The formation of the fibres of areolar tissue from cells, having been typified already in the second class, it follows that organization, 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 unorganized 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, therefore, 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: independent 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
