534 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
the individual cells as such, be their origin early from the embryonic 
cells or later from the parenchyma, are the histological basis for 
regeneration in the Rhabdocoeles and Triclads which have been 
carefully studied and that these studies have extended over a suffi¬ 
cient number of forms to show their general occurrence. 
In my description of the parenchyma of P. maculata, I have 
heretofore referred only to what is found in worms from a locality 
where there is practically no asexual reproduction. In localities 
where normal regeneration is going on as a result of the normal 
amputation occurring in fission, the condition of the ‘‘Bildungs- 
zellen ’’ is of particular interest. The number of these “ Stamm 
or “ formative ” cells, as I shall call them, using a translation of 
Keller’s term, is so enormously increased in the dorsal region of the 
body (pi. 14, fig. 38, els) above the gut that they are far more 
numerous than any of the other mesodermal elements. There 
seem to be more of them throughout the body than are found in 
worms from localities where fission is not occurring, but this would 
probably not have attracted attention were it not for the very great 
number in the dorsal region (pi. 14, fig. 38, els). These cells both 
dorsally and ventrally and in all parts of the body in all the speci¬ 
mens during the entire season of normal fission are at all times 
dividing very rapidly, as shown by the mitotic figures which are 
everywhere abundant (pi. 14, fig. 38 ; pi. 13, fig. 35, k). It is al¬ 
ways the large individual cells which are dividing, and never the 
smaller nuclei of the parenchyma proper. The daughter cells 
resulting from these divisions, have nuclei exactly like those of the 
parenchyma in size and appearance, but have their own cytoplasm, 
though this is of very small amount. These are the only cells in 
which mitotic figures have been discovered. Cell division may 
occur in the endoderm, but I have never seen any indications of it 
though I have searched through many sections. The same result 
came from the examination of the ectoderm, though here the 
rhabdites greatly obscure the nuclei and, as noted above, the small 
nuclei of the parenchyma proper have never been found dividing. 
This makes it evident that it is the formative cells which are 
increasing far more than any others at the time when the worms 
are making good the parts lost by fission and growing so fast that 
they frequently double in all their proportions in twelve days 
(compare pi. 10, fig. 19, and pi. 11, fig. 32). In worms taken at the 
