108 STRUCTURE OF TPIE VERTEBRATES 



ghii) which is in constant cellular division. Above this is a pig- 

 mented area, not well defined in most vertebrates, and then a 

 various number of layers considered together as the stratum 

 corneum. The cuboidal cells of the stratum germinativum be- 

 come flattened in the corneous layer, the outer ones dead. Due 

 to continual mitoses the skin would become progressively thicker 

 if the outer dead cells were not discarded. In the amphibia and 

 reptiles this is accomplished by periodic ecdyses or moults, de- 

 pending to a large extent upon the amount of food and the con- 

 sequent more rapid growth. In the birds and mammals the outer 

 cells are shed as small flakes. Dandruff and the minute rolls of 

 epidermis removed with a rough towel are illustrations of the 

 mechanism. Death in these cells is in large part due to lack of 

 nutrition, nerves and blood vessels being lacking in the. epi- 

 dermis. 



In the making of leather the epidermis is lost. The remaining 

 thick mat of dermal fibers is softened, cured and used in manu- 

 facture. Practically all commercial leather is from the mammal, 

 though alligator, snake and lizard skins are widely used for 

 ornamental leathers; and ostrich skin is frequently used for 

 bill-folds and other small articles. The dermis retains prac- 

 tically all of the original structure, the scales of the reptile or 

 the feather pits of the bird. 



A. Dermal Derivatives of the Skin 



Structures derived from the skin mesoderm appeared first 

 in the most primitive fish. The primitive condition has been 

 retained by the sharks (Elasmobranch fish) where they are 

 found as isolated dermal denticles. These minute bony plates 

 cover the body, varying slightly in size, and continue over the 

 lips where they have become modified into teeth. They are, 

 therefore, homologous with the teeth of all vertebrates. Embry- 

 ologically dermal denticles and teeth are composed of both 

 mesoderm and ectoderm, the basal plate and body of the den- 

 ticle being mesoderm, while the outer covering, or enamel, is of 

 ectodermal origin. The student should study the diagrams of 

 the denticles and compare them with the drawings of developing 

 teeth on page 142. 



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