ELASTIC TISSUE. 129 
The formation of the fibres of areolar tissue from cells, having 
been typified already in the second class, it follows that orga- 
nization, or the presence of blood-vessels, does not establish 
any essential difference in the growth of the elementary par- 
ticles ; for this class belongs to the perfectly organized tissues, 
and areolar tissue is highly vascular. The unorganized tissues 
were formerly said to grow by apposition, and the organized 
by intussusception. We have already discussed this distinction 
at page 95. It is so far correct, that the young cells of unorga- 
nized tissues are not formed throughout the entire thickness of 
the tissue, but only in the neighbourhood of that surface, on 
which they are in contact with vascular substance, and where 
they therefore obtain the freshest cytoblastema. But if this 
distinction between the surface and parenchyma of the tissue 
be not present, in consequence of the blood-vessels being dis- 
tributed throughout its whole thickness, the young cells are 
then also generated in every part of the tissue ; and such is 
the case with areolar tissue. The primary distinction, there- 
fore, merely consists in the absence or presence of vessels, the 
difference in the place of formation of the new cells being but 
a secondary distinction. The elementary particles grow in both 
instances and by the same powers. We shall see hereafter how 
far the presence of vessels facilitates certain processes which 
occur during growth. The essential phenomena of growth, 
and, therefore, also the fundamental powers called into activity 
by it, are similar in both. But why a formation of vessels 
should take place in areolar tissue and not in epithelium, is a 
question for future discussion. 
CLASS V. 
Tissues, generated from cells, the walls and cavities of which 
coalesce together. 
The following is the type of formation in this class: inde- 
pendent cells, by which we mean such as have a special wall and 
cavity, are present in the first instance; these we shall call 
primary cells. They are either round or cylindrical, or of a 
stellate figure. When round or cylindrical, the primary 
cells are applied together in rows, the contiguous portions of 
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