Parodying the celebrated expression of Harvey, viz., Omne animal ex ovo, it has been attempted to formularise the law of development by the expression omnis cellula e cellula, and to maintain “that we must not transfer the seat of real action to any point beyond the cell.” In the attempts which have been made to support this exclusive doctrine, and to give all the tissues and all vital properties a cell origin, the great importance of the molecular element, it seemed to the author, had been strangely overlooked. It becomes important, therefore, to show that real action, both physical and vital, may be seated in minute particles, or molecules much smaller than cells, and that we must obtain a knowledge of such action in these molecules if we desire to comprehend the laws of organization. To this end the author directed attention: 1st, To a description of the nature and mode of origin of organic molecules; 2d To a demonstration of the fact that these molecules possess inherent powers or forces, and are present in all those tissues which manifest vital force; and 3d, To a law which governs the combination, arrangement, and behaviour of these molecules during the development of organised tissue.