ed into bone or
cartilage, or vice versa; while, again, it may be laid down that among
epiblastic and hypoblastic tissues, on the one hand, and mesoblastic
tissues on the other, there is no new development or _metaplasia_ of the
most highly specialized tissues from less specialized tissues; a simple
epithelium cannot in the vertebrate give rise to more complex glandular
tissue, or to nerve cells; in regeneration of epithelium there is no new
formation of hair roots or cutaneous glands. The cells of white fibrous
connective tissue have not been seen to form striated or even
non-striated muscle."[13]

[Footnote 12: _Science_, March 29, 1901, p. 490.]

[Footnote 13: J.G. Adami, "Principles of Pathology," pp. 641-642.]

As implied by these quotations, a constant and progressive
differentiation of cells prevails in the developing embryo; and when
complete, certain groups of cells act as specialists in doing only
certain kinds of work for the body. These cells maintain their specific
characters in a very remarkable degree under normal conditions. Under
various abnormal conditions, however, these cells may become modified as
to functions, so that cells or tissues of one type may assume more or
less completely the characters of another type. "But," as a very high
authority declares, "the limitations in this change in type are strictly
drawn, so that one type can assume only the characters of another which
is closely related to it. This change of one form of closely related
tissue into another is called _metaplasia_....

"When differentiation has advanced so that such distinct types of tissue
have been formed as connective tissue, epithelium, muscle, nerve, _these
do not again merge through metaplasia. There is no evidence that
mesoblastic tissues can be converted into those of the epiblastic or
hypoblastic type, or vice versa_."[14]

[Footnote 14: Delafield and Prudden, "Text-Book of Pathology," pp. 62,
63.]

This modification of function among the cells which sometimes goes on in
the developing embryo, or under pathologic conditions, is very closely
analogous to the variation which goes on among species of animals and
plants. But, as we shall see later, there is a well marked limit to this
variation among species, just as we see there is in the variations among
the cells. Practically the same general laws hold good in each case.

If cells did not maintain their ancestral characters in a very
remarkable way, what would