Morphology of the Fins 



7S 



firmly in the body-wall, and beneath its surface, into an organ 

 of locomotion. 



"May I express the hope that what I have said is suflicient 

 to show in what a state of uncertainty our views are regarding 

 the morphological nature of the paired fins, and upon what an 



vi(\'; 



\^ 



•^ 



-^ 



a 



Fig. 61. — Embryos of Heterodnntus japonicus Maclay and Macleay, a Ces- 



traciont shark, showing the backward migration of the gill-archfs and the fonvard 



movement of the pectoral fin. a, b, c, representing different stages of growth. 



(After Dean.) 



exceedingly slender basis rest both of the two views which at 



present hold the field ? ' ' 



As to the backward migration of the ventral fins. Dr. Bash- 

 ford Dean has recently brought forward evidence from the 

 embryo of a very ancient type of shark {Heterodoutits japonicus) 

 that this does not actually occur in that species. On the other 



