308 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



cement plates, as has been shown by Hertwig, 1 but in the process of 

 time, by a shortening of their developmental history, the bone came 

 to be deposited directly in membrane, without any previous tooth- 

 formation. The same thing may happen with certain perichondral 

 bones, as, for instance, the palatine and the branchial arches. These 

 in some Teleosts are formed from cement-bone, but in Amiurus are 

 developed perichondrally, a shortening of the development again 

 taking place. 



But not only are these different varieties of bone identical in their 

 histological features, and not only are they able to replace each other, 

 but they also are identical in their histogenesis. In all osteoblasts 

 are present (either transformed cartilage or connective tissue cells) 

 and secrete the calcareous matter which is deposited in an organic 

 non-cartilaginous substance. This is very evident in the case of the 

 ' Deck-knochen' and mucous-canal bones. It is also the case with 

 cement bones which are formed of osseous trabecular deposited in 

 membrane by means of osteoblasts, the cement plates of the teeth 

 themselves arising, " theils direct als Abscheidung einer zelligen 

 Anlage (cement membran), theils durch Verknocherung des den Zahn 

 ungebenden Bindegewebes 2 ;" so that the formation of the subsequent 

 osseous trabecular is merely a continuation of the original process 

 which formed the individual cement plates. And again, with regard 

 to the perichondral bone the same thing may be shown to obtain. 

 With the growth of the bone secreted by the osteoblasts there is a 

 concomitant absorption of the cartilage, the cartilage cells probably 

 being partially transformed into osteoblasts, by whose agency new 

 trabecule are formed occupying the place of the lately absorbed 

 cartilage, there being no deposition of the calcareous matter in the 

 cartilaginous matrix. This is what occurs in centripetal perichon- 

 dral bone 3 . The processes in centrifugal perichondral bone are simi- 

 lar to those to be seen in the formation of cement-bone. 



In the dentary portion of the mandible there is a combination of 

 the cement process with the centrifugal perichondral process, in which 

 union of processes is seen the close relationship between perichondral 

 and cement-bone. For exactly the same process goes on as in the 

 premaxilla? and the palatines. The osteoblasts which have given 

 rise to one individual cement plate carry on their work of bone 



1 0. Hertwig — loc. cit. * 0. Hertwig — loc. cit. 3 See Schmid-Monnard — loc. cit. 



