334 EMBRYOLOGY OF THE LOWEE VERTEBRATES oh. 



ences may become conspicuous and highly characteristic — for 

 example the difference in relation to the calcified material — whether 

 the cell elements are completely surrounded by it as in the ordinary 

 bone of the higher "Vertebrates, or have merely a prolongation of 

 the cell-body embedded in it as in ordinary dentine. 



Such differences in detail may be of great interest in themselves. 

 For example the bony tissue forming the scales of Lepidosteus is 

 characterized by the fact that some of the bone - cells show the 

 dentinal characteristic that the main part of the cell-body lies on the 

 surface of the calcified material and only a prolongation of it is 

 enclosed within the hard substance. Now Goodrich (1913) has made 

 out the important fact that this peculiarity is not confined to the 

 scales but extends to the whole of the bony skeleton. Such a fact 

 is obviously a strong additional evidence of intimate evolutionary 

 relationship between the scales and the rest of the bony skeleton. 



Again such detailed differences may raise interesting problems, 

 for example whether the " ordinary bone " type or the dentinal type 

 (as is perhaps probable) is the more primitive type of bony tissue. 



Interest in such details must not be allowed to obscure the main 

 conception of bony tissue as contrasted with cartilaginous, or the 

 problem of its evolutionary origin. As regards that origin we seem 

 justified in believing that bone formation has during the evolution 

 of the Vertebrata spread from the dermis — from the neighbourhood 

 of the placoid scale bases — into the deeper tissues and so given rise 

 to the deeper portions of the bony skeleton. On the other hand we 

 do not appear to be justified in regarding the evolution of the deeper 

 parts of the skeleton as being due to a sinking downwards of actual 

 individual placoid elements. Nor, in the author's opinion, is there 

 reliable evidence, so far, bearing on the further problem whether or 

 not the first scleroblasts or bone-forming cells of the Vertebrata were 

 immigrants from the ectoderm. This view, which was supported by 

 Gegenbaur, has a considerable amount of a' priori probability in 

 its favour in view of the facts of skeleton formation in the lower 

 invertebrates. 



It is no longer possible in the present state of knowledge to 

 classify bones, as did the older workers, simply into two sharply 

 defined sets — membrane bones and cartilage bones. The most that 

 we can do is to recognize various stages in the process of shifting 

 inwards from the skin, from which as already indicated they probably 

 arose in the early stages of their evolution. 



Firstly we have the most primitive type which may be termed 

 dental bones, which are superficial in position and which still are 

 connected at one period or other with teeth. Typical examples are 

 the bones already referred to in the roof of the mouth in Amphibians. 



A second category consists of bony plates which have lost their 

 tooth structures and have sunk down to a deeper level. These 

 frequently become applied to the surface of the cartilaginous skeleton, 

 remaining separated however from the cartilage by a layer of un- 



