The middle germ layer (mesoblast) is derived from the upper (epiblast) and lower (hypoblast), the relative amount contributed by each being doubtful. From the earliest period the middle layer has distinctive characteristics, and ultimately gives rise to a set of tissues which can always be distinguished from those which originate from the upper and lower layers.
From the inner and outer germ layers are formed several connective tissues, which, in a more or less perfect degree, retain the activity of the original protoplasm, and hence may be called active tissues. From the middle germinal layer is developed a set of textures, in the majority of which the protoplasmic elements are reduced to a minimum, and are therefore grouped together as supporting tissues.
The tissues formed in the adult may be classified into four groups:
1. Epithelial Tissues. The primitive surface tissue of the epiblast and the hypoblast, which are variously modified for several distinct functions.
2. Nerve Tissues. Springing from the former, are modified for receiving, conducting, controlling and distributing impressions.
3. Muscle, or Contractile Tissues. In close relation to both the previous and the next groups.
4. Connective Tissues formed only from the middle germ layer. They are much modified in different parts, so as to give shape to the body, and to support and hold the various organs and parts firmly together. They are, in fact, the materials used in the general body architecture.
Epithelial Tissue, although the oldest kind of tissue both in the animal series and in the germinal layers, retains the embryonic character of being entirely composed of cells placed in close relationship on the interal and external surfaces of the body. The individual cells retain the embryonic character in form and function, being soft, rounded masses of protoplasm, only altered in shape by the pressure of their neighbors. The cells which lie next the nutrient vessels of the mesoblast are endowed with energetic powers of growth and reproduction. As the young cells are produced they take the place of the parent cell, whose future life history determines the special characters of the different kinds of tissues.
Sometimes the cells are retained, as in the skin, and are arranged in several layers, one over the other. As the cells are conveyed from the deeper layer, where they take their origin, toward the surface, the efforts of the waning nutritive power of the protoplasm are devoted to the manufacture of a tough, insoluble substance. The cells thus gradually lose their vital activities, and are converted into horny scales, which form the external protecting skin, and its many modifications that give rise to the different dermal appendages, such as hair, feathers, etc. Instead of a horny substance, the protoplasm may manufacture fat in the bodies of the cells, as seen in the mammary and the sebaceous glands of the skin. In other cases the reproductive activity of the cell is in abeyance, and its nutritive energy is devoted to the manufacture of a material which is poured out of the cell at certain periods. Thus we have another function performed by the epithelial tissues, namely, that of manufacturing certain materials which, being collected by suitable channels, appear as secretions.
Fig. 10. Section of the epiderm of the prepuce showing the superimposed layers of cells of a stratified epithelium. (Cadiat).
a. Young proliferating cells, b - d. Cells advancing toward surface, e. Flattened cell of horny layer, f. Basement membrane, g. Connective tissue.
Fig. 11. Two cells of scaly epithelium from the inside of the cheek. (Ranvier).
Fig.12. Section of milk gland of cat, show-ing; secreting cells containing fat globules, and some secretion in alveoli.
Fig. 13. Ciliated epithelial cells from the gills of mussel. (Cadiat).
Fig. 14. Stratified ciliated epithelial cells from the trachea of man. (Cadiat).
a. Large surface cells, with cilia on surface.
b. Lower cells in earlier stage of development. e. Cell charged with mucus.
The active elements of glandular tissue are epithelial cells whose nutrition leads to the formation of specific chemical products within their protoplasm. These products pass out commonly as fluids, and form various substances of great importance in the economy. A gland is simply a special arrangement of epithelial cells lining the sacs or tubes into which the secretion is poured. Some tracts are covered with fine, moving, hair-like processes, called cilia, which give rise to a slight motion of the fluids in contact with them.
The epithelium in various places is thus seen to be modified in different ways, so as to make it suitable for the special function of the part in which it is placed.
Other differences will be given in detail with the description of the uses of the many mucous surfaces. The most interesting modifications are those in the special sense organs, where the cells are in immediate connection with nerves, and aid in forming the special nerve terminals.*