570 



INTEGUMENTARY SYSTEM 



carapace gradually fuse with the flattened trunk vertebrae and the flattened 

 ribs. In figure 268G is shown a longitudinal section through a part of the mid- 

 dorsal area of a turtle just before hatching. It is to be observed that the epi- 

 dermal horny scales or scutes are well formed, while the dermal mesenchyme 

 of the carapace is wrapped intimately around the flattened, dorsal, spinous 

 processes of the vertebrae. 



Epidermal scales and thickened horny skin pads, together with an armor 

 of bone, in turtles, demonstrate the types of dermal and epidermal differen- 

 tiations which form a protective coat in the reptilian group. The "shed skin" 

 of the snake represents a sheet of horny epidermal scales which is peri- 

 odically cast off. New scales are reformed repeatedly throughout the life of 

 snakes. The rattles on the terminal end of the tail in the rattlesnake represent 

 horny rings, developed proximal to the horny spine, prevalent as the end piece 

 of the tail of many serpents. Lizards are well protected with thick epidermal 

 scales, and in some species these scales are reinforced with dermal bony 

 plates. The crocodiles are tough-skinned animals, possessing thick epidermal 

 scales; the dorsal scales are supported underneath by corresponding dermal 



Fig. 269. Development of skin in the chick. (C after WiUiams: Am. J. Anat., 11.) 

 (A) Epidermis of 48-hr. chick. (B) Epidermis of 72-hr. chick. (C) Dermal mesen- 

 chyme, arising from dermatome of embryo of 40 somites. (D) Skin of chick embryo, 

 incubation six days. (E) Skin of eight-day embryo, showing beginning of feather rudi- 

 ment. (F) Eleven-day embryo, feather rudiment. (G) Section of mature skin between 

 feather outgrowths. Observe that the epidermis is thin, and that the dermis is composed 

 of two compact layers separated by a vascular layer. 



