A.NIMAL.] HISTOLOGY to consist of large branched cells (fig. 12) connected to gether by their processes into a network. The meshes of this are occupied by clear intercellular substance within which the calcareous or horny matter which forms the skeleton is deposited. FIG. 12. Connective tissue of sponge. (F. E. Schulze.) When the development of the sponge is traced it is found that the first part of the tissue to be seen is the clear inter mediate substance, and the skeletal spicules begin to appear in this before the cellular elements. These wander subse quently into it from one of the primary layers. There can be no doubt that this jelly-like tissue of the sponge repre sents a primitive form of connective tissue, although, so far as has at present been ascertained, no fibres are developed in it. In the Cceknterata, as in the sponges, the connective tissue makes its first appearance in the form of a clear intermediate substance, which may be so small in amount as to be almost imperceptible, or so large in amount as to form the main bulk of the organism. In the former case, as in the developing sponge, there is an entire absence of both fibres and cells, whereas in the latter case both kinds of elements are found. The fibres are the more con stant, and are of the elastic kind (fig. 1 3) ; they have for the most part a direction across the thickness of the tissue stretching from ento- derm to ectoderm, branching and uniting with their neigh bours to form the character istic network which, enclosing watery fluid in its meshes, produces the jelly-like con sistence of the tissue. Fibres of the white variety are also found as low down in the Metazoa as the Ccelentcrata. In some of the acraspedote Mediisce they occur in the form of bundles of indistinct wavy fibres situated near the surface of the jelly, and in the Actiniae. similar fibres are found forming membranes which bear a strong resemblance to some of the forms of membranous connective tissue of the Vertebrata (fig. 14). As before FIG. 1 3. Cells and clastic fibres from connective tissue of Aurdia. FIG. 14. Fibres and cells of connective tissue of an Actinia. (Kolliker.) mentioned, in the lower forms of Coslenterata cells are en tirely absent from their jelly-like connective tissue, but in the higher forms scattered cells (fig. 13) of indeterminate shape and position, but generally in the neighbourhood of the entoderm, begin to make their appearance. Some of these cells are amoeboid, but others become fixed, and arranged in a network which pervades the jelly. We see then that the cells, the intercellular substance, the white fibres, and the elastic fibres of the vertebrate connective tissue are represented in these low forms of the Metazoa in a perfectly recognizable manner. It is not surprising, therefore, to find in all the higher classes of the Invertebrata that similar elements characterize the connec tive tissue, although there are undoubtedly certain modifica tions and exceptions. The most noteworthy modifications occur in the chemical constitution of the ground substance and of the fibres. Thus, as before mentioned, there is for the most part an absence of the gelatin-yielding substance of the vertebrate connective tissue. On the other hand, the intercellular substance may become infiltrated with chemical principles unknown in vertebrate histology, as in the tunic of the Tunicata, where cellulose is found. 1 There are modifications also in the appearance of the connective tissue fibres which are often accompanied by modifications in the chemical constitution. For example, in the Arthro- poda the tissue often undergoes extensive chitinization, and the fibres in it present a straight, stiff appearance, very unlike the soft, wavy look which is exhibited by the fibrous tissue of the Vertebrata. Although the ramified cell may be looked upon as on the whole the most characteristic form of cell met with in connective tissue, and although this is the first modification in shape which the rounded embryonic cells of the develop ing vertebrate connective tissue take on, nevertheless it gives place in many parts both in invertebrates as well as vertebrates to other forms. One of the commonest of these is the flat cell, and we almost invariably find cells of this description lying on or in connective tissue membranes, and lining cavities which may have become formed in the con nective tissue. In the latter case the flat cells may be and most commonly are spread over the whole inner surface of the cavity which they line, and assume the appearance of a pavement epithelium. Such cells, which are termed epithelioid (or by some endothelial), are found lining the body cavity and the vascular canals and heart (where these exist) of all invertebrates just as they do the similar cavities and canals in vertebrates, and they are derived like the rest of the cells of the connective tissue from the meso- derm, and therefore only indirectly from the primary blastodermic layers. But in the holothurians, and some other animals, the cells in question are derived directly from the entoderm. In the Mollusca (fig 15) a peculiar type of connective tissue cell makes its appearance in addition to the rounded, the ramified, and the flattened forms. This takes the FIG. 15. Connective tissue of slug, r, ramified cell ;/, flattened cell; r, vesicular cell. The fibres in the ground substance are also indicated. shape of a large clear, vesicular, double-contoured cell-body (v) with a relatively small nucleus. Cells of this character are in some cases only to be found scattered here and there in the tissues, but in others they are closely col lected masses, and by their aggregation confer an almost cartilaginous consistency upon the tissue. This is 1 This tunic seems, however, from its development to he an epithelial
structure, although resembling in many respects connective tissue.
