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The skin dermoid, or derma-cyst as it has been called by Askanazy, arises from a portion of epiblast, which has become sequestrated during the process of coalescence of two cutaneous surfaces in development. This form is therefore most frequently met with on the face and neck in the situations which correspond to the various clefts and fissures of the embryo.
In all the higher animals a layer of cells makes its appearance between the hypoblast and the epiblast, and is termed the mesoblast.
The striking facts to which this description is a necessary introduction, may now be stated. From the outer layer, or epiblast, are developed the permanent epidermis and its out-growths, the nervous system, and the organs of sense.
In the further course of development the epiblast becomes the ectoderm or epidermic layer of the body; the hypoblast becomes the epithelium of the middle portion of the alimentary canal; and the mesoblast gives rise to all the other tissues, except the central nervous system, which originates from an ingrowth of the epiblast.
We may accept this, and yet ask why it takes on a form of growth familiar to us only in connection with epiblast? The reply is: "Young cells when put into the optic cup always become transparent, no matter what their origin; it looks as if this were due to a chemical influence, exercised by the optic cup or by the liquid it contains.
While the above-described introversion is taking place, and before the inner surfaces of the resulting epiblast and hypoblast have come into contact, cells, or amoeboid units equivalent to them, are budded off from one or both of these inner surfaces, or some part of one or other; and these form a layer which eventually lies between the other two a layer which, as this mode of formation implies, never has any converse with the surrounding medium and its contents, or with the nutritive bodies taken in from it.
Here the fact which it chiefly concerns us to remark, is that of these two layers the outer, called in embryological language the epiblast, continues to carry on direct converse with the forces and matters in the environment; while the inner, called the hypoblast, comes in contact with such only of these matters as are put into the food-cavity which it lines.
The wall of the planula is next pushed in on one side, or invaginated, whereby it is converted into a double-walled sac with an opening, the blastopore, which leads into the cavity lined by the inner wall. This cavity is the primitive alimentary cavity or archenteron; the inner or invaginated layer is the hypoblast; the outer the epiblast; and the embryo, in this stage, is termed a gastrula.
The most remarkable feature here is the class of Liorhizal Dicotyledons, which includes only the families of Nymphaeaceae and Gramineae. It is based upon the fact that the histological differentiation of the epidermis of their root is that generally characteristic of Monocotyledons, whilst they have two cotyledons the old view of the epiblast as a second cotyledon in Gramineae being adopted.
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