152 Barber . — The Structure of P achy thee a. II. 
observed from the two circles, through a pair of ellipses, to the 
black line. It is assumed that the black lines represent the 
double circles in optical section, and therefore that they are 
the transverse walls of the cell-filaments (see Fig. 5). 
For the observation of these particulars section 6 is better 
than section 5. I have drawn a small portion of the clear 
border of the medulla, just inside the zone of oval bodies, and 
obviously formed by the fusion of the tubes in that region. 
And I have taken advantage of the use of the micrometer 
screw to focus all parts of the thickness of the section, so as 
to include as many of the transverse walls as possible. 
In the medulla of section 5 (see Fig. 4), and, better still, in 
the corresponding part of section 6, the central part is filled 
up by a number of sections and pieces of tubes ; and, if we 
examine these tubes carefully, we shall find the transverse 
walls in various positions and at various distances. The 
course of the cell-rows may be readily traced by following 
the black lines and the general direction of the outer tubes. 
It is seen in longitudinal sections of the larger tubes that the 
short black lines occur at regular intervals, and that they only 
stretch a short distance across the diameter of the tubes. Oc- 
casionally there are more than one set of dark lines in one 
and the same tube; and this probably represents the con- 
dition described as anastomosis of tubes in the transverse 
sections (Fig. 2). From the occasional regular distance of 
the black lines, from one another, cells of the filaments may 
be regarded as from ten to twenty times as long as broad. 
The double circles are only found in transversely cut tubes, 
of which they occupy the centres. 
There are traces, here and there, on each side of a trans- 
verse wall, of the adjacent longitudinal walls ; but these would 
probably not be detected had we not a previous suspicion of 
their existence. 
We are therefore entirely dependent, for our knowledge of 
the exact distribution of the algal filaments, upon their 
transverse walls, which are, indeed, the only parts preserved. 
This is peculiarly interesting because of the attention 
