532 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
de petits espaces laounaires, ainsi qu’on le sait deja. Mais a c6t4 
de ces cellules a prolongemeuts, il ii’est pas rare d’en reiicontrer 
d’autres, qui en sent depourvues, revetant une forme plus ou moins 
roiide, et poss4dant un noyau avec nucleole bien visible; ce dernier, 
du reste, se trouve dans toutes les cellules parencbymateuses.” Far¬ 
ther on he says that the cellules sans prolongenients ” are found in 
greatest abundance at the peripheral parts of the body. Chichkoff, 
therefore, sharply separates the cells witliout branching processes 
from the cells of the parenchyma proper and says nothing of any 
intermediate stages. 
F. von Wagner (’90) describes in the parenchyma of Rhabdo- 
coeles what he calls “ Bildungszellen,’’ which he says are parenchy¬ 
matous in origin and which are the starting points of all the new 
'Oro:ans in reo:eneration. From his own work and that of others he 
makes the statement (p. 385), that, “bei den Turbellarien die Regen- 
erationen v.om Parenchym (Mesoderm) ihren Ausgang nehmen, das 
Regeneration svermogen dieser Thiere somit an die Bilduhgsfahig- 
keit diqses letzteren gebunden erscheint.” That is, he goes back to 
the parenchyma as the source of his Bildiingszellen and hence of all 
the rest and what he calls Bildungszellen are homologous with the 
imverdstelte Bindegeioebszellen of Jjima, the cellules sans prolonge- 
m.ents of Chichkoff, and the large cells which I have described in 
the parench^^ma of B. maculata. 
That the parenchyma is the tissue from which the new parts come 
is, I think, agreed by all who have studied the histological details of 
regeneration in planarians (von Wagner, ’90; Keller, ’94; van 
Duyne, ’96; Flexener, ’98; Bardeen, :01). 
Keller (’94), in his stud^q wliich is i^rincipally upon Rhabdocoeles, 
although he examined Triclads for comparison, has gone a stej) 
farther than von Wagner felt justified in doing, and instead of 
deriving the “ Bildungszellen ” from the parenchyma, he believes 
that the unbranched parenchyma cells, which von Wagner called 
^^Bildungszellen” and of parenchymatous origin, are almost like a 
fourth germ layer which remains undifferentiated and from which 
most of the adult body originally came. He interprets the transi¬ 
tional stages between these “Bildungszellen” or “ Stammzellen,” as 
he calls them, and the branched cells of the parenchyma reticu¬ 
lum as the origin of parenchyma from these cells. He explains 
his theory by a diagram which represents the development of a 
