Two modern hypotheses of heredity demand attention--Weismann’s hypothesis of the continuity of the germ-plasm, and Semon’s “Mnemic” hypothesis. In the latter it is assumed that the basis of heredity is the unconscious memory of the organism: modes of functioning are “remembered” by the germ-plasm and are transmitted. This notion presents many points of similarity to that which we consider later on, so that it need only be mentioned here. Weismann’s hypothesis--like Darwin’s hypothesis of Pangenesis--is a corpuscular one, and has obviously been suggested by the modern development of the concepts of molecules and atoms in the physical sciences. It supposes that that which is handed down is a material substance of a definite chemical and physical structure. This is not the germ-cell, nor even the nucleus of the latter, but a certain material contained in the nucleus. The latter contains protein substances containing a greater proportion of phosphoric acid than does the cytoplasm of the cells in general; these proteins are known as nucleo-proteins, though our knowledge of their chemical structure is, so far, not very exact. It is not, however, these that are the germ-plasm, but a substance in the nucleus _which becomes visible when the cell is killed in certain ways, and which becomes stained by certain basic dyes_. It is distinguished by this character alone and on that account is loosely called “chromatin.” This substance Weismann identifies as “the material basis of inheritance.”