Barber . — The Structure of P achy theca. II. 151 
Returning now to the smaller tubes in section 4, it is notice- 
able that some are much more distinct than others. There 
are all transitions between distinct double circles and an in- 
definite brownish core. This I regard as suggesting that the 
clear double circles are not merely transverse sections of the 
cell-tubes, but that they are the transverse walls of the cell- 
rows. 
It is frequently found that, where the central portion of 
a larger tube is indistinct, a double circle is brought into view 
by focussing deep into the thickness of the section. In the 
outer obliquely cut part of the preparation there are more of 
these double circles present, and they frequently present the 
flattened or oval shape which would be expected in transverse 
walls of tubular cells viewed obliquely. 
Slides 5 and 6 are similar to one another as regards pre- 
servation. The sections are rather obliquely cut, but they 
nevertheless include a considerable portion of the medulla. 
In No. 5 (Fig. 4) the medulla consists of portions of tubes 
cut in all directions — transversely, obliquely, longitudinally. 
Towards the zone of oval bodies these fragments are fused 
together to form a more or less coherent light yellow mass, 
which will presently receive a closer attention. From this 
mass the radiating tubes arise. The latter, however, owing 
to the slight obliqueness of the section, form a clear anasto- 
mosis between numerous oval bodies, instead of being straight 
radiating tubes (as in Fig. 1). The tubes, in all parts of the 
section, have a clear, yellow centre, and a darker, brown 
4 wall 5 similar to that in section 4. The spaces between the 
tubes are unoccupied by any mineral, excepting the occasional 
presence of the dark brown substance hitherto met with (Fig. 
a a). 
Upon carefully examining the outer, fused portion of the 
medulla, two evidences of structure may be detected. There 
are, in the first place, numerous ‘ double circles ’ similar to those 
of section 4. There are also short black lines of surprising 
sharpness, equal in length to the greater diameter of the 
double circles ; and, with care, all stages of transition may be 
