1889.] ^^ * [Kydcr. 



only in the embryo with fine processes sometimes extending in cross sec- 

 tions a little distance into the here more developed hyaline matrix of the 

 massive cartilaginous investment. 



When the irruptive processes of bone formation or neoplastic ossifica- 

 tion occur they are followed by the deposition of dense homogeneous or 

 fibrous matrices around the marrow cavities so formed, within the original 

 homogeneous calcareous tube, which was formed by moulding a deposit of 

 collagen-like substance upon a matrix of cartilage. In such new homo- 

 geneous deposits new infiltrations of calcareous matter occur within and 

 superimposed upon the old. In such endosteal deposits and in the later 

 periosteal deposits the homogeneity of the bone substance is interrupted 

 only by the intercalation of the osteoblasts which are left imbedded in the 

 homogeneous matrix as the latter is laid down, layer after layer. 



If time permitted, the mechanical advantages and expedients of neo- 

 plastic bone formation, as well as the intricate series of substitutions which 

 can be traced in the evolution of the vertebrate skeleton, might be dis- 

 cussed in full, but my intention has been fully realized, at present, if it 

 has been made clear that the physical, chemical and physiological proper- 

 ties as well as the mechanical space relations of the parts involved, must 

 be considered in order to arrive at a sound theory of the origin of the 

 skeleton. It is especially to be insisted upon that the physical properties 

 of the substances involved in calcification are to be taken into account 

 quite as much as their morphological characters, in working out a general 

 theory of the history of the skeleton. Morphology may serve to aid in in- 

 terpreting phylogenies, but it is evident from what has preceded, that 

 morphology alone is incapable of grappling with the question of the true 

 causes of the genesis of the skeleton, ciuite as much so as the overstrained 

 hypothesis of natural selection. 



Note. — The only portion of the ski-leton of vertebrates, the matrix of 

 which is of exoskelelal origin, is the enamel. The dentine is endoskeletal. 



The elastic fibres found by Kolliker in bone, belong to the category of 

 substances denominated colloidal. The later fibrous character of the 

 matrix of many if not the majority of bones of higher types, as a result 

 of which, while living and moist they have a certain elasticity, does not 

 any the less exclude their constituent fibres from the group of colloidal 

 substances. . In the same way the homogeneous actinotrichia of the fins 

 of fishes and their investing matrix of calcified matter is colloidal. The 

 precipitation of calcareous matter follows these actinotrichia to the very 

 margin of the fin folds, Avhere they diverge more and more like the ribs of 

 a fan, and thus give rise to the characteristic dichotomous character of the 

 soft rays of all malacopterygian forms of fishes. 



The deposition within the middle of the notochord of an axial median 

 cord of fibrous collagen-like substance is a further instance of the slow 

 degeneration and metabolism which may go on within even a relatively 

 indifferent tissue. This axial cord is an intercellular deposit, and has been 



