CHAPTER XIV.

THE EPIDERMIS AND ITS DERIVATIVES.

In many of the Coelenterata the outermost layer of the blastoderm is converted as a whole into the skin or ectoderm. The cells composing it become no doubt in part differentiated into muscular elements and in part into nervous elements, &c.; but still it may remain through life as a simple external membrane. This membrane contains in itself indefinite potentialities for developing into various organs, and in all the true Triploblastica these potentialities are more or less realized. The embryonic epiblast ceases in fact, in the higher forms, to become converted as a whole into the epidermis, but first gives rise to parts of the nervous system, organs of special sense, and other parts.

After the formation of these parts the remnant of the epiblast gives rise to the epidermis, and often unites more or less intimately with a subjacent layer of mesoblast, known as the dermis, to form with it the skin.

Various differentiations may arise in the epidermis forming protective or skeletal structures, terminal sense organs, or glands. The structure of the epidermis itself varies greatly, and for Vertebrates its general modifications have been already sufficiently dealt with in