4 VERTEBRATE SKELETON 



extending from the median septum to the skin. In the tail the rela- 

 tions are much the same, except that the body cavity and its viscera 

 are absent. 



In the formation of the skeleton three different conditions are 

 recognized. The first is the membrano-skeleton, just noticed, 

 most of which becomes transformed into several tissues, grouped by 

 histologists as connective tissues. The other conditions are the 

 cartilaginous and the bony (osseous) skeletons which differ markedly 

 in character. There are less differences between the bones which 

 form the internal skeleton and the scales of the dermal skeleton, 

 there being a close connection between these, as it is probable that 

 most, if not all bones had their phylogenetic origin directly or 

 indirectly from dermal ossifications. 



The corium is concerned chiefly in the formation of dermal scales, 

 but in the head it gives rise to certain of the cranial bones. The 

 median septum has to do with the development of the vertebral 

 column, skull, sternum and some other structures of less importance. 

 Ribs may form at the intersection of myosepta and the horizontal 

 septum. The skeleton of the median fins of fishes arises in the dorsal 

 and ventral extensions of the median septum; that of the paired 

 appendages is not so easily located in the scheme, although, like the 

 rest, it is mesenchymatous in origin. 



Dermal scales are calcified structures which, as the name implies, 

 arise in the deeper layer (derma or coriimi) of the skin. They have 

 a special importance since from them teeth and membrane bones 

 have been derived, and from the further possibility that all ossifica- 

 tion had its origin from them. Cartilage is never concerned in their 

 history; they arise as direct ossifications in the corium, often with 

 additions from the ectoderm which have no significance in the general 

 morphology of the skeleton. 



The typical dermal scale, the placoid scale, occurs in the skin of 

 Elasmobranchs, all other dermal scales being thought to be deriva- 

 tives of these. Hence an outline of their development is given, 

 details of structure being found in the section dealing with the 

 dermal skeleton in different Vertebrates (p. lo). 



The first step in the development of a placoid scale is a multiplica- 

 tion of mesenchyme cells in restricted areas of the corium, immediately 

 beneath the ectoderm. These cell aggregates thicken the corium 

 in spots, resulting in the elevation of the overlying ectoderm, this 



