Section 56. Figure IV., Sheet 3 represents, diagrammatically,
embryonic tissue, of which, to begin with, the whole animal
consists. The cells are all living, capable of dividing and similar, but as
development proceeds, they differentiate, some take on one kind of
duty (function), and some another, like boys taking to different
trades on leaving school, and wide differences in structure and
interdependence become apparent.

Section 57. It is convenient to divide tissues into three classes,
though the divisions are by no means clearly marked, nor have they
any scientific value. The first of these comprises tissues composed
wholly, or with the exception of an almost imperceptible cementing
substance, of cells; the second division includes the skeletal tissues,
the tissue of mesentery, and the connective and basement tissue of
most of the organs, tissues which, generally speaking, consist of a
matrix or embedding substance, formed by the cells and outside of
them, as well as the cells themselves; and, thirdly, muscular and
nervous tissue. We shall study the former two in this chapter, and
defer the third division until later.

Section 58. The outer layer of the skin (the epidermis), the inmost
lining of the alimentary canal, the lining of the body cavity, and the
inner linings of blood-vessels, glands, and various ducts constitute our
first division. The general name for such tissues is epithelium. When
the cells are more or less flattened, they form squamous epithelium
(Figure VI.) such as we find lining the inside of a man's cheek (from
which the cells sq.ep. were taken) or covering the mesentery of
various types-- sq.end. are from the mesentery (Section 16) of a frog.
A short cylindroidal form of cell makes up columnar epithelium, seen
typically in the cells covering the villi of the duodenum (Figure V.).
This epithelium of the villi has the outer border curiously striated, and
this is usually spoken of as leading towards "ciliated" epithelium, to
be described immediately. The epithelium of the epididermis is
stratified-- that is to say, has many thicknesses of cells; the deeper
layers are alive and dividing (stratum mucosum), while the more
superficial are increasingly flattened and drier as the surface is
approached (stratum corneum) and are continually being rubbed off
and replaced from below.

Section 59. In the branching air-tubes of the lung, the central canal of
the spinal cord, and in the ureters of