SUMMARY. 305 



there is no real distinction between the two layers, or else that both structures arise 

 from the same layer. As there is no doubt about the origin of the enamel layer, 

 and as there is no conclusive evidence that the odontoblasts have not arisen at a 

 very early period from the ectoderm, it may be assumed that both enamel cells and 

 odontoblasts were primarily derived from the ectoderm. 



We may, therefore, regard the dermal skeleton of primitive vertebrates as 

 consisting of two parts, viz. i. the subdermal trabeculae that develop into the char- 

 acteristic bony plates, and that consist of concentric lamellae, true bone corpuscles, 

 and vascular chambers, or canals; and 2. the more superficial skeleton, that arises 

 either from the outer surface of the ectoderm, or from cells in intimate relation 

 with it, and that consists of parts having a chitenoid, enamel-like, or dentine-like 

 structure. 



X. The general nature of the process by w r hich the continuous epidermal 

 armor of the arachnids becomes fragmented and divided into two overlying 

 systems, and the method of substituting the inner system for the outer, is shown in 

 a diagrammatic way in Fig. 208. 



The ostracoderm skeleton, C, clearly represents a transitional stage between 

 the vertebrate and the arachnid type. In the vertebrates themselves, the principal 

 events in the evolution of the dermal skeleton are the gradual elimination of the 

 epidermal structures, except those retained in the teeth; the reduction of the sub- 

 dermal bones to relative insignificance; and the substitution for them of an endo- 

 skeleton of true mesodermal origin. 



2C 



