
HISTOLOGICAL CHANGES IN HYDRA VIRIDIS 
DURING REGENERATION. 
HANNAH TERESA ROWLEY. 
ALTHOUGH a great deal of work has been done on the regen- 
eration of hydra, no one has as yet attempted to make out the 
histological changes that take place. This point would seem 
to be one of special interest, since the old piece appears to 
change its form as a whole to produce a new.animal. The 
principal question to which I wished to find an answer was 
whether, during the period of regeneration, the old cells go 
over without change into the tissue of the new animal, or 
whether new cells are formed, and if so, in what part or parts. 
Green hydras were used almost exclusively, since they regen- 
erate more readily and with fewer abnormalities than does the 
brown species. Hydras of various lengths were taken, the 
foot end and circle of tentacles were cut away, and the remain- 
ing middle parts of the body wall were each divided by cross 
cuts into four——in some cases two—small rings. These 
were allowed to regenerate, and at different periods, ranging 
from immediately after the cutting to some days after the 
formation of new tentacles and foot, were killed in a solution 
of corrosive acetic, hardened, sectioned lengthwise, and stained 
in Delafield haematoxylin. In some cases the slides were 
dipped for a moment into a weak solution of picric acid in 
absolute alcohol, in order to differentiate the endodermal 
‘tissue. 
Observation of the living piece showed that the ends became 
‘rounded, closing in from fifteen to sixty minutes after having 
been cut ; that the piece then remained without permanent 
‘change: of form for thirty or forty hours, although at intervals 
it might change its shape by expanding and contracting. At 
about this time small knoblike outgrowths, the tentacles, began 
to appear. at one end, the other end fixed itself to the nearest 
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