44
The Body Electric
At first this idea wasn't well received. It was strenuously opposed by
Thomas Hunt Morgan, a respected embryologist at Columbia University
and the first American participant in this saga. Later, when Morgan
found that the results of his own experiments agreed with Boveri's, he
went on to describe chromosome structure in more detail, charting spe-
cific positions, which he called genes, for inherited characteristics. Thus
the science of genetics was born, and Morgan received the Nobel Prize
in 1933. So much for Boveri.
Although Morgan was most famous for his genetics research on fruit
flies, he got his start by studying salamander limb regeneration, about
which he made a crucial observation. He found that the new limb was
preceded by a mass of cells that appeared on the stump and resembled
the unspecialized cell mass of the early embryo. He called this structure
the blastema and later concluded that the problem of how a regenerated
limb formed was identical to the problem of how an embryo developed
from the egg.
Morgan postulated that the chromosomes and genes contained not
only the inheritable characteristics but also the code for cell differentia-
tion. A muscle cell, for example, would be formed when the group of
genes specifying muscle were in action. This insight led directly to our
modern understanding of the process: In the earliest stages of the em-
bryo, every gene on every chromosome is active and available to every
cell. As the organism develops, the cells form three rudimentary tissue
layers—the endoderm, which develops into the glands and viscera; the
mesoderm, which becomes the muscles, bones, and circulatory system;
and the ectoderm, which gives rise to the skin, sense organs, and ner-
vous system. Some of the genes are already being turned off, or re-
pressed, at this stage. As the cells differentiate into mature tissues, only
one specific set of genes stays switched on in each kind. Each set can
make only certain types of messenger ribonucleic acid (mRNA), the "ex-
ecutive secretary" chemical by means of which DNA "instructs" the