Production of New Hydranths in Hydra 19 
think, by the following experiments: A white and a green hydra 
were both cut a little beneath the tentacles and the two cut oral 
surfaces grafted together. A few hours later, after the graft had 
become secure, the green portion was cut off leaving a circle of 
green tissue with aboral end exposed and attached by its oral 
end to the oral surface of the headless white hydra (Fig. 62). In 
three cases, tentacles appeared at the free aboral end and these 
tentacles were formed of green material, the oral material of the 
white stock having no part in their composition (Fig. 63). Ina 
few other cases, one or two of the tentacles were formed of white 
material, the rest of green (Fig. 64). Whether this result was due 
to a somewhat oblique cut, so that the circle of green was minimal 
in one place, or whether the material rearranged itself, some of the 
green migrating posteriorly and the white anteriorly was not 
determined. In the former cases, where all the tentacles were 
green, there was no migration of the white oral material. These 
experiments did not show, however, that there might not have 
been a migration of material in the green tissue itself, the oral 
material going anteriorly and the aboral posteriorly. Inoneexperi- 
ment, it was conclusively shown that such migration did not take 
place. In this case tentacles formed not only on the exposed green 
surface, but also at the union of graft and stock (Fig. 48). More- 
over, both sets of tentacles were composed entirely of green mate- 
rial. One set must therefore have come from the oral material 
which still remained at the junction of graft and stock, and the 
other set from the aboral material which lay at the exposed surface. 
This was an undoubted case of heteromorphosis in the strictest 
sense of the word, and throws some light on the meaning of reversal 
of polarity in connection with heteromorphic structures. We find 
in this experiment, that although polarity has been reversed in so 
far as normal foot-producing material becomes head-forming mate- 
rial under the influence of the larger piece, the original polarity 
which determined that the head-forming material should be head- 
forming has not been lost. It would seem that the original polar- 
ity of the reversed piece is not altered, but that on account of the 
relation of the graft to the stock, a secondary polarity has been 
assumed. This secondary polarity is that of the stock which 
