This well-adapted structure is now interpreted through the _Struggle of the Parts_ as a self-differentiation, for if there be in the rudiments or primordia of the bone differently endowed elements[10], that is, cells which respond in diverse ways to different stimuli, these must arrange themselves locally, owing to the struggle for space and food, in a manner corresponding to the distribution of the different stimuli in the bone. The largest amount of bone substance will be formed in the directions of the strongest strain and the greatest pressure, because the bone-forming cells are excited by this, their functional stimulus, to growth and multiplication. Thus the buttress and arch structure comes about, and between the delicate bone lamellæ spaces will remain free, and these, being relieved from the burden of strain and pressure by the aforesaid bony lamellæ, will offer suitable conditions of life to cells with other functional properties, such as connective tissue cells or vascular cells. [10] I do not here enter into the question whether we have not in this case to do with similar elements, which have the power of differentiating into one or another kind of cell according to the nature of the external stimuli by which they are influenced. The structure of the bone spongiosa is not everywhere the same, and it is demonstrably related with precision to the conditions of strain and pressure at each particular region. Thus, just below the soft cartilaginous covering of the joints there are no long pillars with short arches, but only rounded meshes, because the pressure is here almost equally strong from all sides. The long parallel pillars only occur further down in the bone, and they lie in two directions which intersect each other obliquely, corresponding to the two main directions of pressure. But it is only under the functional stimulus of pressure that the bone-forming cells have an advantage over the others, and multiply more quickly, thus crowding out those that are not attuned to the appropriate functional stimulus. In a similar manner Roux interprets, in the light of the struggle of the parts, the striking adaptations in the course, the branching, and the lumen-formation of the blood-vessels, in the direction of the intersecting connective tissue strands in the tail-fin of the dolphin, in the direction of the fibres in the tympanum, and in many other adaptations in the histological structure of complex tissues.