66 CARTILAGE AND BONE 



Such hard-and-fast rules rarely, if ever, hold good in Zoology. 

 Gegenhaur, to whom we owe so much of our knowledge of the 

 morphology of the vertebrate skeleton, adopted a wider view, and 

 held that bone originated in the skin, and only secondarily came 

 into connection with the more deep-lying cartilage — that, in fact, 

 ' secondary ' bones have become gradually converted into 

 ' primary.' 



Both Gegenbaur [159] and Schmid-Monnard state that the 

 ' squamosal ' (pterotic) of certain Teleostei arises as a membrane- 

 bone, then becomes closely connected with the cartilaginous cranium, 

 and finally continues to develop as an endochondral bone. Histo- 

 genesis would, in such a case, be no criterion for homology. In 

 the course of phylogeny, bones, originally intramembranous, might 

 develop more and more directly as cartilage-bones, so that they 

 could no longer be distinguished from them (p. 266). There is also 

 reason to believe that, in some cases, the reverse may happen, the 

 cartilaginous stage being suppressed. Thanks to the researches 

 of Williamson [496a], Ley dig, Gegenbaur, and more especially 

 0. Hertwig [211-12], the ontogenetic and phylogenetic connection of 

 the dermal bones with denticles, like those developed in the skin of 

 Elasmobranch fish, has been traced. Williamson considered that the 

 plates and scales of fish were formed by the combination of super- 

 ficial denticles with underlying dermal bone. Hertwig supposed 

 that by the enlargement of the basal plate, whereby the hollow 

 tooth-like denticle is fixed in the dermis, or by the fusion of 

 adjacent basal plates, such superficial bones are developed as are 

 found covering the skull or the roof of the mouth in the lower 

 Gnathostomes (see, however, p. 215). 



Now Hertwig contended that true dermal bones can always be 

 traced back, in the lower forms, to the denticles from which they 

 were derived ; even when in the higher vertebrates such bones 

 have sunk below the skin, and become closely united to the 

 endoskeleton. Further : that dermal bones always lie outside the 

 perichondrium, and may cover over cartilage-bones ; and that 

 cartilage-bones can never be traced back to denticles, and are 

 developed entirely in direct relation to the cartilaginous skeleton. 

 The same result, therefore, was reached on this theory, from a 

 study of comparative anatomy, as had previously been reached by 

 Kolliker and others, from a more strictly histological point of view. 

 These conclusions are opposed to those of Gegenbaur and his pupils. 

 While adhering to the view that bone was originally developed in 

 relation to dermal denticles, the latter school holds that in the case 

 of the cartilage-bones, both perichondral and endochondral, the 

 osteoblasts derived from the surface have migrated farther inwards, 

 and finally come to invade the cartilage itself. Whether ossifica- 

 tion extends inwards by such migration, or by a sort of infection, 



