VERTEBRATA. 261 



Cartilage Bones, and Membrane Bones. — It 



has been seen that, as we pass upwards in the series of 

 fishes, we usually find the cartilage of the skeleton 

 more and more replaced by bone. In the higher verte- 

 brates it is practically entirely so replaced in the adult, 

 cartilage remaining only in places where it is useful , as 

 a soft pad. The cartilage becomes directly trans- 

 formed into bone, undergoing certain histological 

 changes, and the deposition of a quantity of mineral 

 matter. Bones so formed are called cartilage bones. 

 But there are bones of another kind, called mem- 

 brane bones, because the tissue which exists before 

 ossification sets in is of a fibrous character, or der- 

 mal bones, because they are derived from the dermis. 

 From the manner in which these bones are seen to be 

 formed in the embryo, and from the comparison of 

 different types, it appears that they are really scales 

 (placoid scales, see 'p. 255) belonging to the deeper 

 layer of the skin, which have sunk in, till in the course 

 of development they have joined on to the cartilage 

 bones, and come to form part of the skeleton. Thus, 

 in the case of the Sturgeon, we find a skull of cartilage, 

 with bones of this kiad lying outside it. In the 

 skull of a Teleostean fish the cartilage of the skull is 

 replaced in the adult by cartilage bones ; and : the 

 membrane bones, instead of merely lying outside 

 them, are "fastened to them. They are, however,, not 

 fastened very firmly ; for in these fishes the different 

 bones of the skull are irregular in form, and very 

 loosely joined, instead of being firmly united into a 



