308 PROCEEDINGS OF THE CANADIAN INSTITUTE. 
cement plates, as has been shown by Hertwig,’ but in the process of 
time, by a shortening of their developmental history, the bone came 
to be deposited directly in niembrane, 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 trabecule 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’ ;” so that the formation of the subsequent 
osseous trabecule 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*. 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 perichondra] 
and cement-bone. For exactly the same process goes on as in the 
premaxille and the palatines. The osteoblasts which have given 
rise to one individual cement plate carry on their work of bone 
' 0. Hertwig—loc. cit. 20. Hertwig—loc. cit. 3 See Schmid-Monnard—loc. cit. 
