When a new individual develops, a certain portion of the germ-plasm of the egg is set aside and remains unchanged in structure.  This, increasing in quantity, forms the reproductive elements for the next generation.  The germ-plasm, which does not form the whole of each reproductive element, but only a part of the nucleus, is thus an exceedingly stable substance.  And there is a just as real continuity of germ-plasm through successive generations of volvox, or of any higher plants or animals, as in successive generations of protozoa.

In certain plants there is an underground stem or rootstock, which grows perennially, and each year produces a plant from a bud at its end.  This underground rootstock would represent the continuous germ-plasm of successive generations; the plants which yearly arise from it would represent the successive generations of adult individuals, composed mainly of somatoplasm.  Or we may imagine a long chain, with a pendant attached to each tenth or one-hundredth link.  The links of the chain would represent the series of generations of germ-cells; the pendants, the adults of successive generations.

But any leaf of begonia can be made to develop into a new plant, giving rise to germ-cells.  Here there must be scattered through the leaves of the plant small portions of germ-plasm, which generally remain dormant, and only under special conditions increase and give rise to germ-cells.

A large part of the germ-plasm of the fertilized egg is used to give rise to the somatoplasm composing the different systems of the embryo and adult.  Weismann’s explanation of this change of germ-plasm into somatoplasm is very ingenious, and depends upon his theory of the structure of the germ-plasm; and this latter theory forms the basis of his theory of evolution.  It would take too long to state his theory of the structure of germ-plasm, but an illustration may present fairly clear all that is of special importance to us.

The molecules of germ-plasm are grouped in units, and these in an ascending series of units of continually increasing complexity, until at last we find the highest unit represented in the nucleus of the germ-cell.  This grouping of molecules in units of increasing complexity is like the grouping of the men of an army in companies, regiments, brigades, divisions, etc.

To form the somatoplasm of the different tissues of the body, this complicated organization breaks up, as the egg divides, into an ever-increasing number of cells.  First, so to speak, the corps separate to preside over the formation of different body regions.  Then the different divisions, brigades, and regiments, composing each next higher unit, separate, being detailed to form ever smaller portions of the body.  The process of changing germ-plasm into somatoplasm is one of disintegration.  The germ-plasm contains representatives of the whole army; a somatic cell only representatives of one special arm of a special training.  Germ-plasm in the egg is like Humpty-Dumpty on the wall; somatoplasm, like Humpty-Dumpty after his great fall.