[250] Journal of Anatomy and Physiology, Vol. XI. April, 1877.
[251] Kleinenberg Hydra.
[252] Virchow's Archiv, Vol. XXXI. 1864.
[253] Phil. Trans., 1876. [This Edition, No. VIII.]
[254] Entwicklungsgeschichte der Unke.
[255] Loc. cit. p. 516.
[256] Archiv für Micros. Anat., Vol. XI. 1875.
[257] There are strong reasons for regarding the posterior roots as the primitive ones. These are spoken of later, but I may state that they depend:
(1) On the fact that only posterior roots exist in the brain.
(2) That only posterior roots exist in Amphioxus.