
582 THE AMERICAN NATURALIST. [Vor. XXXVI. 
tissue of the regenerating piece after the normal form had been 
attained, and the tissue of normal or growing hydra is of 
importance in a consideration of regeneration in this form. 
I shall now attempt to make a conservative statement of the 
amount of dividing tissue in regenerating hydra, based on 
undoubted cell division. 
There is very little evidence of the formation of new tissue 
during the early stages; an exception was noted in one piece 
killed five hours after the cutting, in which a number of divi- 
sions were observed in the ectoderm. Some few divisions can 
be observed before twenty hours, but from forty hours on, — 
the time of tentacle formation, — the division is more active 
and the number of dividing cells, especially neuro-muscular 
cells, is considerable. Division continues after the tentacles 
have begun. Spindles were seen in endodermal cells, and in 
neuro-muscular and interstitial cells, These divisions often 
occur in groups, especially in the neuro-muscular cells. In 
one case as many as five dividing nuclei of neuro-muscular 
cells were observed at one spot in a section cut from the side 
of the piece and so affording a surface view of ectoderm tissue 
alone. Fewer divisions were seen in the endoderm than in the 
ectoderm, partly, perhaps, because of the smaller number of 
cells in the endoderm. 
Unlike the majority of cases of regeneration, however, the 
new cells are not formed at the cut surface alone, and the ten- 
tacles do not seem to be regenerated solely from new tissue. 
Divisions were found to occur as well at the sides as at the 
ends of the regenerating pieces, and in almost, if not quite, as 
great number. The first appearance of the tentacle, which in 
the living animal seems to be brought about by an outpushing 
of the tissue of the original piece, appeared under the micro- 
Scope to be due very often to contraction in this region, 
causing a very slight protuberance of ectoderm, which could 
be fully accounted for by an outpushing of old material with- 
out the formation of new tissue. Divisions were sometimes 
seen at this place, sometimes not. Very frequently, however, 
in later stages, when the tentacle had grown to some length, 
there was seen to be a group of neuro-muscular cells dividing 

