DEVELOPMENT OF THE SKIN 



569 



^%i^ 



PRIMITIVE 

 ERTEBRAL BODY 



PERIDERM 

 ATE EPIDERMIS 



NOTOChORD 







EPIOER 



STRATUM CORNEUM 

 SCALE 

 EPIDERMIS 

 ^DERMAL MESENCHYME 

 ^ \5- VERTEBRAL ARCH 



EURAL TUBE 



DERMIS 

 VERTEBRAL BODY 





Fig. 268. Development of turtle skin. (A) Section through turtle embryo, showing 

 early division of epidermis into periderm and germinative stratum. (B) Section show- 

 ing two-layered condition of epidermis in slightly older embryo. (C) Section through 

 dorsal area of embryo, 1 1 mm. long. ( D) Higher power drawing of epidermis of 1 1-mm. 

 embryo. (E) Section of skin of turtle, after hatching, to show horny plates. (F) 

 Higher power sketch of skin shown in square in (E). (G) Section of skin of turtle 

 just before hatching, showing epidermal scales of carapace, dermal mesenchyme, and 

 vertebrae. 



series of interlocking bony paltes, associated with an outer cover, the epidermal 

 skeleton, composed of horny scutes. The latter comprises the so-called tortoise 

 shell of commerce. The dorsal carapace and ventral plastron are united along 

 their lateral edges by a bony ridge, and the carapace is firmly fused with the 

 vertebrae and ribs of the endoskeleton. The skin of the head, neck, tail, and 

 legs is fortified with thick horny plates placed at intervals (fig. 268E). Be- 

 tween these horny plates, the stratum corneum is highly developed (fig. 268F). 



At the 11- to 15-mm. stage, the condensation of dermal mesenchyme already 

 is thickened greatly in the dorsal region of the embryo in the future carapace 

 area. This thickened condition and the intimate association of the mesenchyme 

 with the trunk vertebrae and ribs are shown in figure 268C. The rudiment of 

 the plastron begins to appear in the ventral region at this time. 



After the young hatch from the egg, ossifications occur within the dermal 

 mesenchyme of the carapace and plastron. The bony ossifications of the 



