634 THE AMERICAN NATURALIST. [Vor. XXXVI. 
The greater part of the material for my experiments was 
collected from ponds near Woods Hole during the months of 
June, July, and August, and was preserved for study in the 
following winter. Later in the year more planarians were 
found near Bryn Mawr, and were kept in aquaria until needed. 
Each worm was cut into three pieces, of which the middle, con- 
taining the pharynx, was rejected, and the anterior and posterior 
pieces were allowed to regenerate. The latter were killed at 
intervals of twelve hours for about eight days, at the end of 
which time those remaining had begun to feed. Where it 
seemed advisable, series were killed at closer intervals, but 
as a rule this was not necessary. The pieces remained in cor- 
rosive acetic for about ten minutes; after cutting they were 
stained on the slide in Delafield’s haematoxylin, and then 
dipped for a few seconds into a strong solution of aniline 
orange. The latter differentiates the endoderm, muscle bands, 
yolk cells, and the lining of the reproductive system. 
Soon after the worms are cut, the edges of the cut-end 
draw together and matter from the digestive tract and loose 
parenchyma tissue collect about the injured region. Within 
a few hours the surface opens out again, until only a slight 
depression marked by a dark line of pigment is noticeable. 
In the mean time parenchyma cells collect at the cut surface 
and form a narrow sheet across it, while cells from the margin 
of the uninjured epithelium push out and cover the new growth. 
This overgrowth of epithelium is comparatively rapid, as the 
cells in this region flatten out and cover individually a much 
larger area. By the end of the’ first day the tip has become 
rounded and the anterior end is covered with an epithelial 
layer; later, with the increase in their number, the flattened 
cells resume their columnar form. 
The parenchyma of the normal planarian appears on exami- 
nation as an ill-defined mass filling in the spaces between the 
organs. Its protoplasm stains very slightly, but the tissue is 
conspicuous owing to the many deeply colored nuclei that are 
present in it. These nuclei are occasionally seen in small 
groups, but they are usually scattered and separated from each 
other by the loose spongy tissue. Among the parenchyma 

