Production of New Hydranths in Hydra 21 
amount of body wall tissue, or pure tentacle tissue, or pure hypo- 
stome tissue (tissue anterior to the tentacles) or a small amount of 
foot tissue, it is absorbed. If it is tissue lying at the base of the 
tentacle, whether it includes a tentacle or not, it becomes part of a 
new hydranth which under its stimulation grows out from thestock. 
The fate of a graft depends (2) on the size of the piece grafted. A 
large piece of body wall tissue with the oral end exposed in a 
lateral graft gives rise to a new hydranth, a small piece is absorbed. 
A large piece from the foot may produce a new hydranth when 
grafted laterally, a small piece is absorbed. ‘The fate of a graft 
depends (3) on the position it occupies in thestock. Ifinthecirclet 
of tentacles, a new hydranth is not produced by the graft of a 
tentacle with peristome tissue at its base, but in any other region 
of the body a new hydranth may be produced. In the foot region 
this hydranth is very minute; in the middle region it is of normal 
size. The fate of a graft depends (4) on its polarity. If a band 
of tissue is grafted by its aboral end to the oral surface of a half- 
hydra, leaving the oral surface of the graft exposed, anormal hydra 
is produced, the tentacles growing on the exposed oral surface. If 
a band of tissue is reversed and grafted by its oral end to the oral 
surface of a half-hydra, leaving exposed the aboral surface of the 
graft, unless sufficiently small, tentacles grow out at the line of 
union of the two components, and not at the free end. 
2. No matter how specialized a tissue has become, it can, 
when grafted, be made over into a different kind of tissue and be 
incorporated into the body of the stock. For example, the mate- 
rial that had become differentiated into tentacle tissue can lose 
its differentiation and become body wall tissue and function as 
such. Likewise, differentiated tissue of the stock can, under the 
influence of special grafts, be made over into other kinds of differ- 
entiated tissue. For example, the foot tissue can, under the influ- 
ence of a grafted tentacle with basal tissue attached, be trans- 
formed into the body wall and tentacle tissue of a new hydranth. 
3. A grafted hydranth and a grafted foot, when not absorbed, 
keep their individuality and do not become one with the stock. 
4. Anew hydranth can be stimulated to grow out from a hydra 
by (1) thegraft of the peristome tissue at the base of the tentacle, with 
