6 HISTOLOGY [ANIMAL. which are generally of an elongated columnar form, com monly branch out at their detached end into fine processes which appear to become connected with nerve-fibres. Cells of this character occur even so low in the Mefasoa as the Medusae, in connexion with the nerve-epithelium to be afterwards mentioned. And, indeed, in many cases where cells of this character enter into the constitution of the sense organs, it is probably most consistent with their true nature to regard them as detached portions of nervous tissue, which also in every case is originally of an epithelial nature. Modifications in the Cell Contents. Another chief modification which the cells of an epithelial tissue may undergo consists in the accumulation within the cells of various chemical substances, which may be either taken in bodily as such, or may be formed in the cell from other substances which are sup plied to it by the blood. The substances that are thus accumu lated and formed within the cells of an epithelium are of very various nature, as, for example, the constituents of special secre- tinna ifirr ~d Tnnrin nipmpnt FlG - 10. Epithelium cells of tions ^ng. L(J), mucin, pigmt.nt, rena] organ O f mollusc con- fatty globules, Uric acid, &C., &C. taimng crystals of uric acid. These several substances are toler ably constant in an epithelium of the same kind thus, mucin is a very frequent constituent of columnar epithe lium, and in glands which have the same function in different animals, the same substances are found in the epithelium cells of the gland. Exudation from Epithelium Cells. Formation of Cuti- cular Structures. In many invertebrates the epithelium which covers the surface of the body, and sometimes also that which lines a part of the alimentary canal, forms an exudation which is generally soft at first, but may after wards harden into a horny consistency, or may be rendered still harder and at the same time more brittle by impregna tion with earthy salts. Any such structure is termed a cuticular formation, It may be composed of a single thin layer, or a number of layers may be superimposed, so that a " shell " of considerable thickness is thus formed. " The chitinous or calcareous covering which forms the exoskeleton in many molluscs, arthropods, annelids, and Hydrozoa is of this nature. On the other hand, the firm skeletons of sponges, Actinozoa, and Echinodermata are formed by deposition in the connective tissue. The Connective Tissues. The connective tissues are characterized by the great development of intercellular substance in comparison with the cells ; indeed in those animals in which connective tissue may first of all be said to appear, there is an entire absence of cellular elements properly belonging to the tissue. This is the case in many of the Coelenterata, in which the connective tissue is repre sented merely by a layer, more or less thick, of hyaline substance, which undoubtedly performs a sustentacular function, in addition to connecting together the epithelial layers of the ectoderm and entoderm. The intercellular or ground substance almost invariably takes a prominent part in the formation of connective tissue. It is of a semi-fluid nature, and often contains in addition to albumen a certain amount of mucin. In most cases the cells of the connective tissue separate themselves from the primary layers before the formation of this ground substance ; indeed the mesoderm is at first chiefly formed of these cells. The stages of development are as follows. The mesodermic cells, which are at first in apposition, become separated from one another by the accumulation of intercellular substance, but at the same time main tain a connexion with one another throughout the tissue by their branching cell-processes (see fig. 2, C, c). Presently, in the production of ordinary connective tissue, fibres of two kinds make their appearance in the intercellular sub stance, and to all appearance independent of the cells. Those of the one kind (fig. 11, A) are highly elastic and refracting, not easily affected by reagents, stain deeply with magenta, run singly, always branch, and become united with neighbouring fibres so as to form a network throughout the tissue ; those of the other kind (fig. 11, B) are exces sively fine and indis tinct, never run singly but always in bundles, and generally with a wavy course, are readily affected by reagents, and, in vertebrates, yield gelatin on boiling. In the various kinds of connective tissue the re lative proportion of these two kinds of fibres to one another and to the cellular elements of the tissue varies. Thus in the so-called elastic tissue of the Vertebrata the elastic fibres greatly preponderate ; in tendinous tissue, on the other hand, they are scarcely to be found, and the ground is almost wholly occupied by the white fibres. It may happen that the intercellular substance is so completely occupied by the fibres as to be entirely obscured, but its presence may be always recognized in consequence of the property which it possesses of reducing silver from its salts when exposed to the light. In certain cases the inter cellular substance becomes hardened by the deposit within it either of a substance termed chondrin, which confers upon it the well-known toughness and elasticity of carti lage, or by a deposit of earthy salts imparting to it the firmness of bone. These several changes in the inter cellular substance are accompanied by special modifications in the form and relations of the cells (by whose agency they are in all probability effected). In comparatively rare cases the intercellular substance which is found occupying the meshes of the network formed by the branched cells of the developing connective tissue may disappear entirely, and the meshes may be occupied either by blood or by the lymph or plasma of the blood (spleen and lymphatic glands of vertebrates). It frequently happens that the connective tissue presents the consistence of jelly, and this is generally ascribed to the characters of the intercellular substance. It may, how ever, be due in many cases to the entanglement of fluid in the meshes of the fibres, and not to a gelatinization of the ground substance. This is shown by the fact that the fluid may be drained from out the meshes by means of filter paper. And the possibility of the formation of a jelly in this manner is evidenced in the coagulation of lymph, where the apparently solid gelatinous clot is a tangled meshwork of fine filaments enclosing fluid. The connective tissues of invertebrates are, on the whole, similar to those of the vertebrate ; at the same time it must be admitted that there are not unimportant differ ences in chemical constitution, such as the absence of a substance yielding gelatin, and the absence for the most part of mucin, both of which are characteristic constituents of vertebrate connective tissue. On the other hand the anatomical characters of the elements, both cells and fibres, are in most cases sufficiently well marked to be recognizable. In the sponges the bulk of the animal is made up of a
jelly which, when examined under the microscope, is found
