EPIDERMAL STRUCTURES. 8/ 



from the anterior, the parapinealis from the posterior, out- 

 growth. Besides these, a third outgrowth, the paraphysis, may 

 arise in front of the epiphysis ; it never develops sensory 

 elements. 



The parietal eye has different fates even in the lizards. In 

 some, as Hatteria (Fig. 90), it retains its parts well developed 

 throughout life, and its nervous connection with the brain per- 

 sists, so that here it is apparently to some extent functional. 

 In other lizards it has lost this power, either through the exten- 

 sive deposit of pigment in all parts, or by the degeneration of 

 its nerve supply. In other groups these structures of the pin- 

 eal apparatus are much more rudimentary, and are usually 

 entirely enclosed within the skull ; although at times, as in the 

 anura, the parietal eye may lie between the bones and the skin, 

 but here all nervous connection is lost. In many extinct 

 vertebrates, if we may judge by the large parietal foramen in 

 the skull, the pinealis was well developed and functional. 



EPIDERMAL AND DERMAL STRUCTURES (SKIN). 



The Skin. — That portion of the ectoderm which remains 

 on the surface of the embryo after the differentiation and in- 

 volution of the central nervous system forms the epidermis, 

 which, together with the underlying mesenchymatous tissue, the 

 derma, makes up the skin. In its earliest stage the epidermis 

 is usually but a single cell in thickness, but later, by division of 

 these cells, other layers are formed on the outside of this first or 

 basal layer. In ganoids, teleosts, and amphibia the epidermis 

 is two cells thick from the first, and in the amphibia (the only 

 instance in vertebrates) the outer layer is ciliated in the young. 

 The basal layer is the active portion, producing by cell division 

 the more superficial layers. In those forms with two-layered 

 epidermis, the basal layer also gives rise to nerves and sense 

 organs, and is therefore often spoken of as the nervous layer, 

 the outer one being the cuticular layer. 



The derma (often called cutis or corium) is of mesenchy- 

 matous origin, and consists largely of layers of fibrous connec- 



