CHAP. I. 
CELLULAR TISSUE. 
5 
end, and unquestionably adhering by one of their ends to the 
inside of the sac ; others from polygons passed to a spherical 
form in rounding off their angles. As they grew older, other 
very important changes took place in certain cells of the ordi- 
nary structure, which had not previously undergone any alter- 
ation : in each of these there appeared three or four rings 
placed parallel with each other, adhering to the membrane, 
from which they were distinguished by their opaqueness; 
these were altogether analogous to annular ducts. The cells 
become tubes did not at first differ from other cells in any 
thing except their form ; their sides were uniform, thin, colour- 
less, and transparent ; but they soon began to thicken, to lose 
their transparency, and to be marked all round from end to 
end with two contiguous parallel streaks disposed spirally. 
They then enlarged, and their streaks became slits which cut 
the sides of the tubes from end to end into two threads, whose 
circumvolutions separated into the resemblance of a gun- 
worm.' In these cases there can, I think, be little doubt that 
the changes witnessed by Mirbel were chiefly owing to the 
developement of a spiral thread in the inside of the tissue ; he, 
however, did not consider it in that light. 
But however clearly the origin of the different forins of tissue 
may be shown to be identical, it is obviously important to dis- 
tinguish them for practical purposes. I shall therefore pro- 
ceed henceforward to speak of them as if they were totally 
distinct in their origin. 
Sect. I. Of Cellular Tissue. 
Cellular, Utricular, or Vesicular tissue, generally, con- 
sists of little bladders or vesicles of various figures, adhering 
together in masses. Occasionally it seems to be composed of 
fibre only, unconnected by membrane. It is transparent, and 
in all cases colourless : when it appears otherwise, its colour is 
caused by matter contained within it. 
If a thin slice of the pith of elder, or any other plant, be 
examined with a microscope, it will be found to have a sort of 
honeycomb appearance, as if there were a number of hexagonal 
B 3 
