Transplantation of embryonic limb cartilage and bone to the chick chorioallantois has been carried out, using donors from the various tetrapod classes. The results may be correlated with the degree of phyletic relationship between the donor and host. Avian cartilage and bone continued to grow even when the donor was of a species and genus distinct from those of the chick host. On the other hand, when the donor belonged to another tetrapod class, only one of these embryonic tissues, namely cartilage, grew. There is a differential response of the tissues of a particular donor to a xenoplastic environment. Embryonic cartilage from amphibian, reptilian and mammalian donors will continue to grow xenoplastically when transplanted to the chick chorioallantois, whereas bone from such donors will not grow. The age factor is important in all xenoplastic results. Young donor cartilage grows well, but with increasing age there is a gradual reduction in its ability to do so. Embryonic cartilage from cold-blooded donors shows a high degree of temperature tolerance. Amphibian cartilage continues to grow xenoplastically at the chick incubation temperature (38° C.), whereas a temperature of 25° C. is known to be lethal to tadpoles of the particular donor species. Embryonic reptilian cartilage survives temperatures ranging from at least 4°C. to 38° C. The differential response of embryonic tissues to the transplantation environment is evident in development at stages much earlier than those at which immunological mechanisms are known to operate. It appears to be based on the metabolic requirements of particular tissues. The less specialized the nutritional requirements of a tissue are, the better its chances of obtaining from a xenoplastic environment the appropriate metabolites which are essential not only for its survival, but also for its continuing growth and differentiation.