e organism where the process of utilizing the
body-fluids, etc., follows the same course as in nature, takes on the
character of grafting rather than of cultivating in a culture medium.
As to causing the external growth, it was ten years later before it
seems first to have succeeded. In 1907 Harrison, from Johns Hopkins
University, furnished details of his research in such form as to be
convincing. But his work had reference to the growth of tissues only of
coldblooded animals, he having cultivated artificially, nerve fibers
from the central nervous system of the frog.

Carrel's work consisted in extending Harrison's method to apply to
warm-blooded animals, including, of course, mammals; he having
primarily in view at this time a more precise knowledge of the laws
governing the restoration of tissues, for example, after serious
surgical wounds. He and his assistant worked steadily to this end, and
succeeded. The tissues of the higher animals, including man, can now be
developed in a culture, and such development can be made to correspond
to a rigidly precise technique. The feat is accomplished by putting
minute pieces of living tissue into a plasmatic (blood) medium which
will coagulate. So complicated is this apparently simple matter in its
application that only the most exquisite surgical skill is proof
against incalculable modifications in results.

Having obtained evidence that tissue can be cultivated in accordance
with a formula that may be relied upon to give definite results, the
effort was made to grow artificially the various malignant (cancerous)
tissues, in turn, of chicken, rat, dog, and human being. Cancerous
tissue invariably developed cancer, and so rapidly and extensively that
the growth could be observed with the naked eye.

It now became evident that, under the right circumstances, the
artificial growth of tissues could be utilized in the study of many
problems; such as malignant growth of tissue; certain problems in
immunity, as, for example, the production of antitoxins of certain
organisms; the regulation of the growth of the organism, or of
different parts of the organism; rejuvenation and senility; and the
character of the internal secretions of the glands, such as the thyroid
which plays a role most important in physical and mental development.
The difficulty lay in the fact that the artificial growth was so very
short-lived. It was found that by passing the growth into a new medium,
and repe