f the Pelvic Plc.ms in Acanthias vulgaris. 21 



should expect to find a greater rostral extension in the embryo, and 

 the greater caudal extension is also explicable on the assumption that 

 a contraction of the fin area has also taken place caudally. 



On the migration theory we have already (p. 17) found it necessary 

 to assume that one or two segments just rostral to the collector area in 

 the adult are potential collector segments. The greater number of 

 collector nerves in young embryos lends ample support to this view. 



Again, since on this theory it is held that the formation of a collector 



