The Embryo at the Wound 41
again, featureless goo from which, slowly and magically, an organism
appeared.
After 1850, biology began to break up into various specialties. Em-
bryology, the study of development, was named and promoted by Dar-
win himself, who hoped (in vain) that it would reveal a precise history of
evolution (phylogeny) recapitulated in the growth process (ontogeny). In
the 1880s, embryology matured as an experimental science under the
leadership of two Germans, Wilhelm Roux and August Weismann.
Roux studied the stages of embryonic growth in a very restricted, mech-
anistic way that revealed itself even in the formal Germanic title, Ent-
wicklungsmechanik ("developmental mechanics"), that he applied to the
whole field. Weismann, however, was more interested in how inheri-
tance passed the instructions for embryonic form from one generation to
the next. One phenomenon—mitosis, or cell division—was basic to
both transactions. No matter how embryos grew and hereditary traits
were transferred, both processes had to be accomplished by cellular ac-
tions.
Although we're taught in high school that Robert Hooke discovered
the cell in 1665, he really discovered that cork was full of microscopic
holes, which he called cells because they looked like little rooms. The
idea that they were the basic structural units of all living things came
from Theodor Schwann, who proposed this cell theory in 1838. How-
ever, even at that late date, he didn't have a clear idea of the origin of
cells. Mitosis was unknown to him, and he wasn't too sure of the dis-
tinction between plants and animals. His theory wasn't fully accepted
until two other German biologists, F. A. Schneider and Otto Butschli,
reintroduced Schwann's concept and described mitosis in 1873.
Observations of embryogenesis soon confirmed its cellular basis. The
fertilized egg was exactly that, a seemingly unstructured single cell.
Embryonic growth occurred when the fertilized egg divided into two
other cells, which promptly divided again. Their progeny then divided,
and so on. As they proliferated, the cells also differentiated; that is, they
began to show specific characteristics of muscle, cartilage, nerve, and so
forth. The creature that resulted obviously had several increasingly com-
plex levels of organization; however, Roux and Weismann had no alter-
native but to concentrate on the lowest one, the cell, and try to imagine
how the inherited material worked at that level.
Weismann
proposed
a
theory
of
"determiners,"
specific
chemical
structures coded for each cell type The fertilized egg contained all the
determiners,
both in type and in number, needed to produce every cell
in the body. As cell division proceeded, the daughter cells each received