■1889.] OOd • [Ryder. 



blood and lympli vessels of the marrow and periosteum, to be absorbed 

 and carried to all parts of the thick bone substance by the processes of 

 tlie bone ci'lls or osteoblasts. 



4. That the avidity of tlie absorption of calcareous matter is proportional 

 to the density of Ihe colloidal matrix, and that the gradually increasing 

 brittleness of bone, as age advances, i-s due to such increasing density of 

 the matriv and its still more enfeebled and less active metabolism. 



5. That the feebleness of the metabolism going on in l)ony tissue or 

 matrices is the main cause, together with their avidity for earthy salts in 

 solution in the fluids of the body, of the rapid calcification of young 

 bones as well as of the persistence of their calcified condiiion throughout 

 life. 



6. There are no such things as "bone cells" or "osteoblasts" in tlie 

 sense of makers of bone ; these cells play an absolutely intermediary r61e 

 in bone formation, since they only give rise to the bone matrix, and form 

 a network in the highest forms of bone by which a bond of union is estab- 

 lished throughout its substance, serving for the transmission of calcareous 

 salts from one part to another. The absorption and retention of calcare- 

 ous matter by bones is a physical process, conditioned by the establish- 

 ment and circumscription of colloidal or homogeneous stable matrices at 

 definite points in the organism ; these loci in turn are determined by the 

 soft parts through inheritance. 



7. The soft parts have determined, in the main, the shapes of the hard 

 parts of the endoskeleton, and not vice versa, with the exception only of 

 the teeth. 



The earliest form of a colloidal non-cellular matrix is found in the um- 

 brella of Medusa, of both hydrozoan and scyphozoan types. It here con- 

 fers an elasticity to the umbrella which is sufficient to eflect the recoil of 

 its margins when the animal is in motion. In other words, the systole of 

 the umbrella is effected by the marginal radial and circular muscles, its 

 diastole or recoil is due to the elasticity of the mass of elastic more or less 

 colloidal matter interposed between the epiblast or ectoderm and the 

 hypoblast or endoderm. Its function is here not only tliat of a supporting 

 endoskeleton which gives the umbrella its configuration, but also effects 

 the diastole of the umbrella without involving any extra expenditure ot 

 energy through its own metabolism, as must happen in contracting 

 muscle. 



This gelatinoid matrix is a secretion of the primary embryonic layers 

 and plays an important role in giving form or configuration and a certain 

 rigidity to many larval forms. It is therefore physiologically the oldest of 

 all endoskeletal structures. 



The next element of the endoskeleton which is of equally great signifi- 

 cance in the elaboration of a theory of the genesis of the vertebrate endo- 

 skeleton are the axial cellular cords of the tentacles of many Hydrozoa and 

 Scyphozoa, the oral rods and axial supports of the cirri of Branchiostoma, 

 and the notochord of all vertebrates, ascidians and Hemichordata. Prob- 



PROC. AMER. PHILOS. SOC. XXVI. 130. 3r. PRINTED DEC. 18, 1889. 



