536 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
to be moi'e numerous than elsewhere. They are quite prominent 
because of their large size and their nucleoli. At the exposed part 
of the parenchyma, there are a considerable number of these cells, 
but not enough to indicate their special multiplication at this place 
previous to the fission. The approximation of the dorsal and ventral 
sides sufficiently accounts for the slight massing of parenchyma and 
formative elements at this point. The ectoderm (ec) has been 
broken and shows rough edges and the parenchyma is left exposed. 
This may be compared with any of the figures of the outside at this 
stage (pi. 9, fig. 2; pi. 10, fig. 21; pi. 11, fig. 25). This section 
just passes through the inner side of one gut ramus (^). It is at 
once evident that one could not distinmiish such a section from one 
of a worm which had been artificially cut in two and furthermore 
that there are now no signs of any histological changes which may 
have preceded the fission. The formative cells both dorsally and 
ventrally are actively dividing, as is shown by their numerous mito¬ 
tic figures (/>;), but the amount of division at the exposed tip is no 
more extensive than can often be found in a space of the same size 
anywhere in the dorsal region (pi. 14, fig. 38). In fact the neigh¬ 
boring sections showed more cell division going on in the dorsal and 
ventral regions some distance back of the exposed tip, as is shown 
by the right half of figure 35 (plate 13), than at the tip itself where 
the new ectoderm is to form. 
Flexener (’98), in studying the histological changes occurring in 
the regeneration after an artificial cut similar to this normal one of 
fission, says (p. 340), “At the end of the first twelve hours and 
about equally at the conclusion of eighteen hours, active cell proli¬ 
feration in the divided end is going on. The evidences for this are 
found in the rich mitosis encountered as well as in the accumulation 
of small immature cells at the injured extremity. The most active 
division is found in the tissues immediately adjacent to the epider¬ 
mal.At the end of twenty-four hours cell division in the regen¬ 
erating end is quite over. Long and painstaking search is required 
to discover a single karyokinetic figure.” 
The artificial cutting seems therefore to have stimulated in Flex- 
ener’s maculates ^ a special production of new cells near the cut 
’ The worm which Elexener calls Planaria torva and which is the subject of 
his paper, came from the localities where I have collected my specimens and is 
incorrectly called torva, as I have identified it as P. maculata. 
