history of the fertilized germ-cell, the zygote formed by the union of ovum and spermatozoön. 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. Its "line of descent" produced only germ-cells. The products of division of the other daughter-cell began to differentiate, and soon formed all the necessary kinds of cells to make up the body of the mature worm. In this body, the cells from the first daughter-cell mentioned were inclosed, still undifferentiated: they formed the germ-cells of the next generation, and after maturity were ready to be ejected from the body, and to form new threadworms.
Imagine this process taking place through generation after generation of threadworms, and one will realize that the germ-plasm was passed on directly from one generation to the next; that in each generation it gave rise to body-plasm, but that it did not at any time lose its identity or continuity, a part of