There is another series of experiments made to test the effect of light on regeneration, which gave, however, negative results. Herbst observed that when the eye of certain crustacea[21] is cut off, sometimes an eye and sometimes an antenna is regenerated. A number of individuals from which the eyes had been removed were kept in the light, and others in the dark, in order to see if the presence or absence of light is a factor in determining the kind of regeneration that takes place. It was found that as many individuals regenerated eyes in the dark as in the light. It was discovered later by Herbst and myself, independently, that, when the end only of the eye-stalk is cut off, an eye regenerates, but when the eye-stalk is cut off at the base, an antenna regenerates. The difference in the result has therefore no connection with the presence or absence of light. _GRAVITY_ The only case known amongst animals, in which regeneration is influenced by the action of gravity,[22] is that of the hydroid _Antennularia antennina_. This hydroid lives attached to the bottom of the sea several metres below the surface. The hydroid consists of a single, vertical, central stem, or axis, with two or four series of lateral branches along which the hydranths arise (Fig. 14, _A_). The stem is attached by so-called stolons, or roots. In its normal growth at the free end the hydroid has been shown by Loeb to exhibit marked geotropic changes. If, for instance, the stem is bent over to one side the new growth that takes place at the apex of the stem directs the new part upwards in a vertical direction. If pieces are cut from the stem of antennularia and suspended in [Illustration: FIG. 14.--After Loeb. Normal stalk of _Antennularia antennina_. _B._ Piece regenerating in vertical, normal position. _C._ Piece regenerating in inverted position. _D._ Piece regenerating in inclined, vertical position. _E._ Piece regenerating in inclined, inverted position. _F._ Piece regenerating in horizontal position.]