The facts to be accounted for in any theory of inheritance are that the fertilized ovum at the outset of its career bears in itself the various particulars of the future individual down to the finest details of local structure. It has an image, or plan, or model which it follows. It seems probable that this plan is embodied in material vital particles contained in the germinal vesicle. Such vital units of excessively minute size have been supposed to exist by Herbert Spencer, who called them ' physiological units'; by Charles Darwin, who called them ' gemmules'; and by Francis Galton. The theory of heredity has been very fully worked out by Weismann, who has, with much industry, given it a consistent expression. His view is that the nucleus of the germ-cell contains in it vital units or ' biophors' which are destined to determine the evolution of the individual in all details of structure. This theory must be meanwhile regarded as sub judire, but it implies what appears to be a necessary condition of such a theory, that the various items of the constitution in being transmitted, are represented by actual particles, and it contains a possible suggestion as to the manner and place of the aggregation of these vital particles. (See Weismann's Germ-Plasm; a Theory of Heredity, 1893).