have one feature in common: they are cells, each equivalent to the
amoeba, that have taken on special duties-- in a word, they are
specialists. The amoeba is Jack of all trades and a free lance; the
protective epidermal cell, the current-making ciliated cell, the bile or
urea-making secretory cell, is master of one trade, and a soldier in a
vast and wonderfully organized host. We will now consider our second
kind of cell in this organization, the cell of which the especial aim is
the building round it of a tissue.