Xii THE ANIMAL KINGDOM. 



tine, the stomach of the lobster, and the gizzard of the cricket, the malpigian tubes of 

 insects, etc., are really to be classed among the derivatives of the epiblast. 



The mesoblast, after separating from the hypoblast, grows around between the 

 other two layers. It either contains a cavity, originally a part of the archenteron, or 

 such a cavity soon appears. This is the body cavity or ccelom, the pleuro-i^eritpneal 

 cavity of vertebrates. The outer wall of the ccelom unites with the epiblast, the 

 inner with the hypoblast, and the segmentation cavity is obliterated. From the meso- 

 blast arise most of the structures and organs not already enumerated. The list in- 

 cludes the bones of vertebrates, the skeleton of echinodei-ms, spicules of sponges, mus- 

 cles, connective tissue, blood, and excretory and reproductive organs. 



From an embryological standpoint, as we have just seen, we can arrive at a classi- 

 fication of tissues; but if we turn to structure and function, the result is a different 

 association, which, for all except the pure morphologist, is far more satisfactory. The 

 following classification of the tissues is taken, with modifications, from Kolliker : — 



1. Epithelial tissues (epidermal and glandular). 



2. Connective tissues (mucous, cartilage, elastic, areolar, and osseous tissues, and 

 dentine). 



3. Muscular tissues (smooth and striated). 



4. Nerve tissue (nerve-cells and fibres). 



The epithelial tissues consist of cells placed side by side, forming a layer. All the 

 other tissues arise from one having the cells characteristic of epithelium, as the germ- 

 layers are fonried of it. As Kolliker reiterates in 1884, " In all multicellular organ- 

 isms all the elements and tissues arise directly from the fertilized egg-cell and the first 

 embryonic nucleus. (1) The tissues first differentiated have the characters of epithe- 

 lial tissues, and form the ectoblast and endoblast. (2) All the other tissues arise from 

 these two cell-layers; they are either directly derived from them, or arise by the 

 intermediation of a median layer (mesoblast) which, when developed, takes an 

 important part in foiming the tissues. (3) When the whole of the animal series is 

 considered, each of the germinal laj^ers is found to be, in certain creatures, capable of 

 giving rise to at least three, and perhaps to all tissues ; the germinal layers cannot, 

 therefore, be regarded as histologically primitive organs." 



Epithelial cells form the skin or epidermis of animals, and also the lining of the 

 digestive canal. The cells of the latter may, as in sponges, bear a general resemblance 

 to a flagellate infusorian, as Codosi(/a, or they may each bear many hair-like processes 

 called cilia, which by their constant motion maintain currents of the fluids passing 

 over the surface of the epithelium. The cilia lining the inside of the windjiipe serve 

 to sweep any fluid formed there towards the throat, where it can be coughed up and 

 expectorated. 



Connective tissue and its varieties, and gristle or cartilage, bone, etc., arise from the 

 mesoblast and support the parts of the body. All the supporting tissues are used in 

 the body for mechanical purposes : the bones and cartilages form the hard framework 

 by which softer tissues are supported and protected ; and the connective tissues, with 

 the various bones and cartilages, form investing membranes around different organs, 

 and in the form of fine network penetrate their substance and support their constit- 

 uent cells. 



Connective tissue is formed by isolated rounded or elongated cells with wide 

 spaces between them filled with a gelatinous fluid or protoplasm, and occurs between 

 muscles, etc. Gelatinous tissue is a variety of connective tissue found in the umbrella 



