I 
CURTIS: LIFE HISTORY OF PLANARIA MACULATA. 537 
\ 
end during the first twenty-four hours. There is certainly no very 
well marked period of extremely active cell division after normal 
fission, and this I account for by the fact that an asexually reprodu¬ 
cing jP. maculata always has an excess of the formative cells in all 
parts of the parenchyma and particularlj^ along the dorsal side, and 
that there is no need for any marked increase of cells at the exposed 
end nor would there be if the worm divided itself at anv other 
%■ 
point. There is perhaps a slight increase in the amount of cell divi¬ 
sion among the formative cells at the exposed tip, but not enough 
to be readily noticed. It is the same process which Flexener 
observed, only the animal is prepared for its fission by the increased 
number of formative cells. These cells are alwavs dividing in all 
parts of the body no matter what stage of regeneration is examined. 
Their increase would seem sufticient to account for the continual 
increase in bulk of the whole body and they are present at the nor¬ 
mally cut surface ready to make good the loss. I have often won¬ 
dered if there were not some actual mio-ration of the laro’e formative 
O O 
cells from the more distant portion (see the right side of fig. 35, pi. 13) 
toward the exposed end for they seem to lie free in the parenchyma 
jjroper and are dividing more actively a short distance behind than 
just at the cut surface, where in the third and fourth days (pi. 14, 
fig. 37; pi. 15, fig. there is more increase in the number of small 
nuclei than seems to be accounted for by the division at the tip end 
itself. 
On the second day (pi. 13, fig. 36) the outermost of the large 
formative cells have begun to make the new ectoderm (ec') and back 
of this there is an increasing bulk of syncytial parenchyma with 
small, close set nuclei. As only the large cells divide, and their 
reconstructing nuclei are exactly like these, this collection of nuclei 
has presumably originated from the division, and so reduction in size, 
of the large cells. A considerable number of large cells still remain 
near the tip, and mitotic figures are to be found among them. On 
following the section posteriorly^, the formative cells are as numer¬ 
ous as ever and their divisions as frequent, though this does not 
happen to appear in the section drawn (pi. 13, fig. 36). 
There is no change in the old ectoderm (ec) except possibly the 
loss of some cells at the edge between the new tissue and the old. 
At the exposed surface, the arrangement of the larger formative 
cells shows at once that they are producing the new ectoderm (ec'). 
