Vou. 2] Torrey —Hydroid Differentiation and Senescence. 331 
distal type is therefore conspicuous so far as the pedicels are 
concerned. 
With respect to the stalks, it may be said that they begin 
always with one or two short segments characteristic of the 
earliest formed portion of the normal stem. ‘These segments 
may even be shorter than the parent segment next the wound. 
It would seem that in this particular the regenerating stem de- 
velops according to the embryonic type. But the duration of 
this type of development is so short, lasting through the forma- 
tion of one or two segments only, that it closely resembles what 
has already been seen in the figures of regular regenerations 
in the lower regions of the hydranth-bearing zone, viz., that 
the new structures are almost invariably initiated by the for- 
mation of an annulus or part of one. 
General considerations. In seeking an explanation which 
shall simplify as well as summarize the results presented in the 
foregoing paragraphs, I think we must pass by any hypothesis 
which rests solely upon a basis of morphological determinants. 
That regeneration at a given level may not reproduce the struc- 
tural type characteristic of that level, while it does reproduce 
the type characteristic of a later level of the stem is a fact that 
is hardly simplified by the assumption of a residual germ plasm. 
So, too, does it seem improbable that the structural type is 
the result of a functional balance between an organism possess- 
ing an unmodified regenerative capacity and the conditions sur- 
rounding it. This view would necessitate a change in the envi- 
ronment between the time a polyp first appears and the time it 
is regenerated in a somewhat different form. But no such 
change is evident. The polyps half way up a stem were sub- 
jected during their development to external conditions essen- 
tially identical with those which surrounded the developing indi- 
viduals distal to yet differing from them. 
The facts, however, appear to give strong support to the 
view that the stem, instead of retaining unmodified its regener- 
ative capacity, actually loses with age its ability to produce 
structures which formerly characterized it; and that this is 
owing to a modification of conditions within the organism, which 
