STRUCTURE OF NEPHRIDIA OF THE MEDICINAL LEECH. 297 
In the anterior limb of the main lobe the recurrent duct 
runs parallel with the central duct, and is in this part of its 
course surrounded by cells similar to those which surround 
the central duct adjacent to it. 
All the cells of the main lohe have that form of ductule 
which I distinguished above as arborescent with some of the 
branches continuous (^. e. leading into branches of the duc- 
tule of a neighbouring cell) and some of them ccecal. The 
best general notion of the arrangement of the ductules and 
their continuity in the neighbouring cells is given in the 
upper part of fig. 13. In fig. 10, from a different portion of 
the main lobe, the arborescent ductules are seen to be 
limited to one side of the cell, and a tendency to form larger 
ductules is exhibited. The finest arborescence is that shown 
in the cell drawn in fig. 5. This is from a particular por- 
tion of the main lobe where all the cells have the same 
character. Five continuous branches of the ductule are 
seen, and a dichotomous series of finer branches which 
completely honeycomb the cell-protoplasm. The ultimate 
branches of this system are 4-oio-o inch in diameter. They are 
so disposed as to open into or (to use another metaphor) to 
take their origin in the cortical substance of the cell. The 
rod-like striated structure of this layer has already been 
mentioned as a general character of the nephridial cells. It 
appears that t he ultimate ramifications of the ductules have 
a definite relation to the bacillary structure of the cortical 
substance — although the bacillary structure is observed 
equally in cells which are provided with a simple in place 
of a finely arborescent ductule. 
The cells of the apical lobe differ from those of the main 
lobe in possessing very much larger ductules, varying from 
-o'oo- inch in diameter to as much as 3-^ inch, -whilst the cells 
themselves vary from tto- ii^ch in diameter (figs. 
3 and 6, and lower part of fig. 13, A. L.). The medium- 
sized and the largest ductules of this larger sort have a defi- 
nite position in the gland, the medium-sized gradually pass- 
ing into the largest as we proceed along the apical lobe fro?n 
the apex towards the posterior limb of the main lobe, where 
that middle piece with the recurrent duct is given off (see 
fig. 1). Here the ductules become very wide, and are un- 
branched, so that the cells are mere hollow cylinders. A 
good view of the relation of wide-branched ductules to the 
ceils is given in the drawing (fig. 3), which represents cells 
of the apical lobe in a state of distension. The ductules of 
the apical lobe are also well seen in fig. 2, a. l. 
The ductules of this wider and larger kind arc lined with 
