the early cleavage stages of the egg. In other cases the germ cells are first recognizable at later stages, but in practically every case they arise from germinal or embryonic cells which have not differentiated into somatic tissues. Germinal continuity and somatic discontinuity of successive generations in sexually produced organisms is not a theory but an established fact. In general, germ cells do not come from differentiated somatic cells, but only from undifferentiated germinal cells, and if in a few cases differentiated cells may reverse the process of development and become embryonic cells and even germ cells it does not destroy this principle of germinal continuity and somatic discontinuity.
Thus the problem which faces the student of heredity and development has been cut in two; he no longer inquires how the body produces the germ cells, for this does not happen, but merely how the latter produce the body and other germ cells. The germ is the undeveloped organism which forms the bond between successive generations; the person is the developed organism which arises from the germ under the influence of environmental conditions. The person develops and dies in each generation; the germ plasm is the continuous stream of living substance which connects all generations. The person nourishes and protects the germ, and in this sense the person is merely the carrier of the germ plasm, the mortal trustee of an immortal substance.
This contrast of the germ and the person, of the undeveloped and the developed organism, is fundamental in all modern work on heredity. It was especially emphasized by Weismann in his germ plasm theory and recently it has been given prominence by Johannsen under the terms genotype and phenotype; the genotype is the fundamental hereditary constitution of an organism—it is the germinal type; the phenotype is the developed organism with all of its visible characters—it is the somatic type.
But important as this distinction is between germ and soma it has sometimes been over-emphasized. This is one of the chief faults of Weismann's theory. The germ and the soma are generically alike, but specifically different. Both germ cells and somatic cells have come from the same oosperm, but have differentiated in different ways; the tissue cells have lost certain things which the germ cells retain and have developed other things which remain undeveloped in the germ cells. But the germ cells do not remain undifferentiated; both egg and sperm are differentiated, the former for receiving the sperm and for the nourishment of the embryo, the latter for locomotion and for penetration into the egg. But while the differentiations of tissue cells are usually irreversible, so that they do not again become germinal cells, the differentiations of the sex cells are reversible, so that these cells, after their union, again become germinal cells.
In many theories of heredity it is assumed that there is a specific "inheritance material," distinct from the general protoplasm whose function is the "transmission" of hereditary properties from generation
