Page:Proceedings of the Royal Society of London Vol 69.djvu/29

Rh

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