different type.

This is a very important principle, and it contains so many lessons for
us bearing on the philosophy of life in general that it may be allowable
to establish this fact by several somewhat lengthy quotations from
standard authorities.

The first will be from one of the highest authorities on embryology,
Charles Sedgwick Minot, of Harvard:

"In accordance with this law [of differentiation] we encounter no
instances, _either in normal or pathological development_, of the
transformation of a cell of one kind of tissue into a cell of another
kind of tissue; and further we encounter no instances of a
differentiated cell being transformed back into an undifferentiated cell
of the embryonic type with varied potentialities."[12]

Again, we have the following from one of the foremost pathologists, as
to the strict and rather narrow limits of even pathologic change:

"Epithelium and gland cells ... never become converted 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]