522 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
During the collection at different times of over three thousand 
worms for my experiments, I was constantly on the watch for speci¬ 
mens with some furrow or constriction at the place of division and 
I isolated many specimens which I fancied were thus marked. These 
invariably failed to show any furrow when they were fully expanded 
and could be more carefully observed. Some of them divided sev¬ 
eral days later, but the absence of any furrow had been previously 
determined by several close examinations. 
The examination of a large number of whole worms at the season 
when the fission is at its height did not show any with signs of new 
organs developing as a forerunner of fission. Serial sections of tail 
pieces killed soon after the division also show no trace of the new 
pharynx, or brain, nor any change in the gut.- The making of the 
new organs and rearrangement of the old are hardly apparent exter¬ 
nally until the second or third day after the fission occurs, although 
within twenty-four hours the parenchyma cells at the anterior end 
cover the naked scar left by the fission with a delicate epithelium 
and form a small cap of new tissue which increases in size from day 
to day (pi. 9, figs. 3, 12, 13). 
These facts seem to show that the worm when it is ready for 
division quickly pinches itself in two at a definite place behind the 
pharynx and explain vvhy, out of one hundred and eighty-six divi¬ 
sions which occurred in the laboratory, I saw the process in only 
one case though many individual specimens were examined at fre¬ 
quent intervals for days at a time. 
As I can discover no histological changes at the place of fission 
in the parenchyma or other tissues of the whole worms which I 
have sectioned, I believe that the division is accomplished by a con¬ 
striction of the circular muscles which can take place only when the 
animal is of suitable size and proportions for division, and in this 
latter respect differs from what is found in some of the land 
planarians (von Graff, ’99; Bergendal, ’87), which will almost 
always divide if sufficiently irritated. 
Wilson (:00) has recently explained the fragmentation of a 
Nemertean {^Cerehratulus lacteus) as due to a violent muscular con¬ 
traction not preceded by any discoverable histological processes 
which can aid the separation. The Nemertean fragmentation may 
be quickly brought about by almost any stimulus which irritates the 
animal, and may cut through opposite any one of the gut lobes. The 
