The effects of two very different experimental procedures, (1) chorio-allantoic grafting and (2) dosing with decamethonium, a "curarizing" drug, on the development of adventitious (secondary) cartilage at several articulations, was studied in the chick embryo. Both procedures paralyse the muscles, the first by their physical destruction, the second by the paralysing action of the drug. The quadratojugal-quadrate articulation and that between the surangular bone and Meckel's cartilage, when grafted from 9-day embryos, formed no adventitious cartilage as grafts. Adventitious cartilage is in normal development formed on the bony component of both articulations. In grafts from many 10-day, and from 11-day embryos, adventitious cartilage was formed in the grafts, was resorbed after its differentiation (as happens also in normal embryos), but was not renewed by continued chondrification of derivatives of the germinal cells (as does happen in normal embryos). Decamethonium, as the commercial preparation Eulissin A, injected via the chorio-allantois into 9-day embryos, totally or almost totally paralysed the skeletal muscles of the embryos which, however, survived several days (their respiratory system and their cardiac and smooth muscles not being involved in the paralysis). Treatment with the drug from 9 to 14 days, followed by fixation, almost totally prevented the development of adventitious cartilage in the four articulations studied (three mobile joints: quadratojugal-quadrate, pterygoid-"roller" of quadrate, quadrate-squamosal, and one non-mobile: surangular-Meckel's cartilage). In other experiments with Eulissin it was found that after cessation of dosing only very limited recovery of movement occurred and there was limited formation of adventitious cartilage; it was, however, shown that as late as 16 days cells exist which can form adventitious cartilage. When dosing began at 12 days the adventitious cartilage already then present was found, after fixation at 16 days, buried under a new bony articular surface and in process of resorption. In some articulations in some embryos there had been a development of young articular cartilage in the late stages of the experiment. Earlier work had shown that adventitious cartilage develops from germinal cells appearing to be identical with those which earlier differentiate as osteoblasts, and in the present work the histology left little doubt that germinal cells, which normally would have formed cartilage, in grafted and paralysed embryos formed bone. In the present paper it is concluded that these cells are ambivalent, differentiating as osteoblasts if they are not subjected to the mechanical conditions existing at articulations, and as chondroblasts if they are so subjected, but that cells which have attained to some early stage in chondrogenesis continue and complete their differentiation as cartilage in grafts and paralysed joints in which the mechanical conditions normal at articulations do not exist. The evocation of cartilage in this instance and in general, the nature of the mechanical factor involved, a possible common factor underlying the variety of circumstances which may in different cases induce chondrogenesis, the present instance as an example of modulation or change of differentiation, and the failure of adventitious chondrification in this instance to have become genetically assimilated, are discussed. It is suggested that given bivalent germinal cells with the ability to form cartilage or bone under appropriate conditions, and the invariable existence of such conditions, there is no opportunity for the action of selection and therefore none for genetic assimilation.