Physiology of Regeneration 597 
II RATE OF POSTERIOR REGENERATION 
This marine polychzte (Podarke obscura) is composed usually 
of 40 to 50 segments. ‘The dorsal surface is, as a rule, covered 
by a chitinoid layer, which varies in color from very dark brown 
to a light shade of yellow. Ina few exceptional cases the worms 
are devoid of this covering, appearing then to be “naked,” and 
are quite transparent. 
The worms possess the power of posterior regeneration only; 
as far as could be ascertained, they never regenerate a new head. 
Normal and perfect regeneration will take place from an exposed 
surface produced by the animal itself, which is able through the 
contraction of the dermal muscles to detach a larger or smaller 
number of its posterior segments. Such a condition can be very 
easily reproduced artificially in the following way. The animal's 
back 1s pressed down gently with the flat blade of a scalpel near 
the region where the separation is desired. In response to the 
mechanical stimulation the worm will attempt to crawl away, 
but since a portion of its body is held down by the scalpel, it will 
not be able to do so. A vigorous contraction will then occur result- 
ing in a constricting off of the free portion from the rest of the 
worm’s body. Bythis method it is possible to cause even single seg- 
ments. to be pinched off very readily, and the experimenter is thus 
enabled to control the number of segments in the piece retained 
for the experiment. The wound formed through such a reflex 
contraction of the musculature will soon close over, and the re- 
generation following upon such an operation is invariably perfect. 
In those cases, however, where the cut has been made with a 
knife, and the mutilated segment has not been thrown off by the 
animal afterward, as often happens, the new tail will show 
abnormalities either in size or the direction of its growth, or in 
both. 
As mentioned above, it is possible to remove as many segments 
as desirable with a great degree of accuracy. Besides, Podarke 
also furnishes the opportunity of keeping an exact record not only 
of the number of segments removed, but also of those remaining 
in the old piece, as well as of the regenerated segments, since the 
