THE EVOLUTION OF EPIPHYSES AND OF ENDOCHONDRAL BONE

Abstract

Summary1. The most primitive type of epiphysial mechanism known is found in bony fishes, and from this type all others are easily derivable. The cartilaginous epiphyses contain a mass of undifferentiated cells, a growth zone of flattened cells, and a zone of hyper‐trophied cells derived from them. Endochondral bone and marrow are well developed. Temporary cessation of growth may occur with loss of differentiation in the growth zone and the formation of a closing plate of endochondral bone shutting off the marrow from the epiphysial cartilage.2. In some modern fishes, Chondrostei and Dipnoi, endochondral bone and marrow have been lost, in others a secondary centre of calcification may be found in the epiphysis, but these changes are peculiar specializations.3. In primitive tetrapods, including all typical early fossil forms and the living Crocodilia and Chelonia, the cells of the growth zone are arranged more or less regularly in columns, and the endochondral bone, guided by these columns, again has a regular arrangement.4. Of the modern amphibians the Urodela have lost the regular arrangement of the cells of the growth cartilage, and have reduced the amount of their endochondral bone, while the Anura have developed a peculiar match‐head type of epiphysis with a calcified secondary centre.5. In Sphenodon a large calcified secondary centre is developed between the articular cartilage and the growth cartilage. By separating these cartilages it allows each to adopt the most advantageous position, the one for the formation of the joint surface, and the other for directing the arrangement of the trabeculae of the endochondral bone.6. All other groups are specialized in one direction or another. Bony secondary centres which strengthen the epiphyses are found in typical Lacertilia except in some small forms which have lost them, in the tibia of Aves and in Mammalia. Cartilage canals, developed primarily for the nutrition of the cartilaginous epiphyses, are found in Varanidae, Aves and Eutheria, and possibly in Anura, while the Monotremata have a very peculiar cartilage canal system whose development is not yet understood. These features are the result of very detailed parallelisms in evolution.7. Intratendinous centres of ossification, formed by ossification of tendons where these are inserted into epiphysial cartilage, are distinguished structurally from typical epiphysial centres.