CHAPTER XVII 



THE SKIN, AND ITS DERIVATIVES 



The skin forms the outermost layer of the body, and its 

 functions are protective, excretory, and sensory ; for all 

 information which the animal receives concerning the outer 

 world must come through the skin. Correlated with these 

 functions, it is found that the constituents of the skin may 

 undergo various modifications. 



The skin is formed of an outer ectodermal layer, the 

 epidermis, and an inner mesodermal layer, the dermis. In 

 Amphioxus the epidermis is only one-cell thick (as in most 

 invertebrates) ; while in all Craniates it is several layers of 

 cells in thickness. Of these, the innermost form the stratum 

 germinativum (or stratum Malpighi) which constantly pro- 

 duces new cells, while the outermost layers tend to become 

 horny forming the stratum corneum. As the cells become 

 horny the protoplasm within them dies, and they become worn 

 away by friction with the environment and replaced from the 

 stratum germinativum. In many reptiles and amphibia, it 

 is common for the superficial layer of the epidermis (overlying 

 the horny scales) to be sloughed off all at once and replaced. 



The epidermis may be ciliated in early stages of develop- 

 ment in the lower forms, such as Amphioxus and the frog 

 tadpole. 



The epidermis covering the eye becomes very thin and 

 transparent forming the conjunctiva. Sensory cells are present 

 in the stratum germinativum, and it will be remembered that 

 the sensory epithelium of the nose, of the eye, the ear, the lens 

 and the placodes which contribute nerve-cells to the cranial 

 ganglia, are all formed from the epidermis. 



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