3<D RE GENERA TION 



experiment there were eighty hydranths under the blue glass, but not 

 one had developed in the red light. On the ninth day the red glass 

 was replaced by a dark blue one. Two days later hydranths began to 

 appear, and on the following day thirty-two hydranths had appeared, 

 and in a few days more as many as sixty had developed. 1 Loeb con- 

 cluded that only in the more refrangible (blue) rays does the regen- 

 eration of the hydranth take place, while the less refrangible (red) 

 rays act as darkness does. 2 This hydroid is the only animal yet 

 found that shows the effect of light on regeneration, and it is inter- 

 esting to find that it is one of the few animals known in which light 

 has an influence on the growth, if the heliotropism, or turning towards 

 the light, of the hydranth is looked upon as a phenomenon of growth. 

 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 3 is cut off, some- 

 times 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 there- 

 fore no connection with the presence or absence of light. 



GRA VI TV 



The only case known amongst animals, in which regeneration is 

 influenced by the action of gravity, 4 is that of the hydroid Antennu- 

 laria 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 whieh 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 



1 The dark red glass was fairly monochromatic; the dark blue let a trace of red light 

 through. 



2 The same difference was found in this form in regard to heliotropism. 

 8 Palaeroon and Sicyonia. 



4 The regeneration of the lens of triton may also be affected by gravity. 



