Let us recall in outline the early history
of the fertilized germ-cell, the _zygote_ formed by the union of ovum
and spermatozooen. These two unite to form a single cell, which is
essentially the same, physiologically, as other germ-cells. It divides
in two similar cells; these each divide; the resulting cells again
divide, and so the process continues, until the whole body--a fully
developed man,--has been produced by division and redivision of the one
zygote.
But the germ-cell is obviously different from most of the cells that
make up the finished product, the body. The latter are highly
differentiated and specialized for different functions--blood cells,
nerve cells, bone cells, muscle cells, and so on, each a single cell but
each adapted to do a certain work, for which the original,
undifferentiated germ-cell was wholly unfit. It is evident that
differentiation began to take place at some point in the series of
divisions, that is to say, in the development of the embryo.
Th. Boveri, studying the development of a threadworm, made the
interesting discovery that this differentiation began at the first
division. Of the two daughter-cells produced from the zygote, one
continued dividing at a very slow rate, and without showing any
specialization.
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