530 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
body and at the ends there is a closer resemblance to the condition 
of the parenchyma in an embryo, for here the nuclei are more 
closely set and there is so little cytoplasm between them that large 
vacuoles or clefts are absent (pi. 15, tig. 4ii). 
In P. macnlata^ the adult parenchyma has much more the appear¬ 
ance of a syncytium which has its cytoplasm very greatly vacuo¬ 
lated, than of a large number of multipolar cells which are connected 
by their branching processes. The latter, however, is evidently the 
structure in some forms, for Krsmanovic (’98, p. 188) says, “Das 
Mesenchym ist auch bei meinen beiden Formen von reticularem 
Ban, wie dies die Fig. 9 (me) deutlich zeigt. Die Auslaufer der Zel- 
len verbinden sich zum Theil unter einander, zurn Theil umspinnen 
und umgreifen sie in Form von Fasern und Flatten.” 
Ijima (’84) also described the parenchyma of Planaria polycliroa 
as made up of anastomosing cells. 
AVhether the parenchyma is a vacuolated syncytium or is made up 
of connecting multipolar cells, is of course of little importance, for 
there may be differences in the parenchyma of different species of 
Planaria such as Bohmig (’90) describes in the Rhabdocoeles. I 
have also observed that fixation with plain corrosive sublimate 
makes the vacuoles more irregular and gives an appearance v^ry 
much like Ijima’s (’84) description, while such a fluid as Zenker’s 
gives the more regular appearance shown in my figures with evenly 
rounded vacuoles. 
In the foregoing description I have mentioned as belonging to 
the parenchyma the cells of a syncytial mass. In addition to these 
there are scattered through the syncytium other cells of a distinctly 
different type which have a large nucleus and a nucleolus which is 
surrounded by a clear space (pi. 16, fig. 43; pi. 15, fig. 41, s). 
The outline of the cell cytoplasm is distinct and, if not sharply spin¬ 
dle-shaped, usually extends out much more from either end than 
from the sides of the oval nucleus. While these individual cells 
are of a type distinctly different from the fused cells of the syncy¬ 
tium, there are others which seem (pi. 16, fig. 43 ; pi. 15, fig. 41) inter¬ 
mediate between the two and it seems a fair presumption that there is 
a transition from one to the other, but it is not so clear in which 
order the series can be interpreted. 
There is nothing particularly new in the above description, but I 
give it in detail and also the review which follows in order that 
