 those supplying the fins on the ventral side, paired and unpaired 

 alike, arise in exactly the same manner from the ventral ends of 

 the myotomes. There is nothing in this process to indicate 

 otherwise than that the fins have arisen in situ as outgrowths 

 from the body wall. 



III. The fins on the dorsal side of the body are supplied by 

 branches of the dorsal rami of the spinal nerves, the same in 

 character as those which supply the adjoining parts of the body, 

 while on the ventral side, paired and unpaired fins alike are 

 similarly supplied with branches of the ventral rami. 



IV. In the embryology of Cestracion I have carefully fol- 

 lowed the development of the blood supply in both kinds of fins. 

 In every case the blood-vessels are those which also supply 

 the adjoining body wall, and which take their origin as dorsal 

 branches of the dorsal aorta, or, in other words, are typical 

 body wall blood-vessels. 



V. The earliest support of the fins, paired and median alike, 

 is, as we have already stated, a dense or thickened mesenchyme. 

 The thickening in all cases begins next to the ectoderm and 

 becomes noticeable immediately after the fin fold makes its 

 appearance. As growth progresses this denser mesenchyme ex- 

 tends inward until it occupies all of the region in which, later on, 

 the fin skeleton is formed. The procartilage is differentiated 

 right in place out of the mesenchyme support, and later becomes 

 chondrined to form the cartilaginous skeleton. This sequence 

 in development is just what the fin-fold theory would lead us 

 to expect, and it seems altogether probable that such has been 

 the phylogenetic history of the supporting structures of the 

 fins. 



VI. In Cestracion the first indication of the formation of 

 the skeleton is seen in the region near the middle of the fin, 

 and the bases of the rays and the adjoining portions ~of basalia 

 appear at the same time and are the first structures to be ob- 

 served. The differentiation spreads in all directions, and the 



