56 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



3. Epidermal exoskeletal structures consist of horn and are composed of numerous flat- 

 tened dead comified cells pressed together. These cells arise by proliferation of the lower layers 

 of the epidermis (stratum germinativum). Of such nature are the superficial scales of reptiles, 

 birds, and mammals, feathers, hair, claws, nails, beaks, hoofs, horns such as those of cattle. 



4. Dermal exoskeletal structures consist of bone secreted by the mesenchyme cells of the 

 dermis. Of such nature are the scales of fishes— in whole or large part, the bony plates of 

 reptiles and mammals, the homs of deer and antelope, the fin rays of fishes, the plates of recent 

 and extinct Amphibia. 



5. Certain exoskeletal structures contain both dermal and epidermal constitutents inextri- 

 cably fused. Such are the placoid scales of the elasmobranch fishes and the teeth of all verte- 

 brates. By the identity of their structure and mode of development these two structures are 

 shown to be homologous. 



6. In the formation of exoskeletal parts a dermal papilla is involved, which furnishes 

 nutrition for the proliferating or secreting cells. In the case of dermal structures the cells of the 

 papilla give rise to the structure. In the case of epidermal exoskeleton the papilla takes no 

 part in the formation of the structure but simply brings a blood supply to the developing part 

 which arises solely by proh'feration and subsequent cornification of epidermal cells. 



7. From the foregoing account and from the definition of homology it follows that all exo- 

 skeletal structures of epidermal origin are homologous, and that similarly all exoskeletal 

 structures of dermal origin are homologous, whatever their shape or form or function. 



8. Epidermal structures exhibit more modifications and a greater complexity of structure 

 than do dermal parts. 



9. There has been no definite progressive evolution of the exoskeleton throughout the 

 vertebrate groups, but each group is provided with exoskeletal structures correlated with its 

 habits and mode of life. There is, however, a certain tendency for the higher vertebrates to 

 develop complicated epidermal structures. 



