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ZOOLOGY: L. R. CARY 
afforded by the Rhizostomous medusa Cassiopea xamachana the disks 
of which can be separated from the oral arms and kept in dishes of sea 
water for an indefinite period. Such disks retain their capacity for 
regeneration, while on account of their shape and the location of the 
marginal sense-organs they permit of many types of operation which 
are possible only on some simple organism. 
In order to determine the influence of the nervous system on the rate 
of regeneration the following experiments were carried out. Pairs of 
entire disks of the same size were compared from one of which all of the 
rhopalia were removed while from the other an equal amount of tissue 
was removed from the bell margin between the rhopalia. In these, as 
in all of the following experiments, the amount of regeneration was 
measured inward from the edge of a hole in the center of the disk from 
which a circular piece of tissue had been removed. 
The active member of any pair of such disks, being compared, regen- 
erated faster in 75% of the experiments. In 10% of the experiments 
the two disks regenerated at an equal rate, while in the remaining 15% 
the inactive member of the pair showed the higher rate of regeneration, 
thus confirming the observations of Zeleny ('07) and Stockard ('09) 
that under such conditions sometimes the active and sometimes the 
inactive member of any pair of disks regenerates faster. 
The marked divergency among the results of this series of experi- 
ments indicated that there must be individual physiological differences 
among the medusa disks of sufficient amount to invalidate the conclu- 
sions drawn from the comparison of entire disks. In all of the later 
experiments, therefore, the two halves of one and the same disk were 
compared with one another. This comparison is made possible through 
the fact that in Cassiopea the muscles and nerve fibres are confined to 
the subumbrella surface of the disk so that when two narrow strips of 
the subumbrella ectoderm, on opposite sides of the disk, are removed 
the two halves are insulated from one another until a new layer of 
tissue containing nerve fibres has been regenerated over one of the 
denuded areas. About thirty-six hours would elapse before functional 
nerve fibres would be regenerated, so by scraping over the strips each 
day the insulation of the halves of a disk was readily maintained. 
In the second type of experiments the two halves of a disk were 
insulated in the manner just described; the sense-organs were removed 
from one half while an equal amount of tissue was removed from between 
the sense-organs of the other half. The disks were then allowed to 
regenerate in jars of normal sea water, the insulation between the 
halves being maintained throughout the experiment. The physiologi- 
